PHYSIOLOGICAL RESPONSES OF ECKLONIA RADIATA (LAMINARIALES) TO A LATITUDINAL GRADIENT IN OCEAN TEMPERATURE(1)

Morpho-physiological traits and tissue burdens of Ecklonia radiata linked to environmental variation in an urban estuary

Organisms respond to their environment in various ways, including moving, adapting, acclimatising or a combination of responses. Within estuarine habitats, organisms are exposed to naturally variable environmental conditions. In urbanised estuaries, these natural variations can interact with human stressors such as habitat modification and pollution. Here, we investigated trait variation in the golden kelp Ecklonia radiata across an urban estuary - Sydney Harbour, Australia. We found that kelp morphology differed significantly between the more human-modified inner and the less modified outer harbour. Kelp individuals were smaller, had fewer laminae, and lacked spines in the inner harbour where it was warmer, more contaminated and less light was available. Inner harbour populations were characterised by lower tissue nitrogen and higher lead concentrations. These findings provide insights into how environmental variation could affect kelp morphology and physiology, and the high trait variation suggests adaptive capacity in E. radiata.

Artificial light at night and warming impact grazing rates and gonad index of the sea urchin Centrostephanus rodgersii

Artificial light at night (ALAN) is a growing threat to coastal habitats, and is likely to exacerbate the impacts of other stressors. Kelp forests are dominant habitats on temperate reefs but are declining due to ocean warming and overgrazing. We tested the independent and interactive effects of ALAN (dark versus ALAN) and warming (ambient versus warm) on grazing rates and gonad index of the sea urchin Centrostephanus rodgersii. Within these treatments, urchins were fed either 'fresh' kelp or 'treated' kelp. Treated kelp (Ecklonia radiata) was exposed to the same light and temperature combinations as urchins. We assessed photosynthetic yield, carbon and nitrogen content and C : N ratio of treated kelp to help identify potential drivers behind any effects on urchins. Grazing increased with warming and ALAN for urchins fed fresh kelp, and increased with warming for urchins fed treated kelp. Gonad index was higher in ALAN/ambient and dark/warm treatments compared to dark/ambient treatments for urchins fed fresh kelp. Kelp carbon content was higher in ALAN/ambient treatments than ALAN/warm treatments at one time point. This indicates ocean warming and ALAN may increase urchin grazing pressure on rocky reefs, an important finding for management strategies.

Temperature sensitivity of detrital photosynthesis

BACKGROUND AND AIMS

Kelp forests are increasingly considered blue carbon habitats for ocean-based biological carbon dioxide removal, but knowledge gaps remain in our understanding of their carbon cycle. Of particular interest is the remineralization of detritus, which can remain photosynthetically active. Here, we study a widespread, thermotolerant kelp (Ecklonia radiata) to explore detrital photosynthesis as a mechanism underlying temperature and light as two key drivers of remineralization.

METHODS

We used meta-analysis to constrain the thermal optimum (Topt) of E. radiata. Temperature and light were subsequently controlled over a 119-day ex situ decomposition experiment. Flow-through experimental tanks were kept in darkness at 15 °C or under a subcompensating maximal irradiance of 8 µmol photons m-2 s-1 at 15, 20 or 25 °C. Photosynthesis of laterals (analogues to leaves) was estimated using closed-chamber oxygen evolution in darkness and under a saturating irradiance of 420 µmol photons m-2 s-1.

KEY RESULTS

T opt of E. radiata is 18 °C across performance variables (photosynthesis, growth, abundance, size, mass and fertility), life stages (gametophyte and sporophyte) and populations. Our models predict that a temperature of >15 °C reduces the potential for E. radiata detritus to be photosynthetically viable, hence detrital Topt ≤ 15 °C. Detritus is viable under subcompensating irradiance, where it performs better than in darkness. Comparison of net and gross photosynthesis indicates that elevated temperature primarily decreases detrital photosynthesis, whereas darkness primarily increases detrital respiration compared with optimal experimental conditions, in which detrital photosynthesis can persist for ≥119 days.

CONCLUSIONS

T opt of kelp detritus is ≥3 °C colder than that of the intact plant. Given that E. radiata is one of the most temperature-tolerant kelps, this suggests that photosynthesis is generally more thermosensitive in the detrital phase, which partly explains the enhancing effect of temperature on remineralization. In contrast to darkness, even subcompensating irradiance maintains detrital viability, elucidating the accelerating effect of depth and its concomitant light reduction on remineralization to some extent. Detrital photosynthesis is a meaningful mechanism underlying at least two drivers of remineralization, even below the photoenvironment inhabited by the attached alga.

The challenge of estimating kelp production in a turbid marine environment

Coastal kelp forests produce substantial marine carbon due to high annual net primary production (NPP) rates, but upscaling of NPP estimates over time and space remains difficult. We investigated the impact of variable underwater photosynthetically active radiation (PAR) and photosynthetic parameters on photosynthetic oxygen production of Laminaria hyperborea, the dominant NE-Atlantic kelp species, throughout summer 2014. Collection depth of kelp had no effect on chlorophyll a content, pointing to a high photoacclimation potential of L. hyperborea towards incident light. However, chlorophyll a and photosynthesis versus irradiance parameters differed significantly along the blade gradient when normalized to fresh mass, potentially introducing large uncertainties in NPP upscaling to whole thalli. Therefore, we recommend a normalization to kelp tissue area, which is stable over the blade gradient. Continuous PAR measurements revealed a highly variable underwater light climate at our study site (Helgoland, North Sea) in summer 2014, reflected by PAR attenuation coefficients (Kd ) between 0.28 and 0.87 m-1 . Our data highlight the importance of continuous underwater light measurements or representative average values using a weighted Kd to account for large PAR variability in NPP calculations. Strong winds in August increased turbidity, resulting in a negative carbon balance at depths >3-4 m over several weeks, considerably impacting kelp productivity. Estimated daily summer NPP over all four depths was 1.48 ± 0.97 g C · m-2 seafloor · d-1 for the Helgolandic kelp forest, which is in the range of other kelp forests along European coastlines.

The Influence of Abiotic Factors on the Induction of Seaweed Callus

Seaweeds are a major source of functional foods, nutraceuticals, and pharmaceuticals. Seaweed can be sustainably harvested through callus culture, which yields homogenous cells and bioproducts under controlled conditions. Callus induction is a crucial early step in callus culture and is influenced by several abiotic factors. This review aims to discuss the influence of abiotic factors on callus induction in seaweeds, a prerequisite for the application and development of seaweed callus culture. We used three online databases (Springer, Science Direct, and Wiley) to search for the literature on seaweed callus induction published between 1987 and 2020. Thirty-three articles for review were identified and analyzed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The analysis covers 56 seaweed species (3% Chlorophyta, 44% Phaeophyta, and 53% Rhodophyta) under various abiotic treatments, including light irradiance (23%), temperature (15%), media type (21%), plant growth regulators (26%), gelling conditions (9%), and other factors (6%). The information on these abiotic factors is intended to be a practical reference and to foster the further study of the callus culture of seaweed. More studies are needed to determine how to maintain and increase callus mass in suspension culture for the industrial production of seaweed and its metabolites.

Response of Cymodocea nodosa to ocean acidification and warming in the Canary Islands: Direct and indirect effects

As detected in warming and ocean acidification, global change can have profound impact on marine life. Its effects on seagrasses are becoming increasingly well-known, since several studies have focused on the responses of these species to global change conditions. However a few studies have assessed the combined effect of temperature and acidification on seagrasses. Overall in this study, the combined effects of increased ocean temperature and pH levels expected at the end of this century (+5 °C and pH 7.5) on Cymodocea nodosa from Canary Islands, were evaluated for one month through manipulative laboratory experiments. Growth, net production, respiration, gross primary production, chlorophyll-a concentration and its vulnerability to herbivory were quantified. Results showed a positive effect of decreased pH on growth and gross primary production, as well as greater vulnerability to consumption by the sea urchin Paracentrotus lividus. In contrast, increased temperature limited net and gross primary production. This study shows than in future scenarios, C. nodosa from the Canary Islands may be a losing species in the global change stakes.

Genotype-Environment mismatch of kelp forests under climate change

Climate change is increasingly impacting ecosystems globally. Understanding adaptive genetic diversity and whether it will keep pace with projected climatic change is necessary to assess species' vulnerability and design efficient mitigation strategies such as assisted adaptation. Kelp forests are the foundations of temperate reefs globally but are declining in many regions due to climate stress. A lack of knowledge of kelp's adaptive genetic diversity hinders assessment of vulnerability under extant and future climates. Using 4245 single nucleotide polymorphisms (SNPs), we characterized patterns of neutral and putative adaptive genetic diversity for the dominant kelp in the southern hemisphere (Ecklonia radiata) from ~1000 km of coastline off Western Australia. Strong population structure and isolation-by-distance was underpinned by significant signatures of selection related to temperature and light. Gradient forest analysis of temperature-linked SNPs under selection revealed a strong association with mean annual temperature range, suggesting adaptation to local thermal environments. Critically, modelling revealed that predicted climate-mediated temperature changes will probably result in high genomic vulnerability via a mismatch between current and future predicted genotype-environment relationships such that kelp forests off Western Australia will need to significantly adapt to keep pace with projected climate change. Proactive management techniques such as assisted adaptation to boost resilience may be required to secure the future of these kelp forests and the immense ecological and economic values they support.

Physiological response to warming in intertidal macroalgae with different thermal affinity

Changes in the abundance and distribution of seaweeds have been reported worldwide. In Northwest Iberia, cold and warm affinity seaweeds have respectively decreased and increased their abundance. To improve our understanding of their vulnerability to future warming scenarios, the effects of warming on the photosynthetic and respiratory performance of seaweeds with cold-water (Fucus serratus and Vertebrata lanosa) and warm-water (Padina pavonica and Gigartina pistillata) affinities were compared in a highly resolved temperature gradient (7-31 °C) under controlled laboratory conditions. While neither the optimum temperature nor the photosynthetic rate at the optimum temperature showed consistent differences between water affinity groups, the temperature dependence of net photosynthesis was significantly higher in the warm-affinity group. Photosynthesis was less responsive than respiration to warming in cold-affinity seaweeds while both rates showed similar responsiveness in warm-affinity ones, suggesting that the relative responsiveness of respiration and photosynthesis to temperature may be indicative of warming susceptibility.

Warming intensifies the interaction between the temperate seagrass Posidonia oceanica and its dominant fish herbivore Sarpa salpa

Apart from directly influencing individual life histories of species, climate change is altering key biotic interactions as well, causing community processes to unravel. With rising temperatures, disruptions to producer-consumer relationships can have major knock-on effects, particularly when the producer is a habitat-forming species. We studied how sea surface temperature (SST) modifies multiple pathways influencing the interaction between the foundational seagrass species, Posidonia oceanica, and its main consumer, the fish Sarpa salpa in the Mediterranean Sea. We used a combination of a field-based temperature gradient approaches and experimental manipulations to assess the effect of temperature on seagrass performance (growth) and fish early life history (larval development) as well as on the interaction itself (seagrass palatability and fish foraging activity). Within the range of temperatures assessed, S. salpa larvae grew slightly faster at warmer conditions but maintained their settlement size, resulting in a relatively small reduction in pelagic larval duration (PLD) and potentially reducing dispersion. Under warmer conditions (>24 °C), P. oceanica reduced its growth rate considerably and seemed to display fewer deterring mechanisms as indicated by a disproportionate consumption in choice experiments. However, our field-based observations along the temperature gradient showed no change in fish foraging time, or in other aspects of feeding behaviour. As oceans warm, our results indicate that, while S. salpa may show little change in early life history, its preference towards P. oceanica might increase, which, together with reduced seagrass growth, could considerably intensify the strength of herbivory. It is unclear if P. oceanica meadows can sustain such an intensification, but it will clearly add to the raft of pressures this threatened ecosystem already faces from global and local environmental change.

Effects of climate change factors on marine macroalgae: A review

Marine macroalgae, the main primary producers in coastal waters, play important roles in the fishery industry and global carbon cycles. With progressive ocean global changes, however, they are increasingly exposed to enhanced levels of multiple environmental drivers, such as ocean acidification, warming, heatwaves, UV radiation and deoxygenation. While most macroalgae have developed physiological strategies against variations of these drivers, their eco-physiological responses to each or combinations of the drivers differ spatiotemporally and species-specifically. Many freshwater macroalgae are tolerant of pH drop and its diel fluctuations and capable of acclimating to changes in carbonate chemistry. However, calcifying species, such as coralline algae, are very sensitive to acidification of seawater, which reduces their calcification, and additionally, temperature rise and UV further decrease their physiological performance. Except for these calcifying species, both economically important and harmful macroalgae can benefit from elevated CO₂ concentrations and moderate temperature rise, which might be responsible for increasing events of harmful macroalgal blooms including green macroalgal blooms caused by Ulva spp. and golden tides caused by Sargassum spp. Upper intertidal macroalgae, especially those tolerant of dehydration during low tide, increase their photosynthesis under elevated CO₂ concentrations during the initial dehydration period, however, these species might be endangered by heatwaves, which can expose them to high temperature levels above their thermal windows' upper limit. On the other hand, since macroalgae are distributed in shallow waters, they are inevitably exposed to solar UV radiation. The effects of UV radiation, depending on weather conditions and species, can be harmful as well as beneficial to many species. Moderate levels of UV-A (315-400nm) can enhance photosynthesis of green, brown and red algae, while UV-B (280-315nm) mainly show inhibitory impacts. Although little has been documented on the combined effects of elevated CO₂, temperature or heatwaves with UV radiation, exposures to heatwaves during midday under high levels of UV radiation can be detrimental to most species, especially to their microscopic stages which are less tolerant of climate change induced stress. In parallel, reduced availability of dissolved O₂ in coastal water along with eutrophication might favour the macroalgae's carboxylation process by suppressing their oxygenation or photorespiration. In this review, we analyse effects of climate change-relevant drivers individually and/or jointly on different macroalgal groups and different life cycle stages based on the literatures surveyed, and provide perspectives for future studies.

Heatwaves during low tide are critical for the physiological performance of intertidal macroalgae under global warming scenarios

The abundance and distribution of intertidal canopy-forming macroalgae are threatened by the increase in sea surface temperature and in the frequency and intensity of heatwaves caused by global warming. This study evaluated the physiological response of predominant intertidal macroalgae in the NW Iberian Peninsula (Bifurcaria bifurcata, Cystoseira tamariscifolia and Codium tomentosum) to increased seawater temperature during immersion and increased air temperatures during consecutive emersion cycles. We combined field mensuration and laboratory experiments in which we measured mortality, growth, maximum quantum yield and C:N content of the macroalgae. Air temperature was a critical factor in determining physiological responses and survivorship of all species, whereas high seawater temperature had sublethal effects. Cystoseira tamariscifolia suffered the greatest decreases in Fv/Fm, growth and the highest mortality under higher air temperatures, whereas C. tomentosum was the most resistant and resilient species. Two consecutive cycles of emersion under atmospheric heatwaves caused cumulative stress in all three macroalgae, affecting the physiological performance and increasing the mortality. The potential expansion of the warm-temperate species B. bifurcata, C. tamariscifolia and C. tomentosum in the NW Iberian Peninsula in response to increasing seawater temperature may be affected by the impact of increased air temperature, especially in a region where the incidence of atmospheric heatwaves is expected to increase.

Gene transcription and antioxidants production in Arthrospira (Spirulina) platensis grown under temperature variation

AIM

To investigate the transcription of selected antioxidants and relevant genes under varying temperature conditions, and to identify the optimum temperature for antioxidants production by Arthrospira platensis.

METHODS AND RESULTS

The dry weight (DW), pigment production, antioxidants production and gene transcription were examined in A. platensis growing under three temperatures of 23, 30 and 37°C. The cyanobacterial DW was highest in the high temperatures (30 and 37°C), while the pigments, such as Chl a, carotenoids, C-phycocyanin and total phycobiliprotein contents, showed their maximum value at 30°C. The total soluble protein and carbohydrate contents were highest at 30°C. Lipid peroxidation, as a marker for thermal stress, was high at 23°C, while higher temperatures remarkably reduced lipid peroxidation levels. Antioxidants activity was increased by 1·5-fold at 30°C and temperature fluctuations induced the antioxidant enzyme activities. The transcriptional abundance of heat shock protein (HSP90), glutamate synthase (GOGAT), delta-9 desaturase (desC), iron-superoxide dismutase (FeSOD) and the large subunit of Rubisco (rbcL) genes was measured under the same temperatures.

CONCLUSION

The optimal temperature for growth, biochemical constituents and antioxidants of A. platensis is 30°C while some antioxidant enzyme activity increased at lower and higher temperatures.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study showed the significance of temperature for growth, enzymatic and non-enzymatic antioxidants and gene expression in A. platensis. This contributes to the knowledge of culturing A. platensis to harvest specific antioxidants or as an antioxidant-rich food source.

Genetic tropicalisation following a marine heatwave

Extreme events are increasing globally with devastating ecological consequences, but the impacts on underlying genetic diversity and structure are often cryptic and poorly understood, hindering assessment of adaptive capacity and ecosystem vulnerability to future change. Using very rare "before" data we empirically demonstrate that an extreme marine heatwave caused a significant poleward shift in genetic clusters of kelp forests whereby alleles characteristic of cool water were replaced by those that predominated in warm water across 200 km of coastline. This "genetic tropicalisation" was facilitated by significant mortality of kelp and other co-occurring seaweeds within the footprint of the heatwave that opened space for rapid local proliferation of surviving kelp genotypes or dispersal and recruitment of spores from warmer waters. Genetic diversity declined and inbreeding increased in the newly tropicalised site, but these metrics were relative stable elsewhere within the footprint of the heatwave. Thus, extreme events such as marine heatwaves not only lead to significant mortality and population loss but can also drive significant genetic change in natural populations.

Substantial blue carbon in overlooked Australian kelp forests

Recognition of the potential for vegetated coastal ecosystems to store and sequester carbon has led to their increasing inclusion into global carbon budgets and carbon offset schemes. However, kelp forests have been overlooked in evaluations of this 'blue carbon', which have been limited to tidal marshes, mangrove forests, and seagrass beds. We determined the continental-scale contribution to blue carbon from kelp forests in Australia using areal extent, biomass, and productivity measures from across the entire Great Southern Reef. We reveal that these kelp forests represent 10.3-22.7 Tg C and contribute 1.3-2.8 Tg C year^-1 in sequestered production, amounting to more than 30% of total blue carbon stored and sequestered around the Australian continent, and ~ 3% of the total global blue carbon. We conclude that the omission of kelp forests from blue carbon assessments significantly underestimates the carbon storage and sequestration potential from vegetated coastal ecosystems globally.

Local thermal adaptation and limited gene flow constrain future climate responses of a marine ecosystem engineer

Rising ocean temperatures and extreme temperature events have precipitated declines and local extinctions in many marine species globally, but patterns of loss are often uneven across species ranges for reasons that are poorly understood. Knowledge of the extent of local adaptation and gene flow may explain such patterns and help predict future trajectories under scenarios of climate change. We test the extent to which local differentiation in thermal tolerance is influenced by gene flow and local adaptation using a widely distributed intertidal seaweed (Hormosira banksii) from temperate Australia. Population surveys across ~2,000 km of the species range revealed strong genetic structuring at regional and local scales (global F ST = 0.243) reflecting extremely limited gene flow, while common garden experiments (14-day exposures to 15, 18, 21°C) revealed strong site differences in early development and mortality in response to elevated temperature. Embryos from many sites spanning a longitudinal thermal gradient showed suppressed development and increased mortality to elevated water temperatures, but populations originating from warmer and more variable thermal environments tended to be less susceptible to warming. Notably, there was significant local-scale variation in the thermal responses of embryos within regions which was corroborated by the finding of small-scale genetic differences. We expect the observed genetic and phenotypic differentiation to lead to uneven responses to warming sea surface temperatures in this important marine foundation species. The study highlights the challenges of predicting species responses to thermal stress and the importance of management strategies that incorporate evolutionary potential for "climate-proofing" marine ecosystems.

Nitrogen sufficiency enhances thermal tolerance in habitat-forming kelp: implications for acclimation under thermal stress

Local and global changes associated with anthropogenic activities are impacting marine and terrestrial ecosystems. Macroalgae, especially habitat-forming species like kelp, play critical roles in temperate coastal ecosystems. However, their abundance and distribution patterns have been negatively affected by warming in many regions around the globe. Along with global change, coastal ecosystems are also impacted by local drivers such as eutrophication. The interaction between global and local drivers might modulate kelp responses to environmental change. This study examines the regulatory effect of NO₃⁻ on the thermal plasticity of the giant kelp Macrocystis pyrifera. To do this, thermal performance curves (TPCs) of key temperature-dependant traits–growth, photosynthesis, NO₃⁻ assimilation and chlorophyll a fluorescence–were examined under nitrate replete and deplete conditions in a short-term incubation. We found that thermal plasticity was modulated by NO₃⁻ but different thermal responses were observed among traits. Our study reveals that nitrogen, a local driver, modulates kelp responses to high seawater temperatures, ameliorating the negative impacts on physiological performance (i.e. growth and photosynthesis). However, this effect might be species-specific and vary among biogeographic regions – thus, further work is needed to determine the generality of our findings to other key temperate macroalgae that are experiencing temperatures close to their thermal tolerance due to climate change.

Diverse responses of sporophytic photochemical efficiency and gametophytic growth for two edible kelps, Saccharina japonica and Undaria pinnatifida, to ocean acidification and warming

Ocean acidification and warming represent major environmental threats to kelp mariculture. In this study, sporophytic photochemical efficiency and gametophytic growth of Saccharina japonica and Undaria pinnatifida were evaluated under different pCO2 levels (360, 720, and 980 ppmv) and temperatures (5, 10, 15, and 20 °C for sporophytes; 15 and 20 °C for gametophytes). Sporophytic photochemical efficiencies of both kelps were significantly greater at 720 ppmv than at 360 and 980 ppmv. Female gametophytes of both kelps grew significantly better at 360 ppmv than at higher pCO2 levels. The growth of U. pinnatifida gametophytes was significantly greater at 20 °C than at 15 °C, while no significant difference was observed for the growth of S. japonica. These results indicate that increased pCO2 stimulated sporophytic photochemical efficiency while inhibited gametophytic growth of these kelps, which might negatively affect their seedling cultivation. U. pinnatifida exhibited higher productivity in warmer ocean than S. japonica.

Seasonal and spatial variation in photosynthetic response of the kelp Ecklonia radiata across a turbidity gradient

Understanding the photoacclimation response of macroalgae across broad spatial and temporal scales is necessary for predicting their vulnerability to environmental changes and quantifying their contribution to coastal primary production. This study investigated how the photosynthesis-irradiance response and photosynthetic pigment content of the kelp Ecklonia radiata varies both spatially and seasonally among seven sites located across a turbidity gradient in the Hauraki Gulf, north-eastern New Zealand. Photosynthesis-irradiance curves were derived under laboratory conditions for whole adult E. radiata using photorespirometry chambers. Lab-derived photosynthesis-irradiance curves in summer were also compared with in situ measurements made on kelp at each of the seven study sites. Photosynthetic parameters and pigments showed clear seasonal patterns across all sites as demonstrated by higher photosynthetic pigment levels and photosynthetic efficiency occurring in autumn and winter, and higher maximum rates of photosynthesis and respiration occurring in summer. Lamina biomass was similar across sites, yet thalli exhibited a clear photokinetic response to increasing turbidity. At turbid sites photosynthetic pigment levels and photosynthetic efficiency was higher, and respiration and saturation and compensation irradiances lower, compared to high-light sites. The results presented here further our understanding of low-light acclimation strategies in kelp and highlight the degree of seasonality in photosynthetic parameters. Though E. radiata demonstrates a clear capacity to photoacclimate to a degrading light environment, further research is needed to investigate the extent to which the observed acclimation can offset the likely negative effects of increasing turbidity on kelp forest primary production.

Invasion-mediated effects on marine trophic interactions in a changing climate: positive feedbacks favour kelp persistence

The interactive effects of ocean warming and invasive species are complex and remain a source of uncertainty for projecting future ecological change. Climate-mediated change to trophic interactions can have pervasive ecological consequences, but the role of invasion in mediating trophic effects is largely unstudied. Using manipulative experiments in replicated outdoor mesocosms, we reveal how near-future ocean warming and macrophyte invasion scenarios interactively impact gastropod grazing intensity and preference for consumption of foundation macroalgae ( Ecklonia radiata and Sargassum vestitum). Elevated water temperature increased the consumption of both macroalgae through greater grazing intensity. Given the documented decline of kelp ( E. radiata) growth at higher water temperatures, enhanced grazing could contribute to the shift from kelp-dominated to Sargassum-dominated reefs that is occurring at the low-latitude margins of kelp distribution. However, the presence of a native invader ( Caulerpa filiformis) was related to low consumption by the herbivores on dominant kelp at warmer temperatures. Thus, antagonistic effects between climate change and a range expanding species can favour kelp persistence in a warmer future. Introduction of species should, therefore, not automatically be considered unfavourable under climate change scenarios. Climatic changes are increasing the need for effective management actions to address the interactive effects of multiple stressors and their ecological consequences, rather than single threats in isolation.

Ecophysiological responses of a threatened red alga to increased irradiance in an in situ transplant experiment

The red alga Gelidium corneum is a dominant foundation species in the south-eastern Bay of Biscay, where a decline in its populations has been documented in the few last decades. We investigated the ecophysiological responses of G. corneum to different light conditions by means of an in situ transplant experiment. We found that the stress response measured by physiological and biochemical approaches was higher in G. corneum at higher irradiance levels, for both transplanted and control specimens, than under lower light intensities. In the former case the specimens showed a decrease in maximum quantum yield (F_v/F_m), maximum electron transport rate (ETR_max), photosynthetic efficiency (α_ETR), photosynthetic pigment contents, nitrogen content and thallus length, whereas the C:N ratio, MAAs and bleaching cover increased. In general terms, these responses were more evident in the apical parts of the thallus than in middle ones. Our results suggest that high light stress at depths of 3 m triggered photobiological changes in G. corneum, involving ineffective photoprotection and the occurrence of chronic photoinhibition. Therefore, considering the upward trend in summer mean surface solar radiation in the study area since the 80s, high light conditions may have played a role in the declines observed in G. corneum beds from the south-eastern Bay of Biscay.

Trawl ban in a heavily exploited marine environment: Responses in population dynamics of four stomatopod species

Intensive trawling activities in Hong Kong waters have seriously depleted fishery resources and damaged marine benthic habitats over the last four decades. To minimize further destruction and rehabilitate fishery resources, the Hong Kong Government implemented a permanent territory-wide trawling closure on 31 December 2012. Such a trawl ban creates a unique opportunity to investigate recoveries in ecosystem structure and function following a major shift in disturbance regime by removing impacts from a major gear. This study was designed to test the hypothesis that dominant predatory mantis shrimps, including Harpiosquilla harpax, Miyakella nepa, Oratosquillina interrupta, and Oratosquilla oratoria would show signs of recovery following the trawl ban. Their population dynamics were investigated before and after the trawl ban. The results showed that their mean weight, mean carapace length and proportion of large-sized individuals increased significantly 3.5 years after the trawl ban, whilst their abundance, biomass and maximum length remained unchanged. This study suggests that the stomatopod assemblage in the human-dominated Hong Kong waters shows some initial signs of possible recovery following the trawl ban but also highlights the complexity of implementing fishery management and detecting changes resulted from management measures in a heavily urbanized seascape where many biotic and abiotic factors can influence their population dynamics.

Comparative physiological behaviors of Ulva lactuca and Gracilariopsis lemaneiformis in responses to elevated atmospheric CO2 and temperature

Physiological metabolisms of seaweeds usually suffered climate changes in the field. Gracilariopsis lemaneiformis and Ulva lactuca, collected from Nan'ao Island, Shantou, China, were cultured under ambient and elevated CO₂ supply (390 and 800 μl L^-1), with low and high temperatures (15 °C and 25 °C) for 2 weeks, aiming to compare the difference of the main physiological metabolism between two seaweed species in response to the elevated CO₂ and high temperature. At 15 °C, the pH reduction in the culture medium caused by elevated CO₂ was larger in G. lemaneiformis than in U. lactuca. At 25 °C, elevated CO₂ significantly increased photosynthetic rates (P_n or P_g) and maintained constant respiratory rates (R_d) in G. lemaneiformis. However, for 25 °C-grown U. lactuca, the increment of CO₂ did not enhance the P_n (P_g) rates but rapidly decreased the R_d rates itself. With the higher R_d/P_g ratios in G. lemaneiformis than U. lactuca, the warming thereby promoted more allocation of photosynthetic products to respiratory consumption in G. lemaneiformis. Both P_g and R_d rates exhibited lower temperature acclimation in two seaweeds. In addition, elevated CO₂ markedly increased the relative growth rate (RGR) and phycobiliprotein (PB) contents at 25 °C, but exhibited no enhancement of chlorophyll a (Chl a), carotenoids (Car), soluble carbohydrate (SC), and soluble protein (SP) contents in G. lemaneiformis, with the reduction of SC when temperature increased only. We suggested that climate changes were probably a more benefit to U. lactuca than to G. lemaneiformis, inherently justifying the metabolism during G. lemaneiformis maricultivation.

Seaweed assemblages under a climate change scenario: Functional responses to temperature of eight intertidal seaweeds match recent abundance shifts

Field evidence is essential to assess the consequences of climate change but a solid causal link often requires additional information obtained under controlled laboratory conditions. Additionally, the functional response to temperature may also help to discriminate species potentially more vulnerable to warming. Using a highly resolved temperature gradient, we examined the temperature dependence of photosynthesis and respiration in eight intertidal seaweeds that recently followed opposite abundance trends in NW Iberia. The temperature dependence of photosynthesis was consistently different between the macroalgae that increased and those that decreased their abundance in the last decade and a half, with photosynthesis twice more sensitive in the upward group. Unlike photosynthesis, the temperature dependence of respiration was unrelated to the abundance trend group, implying that the net metabolic scaling with temperature varied between the two groups of seaweeds. Overall, our results provide experimental support to the role of temperate as a likely driver of the changes in abundance recorded by field-monitoring studies. They also suggest that the temperature dependence of photosynthesis and respiration assessed in short-term experiments may serve as a biomarker of the potential vulnerability of some seaweed to the consequences of water warming.

High acclimation potential in floating Macrocystis pyrifera to abiotic conditions even under grazing pressure - a field study

The persistence of floating seaweeds, which depends on abiotic conditions but also herbivory, had previously been mostly tested in outdoor mesocosm experiments. In order to investigate if the obtained mesocosm results of high seaweed persistence under natural environmental conditions and under grazing pressure can be extrapolated to field situations, we conducted in situ experiments. During two summers (2007 and 2008), Macrocystis pyrifera was tethered (for 14 d) to lines in the presence and absence of the amphipod Peramphithoe femorata at three sites (Iquique, Coquimbo, Calfuco). We hypothesized that grazing damage and seaweed persistence vary among sites due to different abiotic factors. By incubating the sporophytes in mesh bags, we were either able to isolate (grazing) or exclude (control) amphipods. To test for a mesh bag artifact, a set of sporophytes was incubated without mesh bags (natural). Mesh bags used to exclude herbivores influenced sporophyte growth and physiological performance. The chlorophyll a (Chl a) content depended largely on grazers and grazed sporophytes grew less than natural and control sporophytes within the two summers. A decrease in Chl a content was found for the sites with the highest prevailing irradiances and temperatures, suggesting an efficient acclimation to these sea surface conditions. Our field-based results of sporophyte acclimation ability even under grazing pressure widely align with previous mesocosm results. We conclude that M. pyrifera and other temperate floating seaweeds can function as long-distance dispersal vectors even with hitchhiking mesoherbivores.

Genetic diversity and kelp forest vulnerability to climatic stress

Genetic diversity confers adaptive capacity to populations under changing conditions but its role in mediating impacts of climate change remains unresolved for most ecosystems. This lack of knowledge is particularly acute for foundation species, where impacts may cascade throughout entire ecosystems. We combined population genetics with eco-physiological and ecological field experiments to explore relationships among latitudinal patterns in genetic diversity, physiology and resilience of a kelp ecosystem to climate stress. A subsequent 'natural experiment' illustrated the possible influence of latitudinal patterns of genetic diversity on ecosystem vulnerability to an extreme climatic perturbation (marine heatwave). There were strong relationships between physiological versatility, ecological resilience and genetic diversity of kelp forests across latitudes, and genetic diversity consistently outperformed other explanatory variables in contributing to the response of kelp forests to the marine heatwave. Population performance and vulnerability to a severe climatic event were thus strongly related to latitudinal patterns in genetic diversity, with the heatwave extirpating forests with low genetic diversity. Where foundation species control ecological structure and function, impacts of climatic stress can cascade through the ecosystem and, consequently, genetic diversity could contribute to ecosystem vulnerability to climate change.

Global and local disturbances interact to modify seagrass palatability

Global change, such as warming and ocean acidification, and local anthropogenic disturbances, such as eutrophication, can have profound impacts on marine organisms. However, we are far from being able to predict the outcome of multiple interacting disturbances on seagrass communities. Herbivores are key in determining plant community structure and the transfer of energy up the food web. Global and local disturbances may alter the ecological role of herbivory by modifying leaf palatability (i.e. leaf traits) and consequently, the feeding patterns of herbivores. This study evaluates the main and interactive effects of factors related to global change (i.e. elevated temperature, lower pH levels and associated ocean acidification) and local disturbance (i.e. eutrophication through ammonium enrichment) on a broad spectrum of leaf traits using the temperate seagrass Cymodocea nodosa, including structural, nutritional, biomechanical and chemical traits. The effect of these traits on the consumption rates of the generalist herbivore Paracentrotus lividus (purple sea urchin) is evaluated. The three disturbances of warming, low pH level and eutrophication, alone and in combination, increased the consumption rate of seagrass by modifying all leaf traits. Leaf nutritional quality, measured as nitrogen content, was positively correlated to consumption rate. In contrast, a negative correlation was found between feeding decisions by sea urchins and structural, biomechanical and chemical leaf traits. In addition, a notable accomplishment of this work is the identification of phenolic compounds not previously reported for C. nodosa. Our results suggest that global and local disturbances may trigger a major shift in the herbivory of seagrass communities, with important implications for the resilience of seagrass ecosystems.

Future warming and acidification effects on anti-fouling and anti-herbivory traits of the brown alga Fucus vesiculosus (Phaeophyceae)

Human-induced ocean warming and acidification have received increasing attention over the past decade and are considered to have substantial consequences for a broad range of marine species and their interactions. Understanding how these interactions shift in response to climate change is particularly important with regard to foundation species, such as the brown alga Fucus vesiculosus. This macroalga represents the dominant habitat former on coastal rocky substrata of the Baltic Sea, fulfilling functions essential for the entire benthic community. Its ability to withstand extensive fouling and herbivory regulates the associated community and ecosystem dynamics. This study tested the interactive effects of future warming, acidification, and seasonality on the interactions of a marine macroalga with potential foulers and consumers. F. vesiculosus rockweeds were exposed to different combinations of conditions predicted regionally for the year 2100 (+∆5°C, +∆700 μatm CO2 ) using multifactorial long-term experiments in novel outdoor benthic mesocosms ("Benthocosms") over 9-12-week periods in four seasons. Possible shifts in the macroalgal susceptibility to fouling and consumption were tested using consecutive bioassays. Algal susceptibility to fouling and grazing varied substantially among seasons and between treatments. In all seasons, warming predominantly affected anti-fouling and anti-herbivory interactions while acidification had a subtle nonsignificant influence. Interestingly, anti-microfouling activity was highest during winter under warming, while anti-macrofouling and anti-herbivory activities were highest in the summer under warming. These contrasting findings indicate that seasonal changes in anti-fouling and anti-herbivory traits may interact with ocean warming in altering F. vesiculosus community composition in the future.

Climate-driven disparities among ecological interactions threaten kelp forest persistence

The combination of ocean warming and acidification brings an uncertain future to kelp forests that occupy the warmest parts of their range. These forests are not only subject to the direct negative effects of ocean climate change, but also to a combination of unknown indirect effects associated with changing ecological landscapes. Here, we used mesocosm experiments to test the direct effects of ocean warming and acidification on kelp biomass and photosynthetic health, as well as climate-driven disparities in indirect effects involving key consumers (urchins and rock lobsters) and competitors (algal turf). Elevated water temperature directly reduced kelp biomass, while their turf-forming competitors expanded in response to ocean acidification and declining kelp canopy. Elevated temperatures also increased growth of urchins and, concurrently, the rate at which they thinned kelp canopy. Rock lobsters, which are renowned for keeping urchin populations in check, indirectly intensified negative pressures on kelp by reducing their consumption of urchins in response to elevated temperature. Overall, these results suggest that kelp forests situated towards the low-latitude margins of their distribution will need to adapt to ocean warming in order to persist in the future. What is less certain is how such adaptation in kelps can occur in the face of intensifying consumptive (via ocean warming) and competitive (via ocean acidification) pressures that affect key ecological interactions associated with their persistence. If such indirect effects counter adaptation to changing climate, they may erode the stability of kelp forests and increase the probability of regime shifts from complex habitat-forming species to more simple habitats dominated by algal turfs.

Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp

Some of the most profound effects of climate change on ecological communities are due to alterations in species interactions rather than direct physiological effects of changing environmental conditions. Empirical evidence of historical changes in species interactions within climate-impacted communities is, however, rare and difficult to obtain. Here, we demonstrate the recent disappearance of key habitat-forming kelp forests from a warming tropical-temperate transition zone in eastern Australia. Using a 10-y video dataset encompassing a 0.6 °C warming period, we show how herbivory increased as kelp gradually declined and then disappeared. Concurrently, fish communities from sites where kelp was originally abundant but subsequently disappeared became increasingly dominated by tropical herbivores. Feeding assays identified two key tropical/subtropical herbivores that consumed transplanted kelp within hours at these sites. There was also a distinct increase in the abundance of fishes that consume epilithic algae, and much higher bite rates by this group at sites without kelp, suggesting a key role for these fishes in maintaining reefs in kelp-free states by removing kelp recruits. Changes in kelp abundance showed no direct relationship to seawater temperatures over the decade and were also unrelated to other measured abiotic factors (nutrients and storms). Our results show that warming-mediated increases in fish herbivory pose a significant threat to kelp-dominated ecosystems in Australia and, potentially, globally.

Exploring the role of temperature in the ocean through metabolic scaling

Temperature imposes a constraint on the rates and outcomes of ecological processes that determine community- and ecosystem-level patterns. The application of metabolic scaling theory has advanced our understanding of the influence of temperature on pattern and process in marine communities. Metabolic scaling theory uses the fundamental and ubiquitous patterns of temperature-dependent metabolism to predict how environmental temperature influences patterns and processes at higher levels of biological organization. Here, we outline some of these predictions to review recent advances and illustrate how scaling theory might be applied to new challenges. For example, warming can alter species interactions and food-web structure and can also reduce total animal biomass supportable by a given amount of primary production by increasing animal metabolism and energetic demand. Additionally, within a species, larval development is faster in warmer water, potentially influencing dispersal and other demographic processes like population connectivity and gene flow. These predictions can be extended further to address major questions in marine ecology, and present an opportunity for conceptual unification of marine ecological research across levels of biological organization. Drawing on work by ecologists and oceanographers over the last century, a metabolic scaling approach represents a promising way forward for applying ecological understanding to basic questions as well as conservation challenges.

Disciplinary reporting affects the interpretation of climate change impacts in global oceans

Climate change is affecting marine ecosystems, but different investigative approaches in physical, chemical, and biological disciplines may influence interpretations of climate-driven changes in the ocean. Here, we review the ocean change literature from 2007 to 2012 based on 461 of the most highly cited studies in physical and chemical oceanography and three biological subdisciplines. Using highly cited studies, we focus on research that has shaped recent discourse on climate-driven ocean change. Our review identified significant differences in spatial and temporal scales of investigation among disciplines. Physical/chemical studies had a median duration of 29 years (n = 150) and covered the greatest study areas (median 1.41 × 10(7) km(2) , n = 148). Few biological studies were conducted over similar spatial and temporal scales (median 8 years, n = 215; median 302 km(2) , n = 196), suggesting a more limited ability to separate climate-related responses from natural variability. We linked physical/chemical and biological disciplines by tracking studies examining biological responses to changing ocean conditions. Of the 545 biological responses recorded, a single physical or chemical stressor was usually implicated as the cause (59%), with temperature as the most common primary stressor (44%). The most frequently studied biological responses were changes in physiology (31%) and population abundance (30%). Differences in disciplinary studies, as identified in this review, can ultimately influence how researchers interpret climate-related impacts in marine systems. We identified research gaps and the need for more discourse in (1) the Indian and other Southern Hemisphere ocean basins; (2) research themes such as archaea, bacteria, viruses, mangroves, turtles, and ocean acidification; (3) physical and chemical stressors such as dissolved oxygen, salinity, and upwelling; and (4) adaptive responses of marine organisms to climate-driven ocean change. Our findings reveal that highly cited biological studies are rarely conducted on scales that match those of physical and chemical studies. Rather, we suggest a need for measuring responses at biologically relevant scales.

Sensitivity and Acclimation of Three Canopy-Forming Seaweeds to UVB Radiation and Warming

Canopy-forming seaweeds, as primary producers and foundation species, provide key ecological services. Their responses to multiple stressors associated with climate change could therefore have important knock-on effects on the functioning of coastal ecosystems. We examined interactive effects of UVB radiation and warming on juveniles of three habitat-forming subtidal seaweeds from Western Australia–Ecklonia radiata, Scytothalia dorycarpa and Sargassum sp. Fronds were incubated for 14 days at 16–30°C with or without UVB radiation and growth, health status, photosynthetic performance, and light absorbance measured. Furthermore, we used empirical models from the metabolic theory of ecology to evaluate the sensitivity of these important seaweeds to ocean warming. Results indicated that responses to UVB and warming were species specific, with Sargassum showing highest tolerance to a broad range of temperatures. Scytothalia was most sensitive to elevated temperature based on the reduced maximum quantum yields of PSII; however, Ecklonia was most sensitive, according to the comparison of activation energy calculated from Arrhenius’ model. UVB radiation caused reduction in the growth, physiological responses and thallus health in all three species. Our findings indicate that Scytothalia was capable of acclimating in response to UVB and increasing its light absorption efficiency in the UV bands, probably by up-regulating synthesis of photoprotective compounds. The other two species did not acclimate over the two weeks of exposure to UVB. Overall, UVB and warming would severely inhibit the growth and photosynthesis of these canopy-forming seaweeds and decrease their coverage. Differences in the sensitivity and acclimation of major seaweed species to temperature and UVB may alter the balance between species in future seaweed communities under climate change.

Phenotypic plasticity and biogeographic variation in physiology of habitat-forming seaweed: response to temperature and nitrate

Southeastern Australian waters are warming at nearly four times the global average rate (~0.7°C · century(-1) ) driven by strengthening incursions of the warm oligotrophic East Australian Current. The growth rate hypothesis (GRH) predicts that nutrient depletion will impact more severely on seaweeds at high latitudes with compressed growth seasons. This study investigates the effects of temperature and nutrients on the ecophysiology of the habitat-forming seaweed Phyllospora comosa in a laboratory experiment using temperature (12°C, 17°C, 22°C) and nutrient (0.5, 1.0, 3.0 μM NO3 (-) ) scenarios representative of observed variation among geographic regions. Changes in growth, photosynthetic characteristics (via chlorophyll fluorescence), pigment content, tissue chemistry (δ(13) C, % C, % N, C:N) and nucleic acid characteristics (absolute RNA and DNA, RNA:DNA ratios) were determined in seaweeds derived from cool, high-latitude and warm, low-latitude portions of the species' range. Performance of P. comosa was unaffected by nitrate availability but was strongly temperature-dependent, with photosynthetic efficiency, growth, and survival significantly impaired at 22°C. While some physiological processes (photosynthesis, nucleic acid, and accessory pigment synthesis) responded rapidly to temperature, others (C/N dynamics, carbon concentrating processes) were largely invariant and biogeographic variation in these characteristics may only occur through genetic adaptation. No link was detected between nutrient availability, RNA synthesis and growth, and the GRH was not supported in this species. While P. comosa at high latitudes may be less susceptible to oligotrophy than predicted by the GRH, warming water temperatures will have deleterious effects on this species across its range unless rapid adaptation is possible.

Canopy interactions and physical stress gradients in subtidal communities

Species interactions are integral drivers of community structure and can change from competitive to facilitative with increasing environmental stress. In subtidal marine ecosystems, however, interactions along physical stress gradients have seldom been tested. We observed seaweed canopy interactions across depth and latitudinal gradients to test whether light and temperature stress structured interaction patterns. We also quantified interspecific and intraspecific interactions among nine subtidal canopy seaweed species across three continents to examine the general nature of interactions in subtidal systems under low consumer pressure. We reveal that positive and neutral interactions are widespread throughout global seaweed communities and the nature of interactions can change from competitive to facilitative with increasing light stress in shallow marine systems. These findings provide support for the stress gradient hypothesis within subtidal seaweed communities and highlight the importance of canopy interactions for the maintenance of subtidal marine habitats experiencing environmental stress.

Seagrass tolerance to herbivory under increased ocean temperatures

Climate change is acknowledged as a major threat to marine ecosystems, but the effect of temperature on species interactions remains poorly understood. We quantified the effects of long-term warming on plant-herbivore interactions of a dominant seagrass, Zostera muelleri. Growth, herbivory and tolerance to damage were compared between a meadow warmed by the thermal plume from a power station for 30 years (2-3 °C above background temperatures) and three control locations. Leaf growth rates and tissue loss were spatially variable but unrelated to temperature regimes. Natural herbivory was generally low. Simulated herbivory experiments showed that the tolerance of Z. muelleri to defoliation did not differ between warm and unimpacted meadows, with damaged and undamaged plants maintaining similar growth rates irrespective of temperature. These results suggest that the ability of temperate Z. muelleri to tolerate herbivory is not strongly influenced by warming, and this species may be relatively resilient to future environmental change.

Temperature response of photosynthetic light- and carbon-use characteristics in the red seaweed Gracilariopsis lemaneiformis (Gracilariales, Rhodophyta)

The red seaweed Gracilariopsis is an important crop extensively cultivated in China for high-quality raw agar. In the cultivation site at Nanao Island, Shantou, China, G. lemaneiformis experiences high variability in environmental conditions like seawater temperature. In this study, G. lemaneiformis was cultured at 12, 19, or 26°C for 3 weeks, to examine its photosynthetic acclimation to changing temperature. Growth rates were highest in G. lemaneiformis thalli grown at 19°C, and were reduced with either decreased or increased temperature. The irradiance-saturated rate of photosynthesis (Pmax ) decreased with decreasing temperature, but increased significantly with prolonged cultivation at lower temperatures, indicating the potential for photosynthesis acclimation to lower temperature. Moreover, Pmax increased with increasing temperature (~30 μmol O2  · g(-1) FW · h(-1) at 12°C to 70 μmol O2  · g(-1) FW · h(-1) at 26°C). The irradiance compensation point for photosynthesis (Ic ) decreased significantly with increasing temperature (28 μmol photons · m(-2)  · s(-1) at high temperature vs. 38 μmol photons · m(-2)  · s(-1) at low temperature). Both the photosynthetic light- and carbon-use efficiencies increased with increasing growth or temperatures (from 12°C to 26°C). The results suggested that the thermal acclimation of photosynthetic performance of G. lemaneiformis would have important ecophysiological implications in sea cultivation for improving photosynthesis at low temperature and maintaining high standing biomass during summer. Ongoing climate change (increasing atmospheric CO2 and global warming) may enhance biomass production in G. lemaneiformis mariculture through the improved photosynthetic performances in response to increasing temperature.

Modeling effects of climate change and phase shifts on detrital production of a kelp bed

The exchange of energy and nutrients between ecosystems (i.e., resource subsidies) plays a central role in ecological dynamics over a range of spatial and temporal scales. Little attention has been paid to the role of anthropogenic impacts on natural systems in altering the magnitude, timing, and quality of resource subsidies. Kelp ecosystems are highly productive on a local scale and export over 80% of kelp primary production as detritus, subsidizing consumers across broad spatial scales. Here, we generate a model of detrital production from a kelp bed in Nova Scotia to hindcast trends in detrital production based on temperature and wave height recorded in the study region from 1976 to 2009, and to project changes in detrital production that may result from future climate change. Historical and projected increases in temperature and wave height led to higher rates of detrital production through increased blade breakage and kelp dislodgment from the substratum, but this reduced kelp biomass and led to a decline in detrital production in the long-term. We also used the model to demonstrate that the phase shift from a highly productive kelp bed to a low-productivity barrens, driven by the grazing activity of sea urchins, reduces kelp detrital production by several orders of magnitude, an effect that would be exacerbated by projected increases in temperature and wave action. These results indicate that climate-mediated changes in ecological dynamics operating on local scales may alter the magnitude of resource subsidies to adjacent ecosystems, affecting ecological dynamics on regional scales.

Ocean acidification alleviates low-temperature effects on growth and photosynthesis of the red alga Neosiphonia harveyi (Rhodophyta)

This study aimed to examine interactive effects between ocean acidification and temperature on the photosynthetic and growth performance of Neosiphonia harveyi. N. harveyi was cultivated at 10 and 17.5 °C at present (~380 µatm), expected future (~800 µatm), and high (~1500 µatm) pCO2. Chlorophyll a fluorescence, net photosynthesis, and growth were measured. The state of the carbon-concentrating mechanism (CCM) was examined by pH-drift experiments (with algae cultivated at 10 °C only) using ethoxyzolamide, an inhibitor of external and internal carbonic anhydrases (exCA and intCA, respectively). Furthermore, the inhibitory effect of acetazolamide (an inhibitor of exCA) and Tris (an inhibitor of the acidification of the diffusive boundary layer) on net photosynthesis was measured at both temperatures. Temperature affected photosynthesis (in terms of photosynthetic efficiency, light saturation point, and net photosynthesis) and growth at present pCO2, but these effects decreased with increasing pCO2. The relevance of the CCM decreased at 10 °C. A pCO2 effect on the CCM could only be shown if intCA and exCA were inhibited. The experiments demonstrate for the first time interactions between ocean acidification and temperature on the performance of a non-calcifying macroalga and show that the effects of low temperature on photosynthesis can be alleviated by increasing pCO2. The findings indicate that the carbon acquisition mediated by exCA and acidification of the diffusive boundary layer decrease at low temperatures but are not affected by the cultivation level of pCO2, whereas the activity of intCA is affected by pCO2. Ecologically, the findings suggest that ocean acidification might affect the biogeographical distribution of N. harveyi.

The footprint of continental-scale ocean currents on the biogeography of seaweeds

Explaining spatial patterns of biological organisation remains a central challenge for biogeographic studies. In marine systems, large-scale ocean currents can modify broad-scale biological patterns by simultaneously connecting environmental (e.g. temperature, salinity and nutrients) and biological (e.g. amounts and types of dispersed propagules) properties of adjacent and distant regions. For example, steep environmental gradients and highly variable, disrupted flow should lead to heterogeneity in regional communities and high species turnover. In this study, we investigated the possible imprint of the Leeuwin (LC) and East Australia (EAC) Currents on seaweed communities across ~7,000 km of coastline in temperate Australia. These currents flow poleward along the west and east coasts of Australia, respectively, but have markedly different characteristics. We tested the hypothesis that, regional seaweed communities show serial change in the direction of current flow and that, because the LC is characterised by a weaker temperature gradient and more un-interrupted along-shore flow compared to the EAC, then coasts influenced by the LC have less variable seaweed communities and lower species turnover across regions than the EAC. This hypothesis was supported. We suggest that this pattern is likely caused by a combination of seaweed temperature tolerances and current-driven dispersal. In conclusion, our findings support the idea that the characteristics of continental-scale currents can influence regional community organisation, and that the coupling of ocean currents and marine biological structure is a general feature that transcends taxa and spatial scales.

Direct and indirect effects of ocean acidification and warming on a marine plant-herbivore interaction

The impacts of climatic change on organisms depend on the interaction of multiple stressors and how these may affect the interactions among species. Consumer-prey relationships may be altered by changes to the abundance of either species, or by changes to the per capita interaction strength among species. To examine the effects of multiple stressors on a species interaction, we test the direct, interactive effects of ocean warming and lowered pH on an abundant marine herbivore (the amphipod Peramphithoe parmerong), and whether this herbivore is affected indirectly by these stressors altering the palatability of its algal food (Sargassum linearifolium). Both increased temperature and lowered pH independently reduced amphipod survival and growth, with the impacts of temperature outweighing those associated with reduced pH. Amphipods were further affected indirectly by changes to the palatability of their food source. The temperature and pH conditions in which algae were grown interacted to affect algal palatability, with acidified conditions only affecting feeding rates when algae were also grown at elevated temperatures. Feeding rates were largely unaffected by the conditions faced by the herbivore while feeding. These results indicate that, in addition to the direct effects on herbivore abundance, climatic stressors will affect the strength of plant-herbivore interactions by changes to the susceptibility of plant tissues to herbivory.

Temperature acclimation and heat tolerance of photosynthesis in Norwegian Saccharina latissima (Laminariales, Phaeophyceae)

Kelps, seaweeds and seagrasses provide important ecosystem services in coastal areas, and loss of these macrophytes is a global concern. Recent surveys have documented severe declines in populations of the dominant kelp species, Saccharina latissima, along the south coast of Norway. S. latissima is a cold-temperate species, and increasing seawater temperature has been suggested as one of the major causes of the decline. Several studies have shown that S. latissima can acclimate to a wide range of temperatures. However, local adaptations may render the extrapolation of existing results inappropriate. We investigated the potential for thermal acclimation and heat tolerance in S. latissima collected from three locations along the south coast of Norway. Plants were kept in laboratory cultures at three different growth temperatures (10, 15, and 20°C) for 4-6 weeks, after which their photosynthetic performance, fluorescence parameters, and pigment concentrations were measured. S. latissima obtained almost identical photosynthetic characteristics when grown at 10 and 15°C, indicating thermal acclimation at these temperatures. In contrast, plants grown at 20°C suffered substantial tissue deterioration, and showed reduced net photosynthetic capacity caused by a combination of elevated respiration and reduced gross photosynthesis due to lowered pigment concentrations, altered pigment composition, and reduced functionality of Photo-system II. Our results support the hypothesis that extraordinarily high temperatures, as observed in 1997, 2002, and 2006, may have initiated the declines in S. latissima populations along the south coast of Norway. However, observations of high mortality in years with low summer temperatures suggest that reduced population resilience or other factors may have contributed to the losses.

Thermal Acclimation of Respiration and Photosynthesis in the Marine Macroalga Gracilaria lemaneiformis (Gracilariales, Rhodophyta)

The responses of respiration and photosynthesis to temperature fluctuations in marine macroalgae have the potential to significantly affect coastal carbon fluxes and sequestration. In this study, the marine red macroalga Gracilaria lemaneiformis was cultured at three different temperatures (12, 19, and 26°C) and at high- and low-nitrogen (N) availability, to investigate the acclimation potential of respiration and photosynthesis to temperature change. Measurements of respiratory and photosynthetic rates were made at five temperatures (7°C-33°C). An instantaneous change in temperature resulted in a change in the rates of respiration and photosynthesis, and the temperature sensitivities (i.e., the Q10 value) for both the metabolic processes were lower in 26°C-grown algae than 12°C- or 19°C-grown algae. Both respiration and photosynthesis acclimated to long-term changes in temperature, irrespective of the N availability under which the algae were grown; respiration displayed strong acclimation, whereas photosynthesis only exhibited a partial acclimation response to changing growth temperatures. The ratio of respiration to gross photosynthesis was higher in 12°C-grown algae, but displayed little difference between the algae grown at 19°C and 26°C. We propose that it is unlikely that respiration in G. lemaneiformis would increase significantly with global warming, although photosynthesis would increase at moderately elevated temperatures.

Extreme climatic event drives range contraction of a habitat-forming species

Species distributions have shifted in response to global warming in all major ecosystems on the Earth. Despite cogent evidence for these changes, the underlying mechanisms are poorly understood and currently imply gradual shifts. Yet there is an increasing appreciation of the role of discrete events in driving ecological change. We show how a marine heat wave (HW) eliminated a prominent habitat-forming seaweed, Scytothalia dorycarpa, at its warm distribution limit, causing a range contraction of approximately 100 km (approx. 5% of its global distribution). Seawater temperatures during the HW exceeded the seaweed's physiological threshold and caused extirpation of marginal populations, which are unlikely to recover owing to life-history traits and oceanographic processes. Scytothalia dorycarpa is an important canopy-forming seaweed in temperate Australia, and loss of the species at its range edge has caused structural changes at the community level and is likely to have ecosystem-level implications. We show that extreme warming events, which are increasing in magnitude and frequency, can force step-wise changes in species distributions in marine ecosystems. As such, return times of these events have major implications for projections of species distributions and ecosystem structure, which have typically been based on gradual warming trends.

Seasonal variation in crassulacean acid metabolism by the aquatic isoetid Littorella uniflora

The seasonal temperature acclimation in crassulacean acid metabolism (CAM) and photosynthetic performance were investigated in the aquatic isoetid, Littorella uniflora. Plants were collected monthly from January to September, and CAM capacity and photosynthesis rates were measured at 5, 10, 15, and 20 °C. Seasonal acclimation was observed for CAM (Q(10) range: 0.6-1.8), and CAM was optimised close to ambient temperature throughout the season. Thus, in winter acclimated L. uniflora, the short-term response to raised temperature resulted in a decline in CAM capacity. Even though the ambient CAM increased from winter to spring/summer, CAM was present in cold acclimated plants, thus indicating an ecophysiological role for CAM even in winter. Similar to CAM, seasonal acclimation was observed in the light and carbon-saturated photosynthesis (Q(10) values ranged from 1.4 to 2.3), and the photosynthetic capacity was generally higher during the winter at all temperatures, indicating compensatory investments in the photosynthetic apparatus. Thus, L. uniflora displayed seasonal temperature acclimation with respect to both CAM and photosynthesis. The estimated in situ contribution of CAM to the carbon budget in L. uniflora was independent of season and varied from 23 to 46 %. A positive correlation between photosynthetic capacity and CAM capacity (both measured in the lab at temperature close to ambient temperature) was found, and the ratio of CAM activity to photosynthetic capacity was higher in summer compared with winter plants. Overall, the results from the present study support the suggested role of CAM as a carbon conserving mechanism of importance for survival in a carbon-limited habitat.

RESPONSE OF TRACHYDISCUS MINUTUS (XANTHOPHYCEAE) TO TEMPERATURE AND LIGHT(1)

The effects of different temperatures and light intensities on growth, pigments, sugars, lipids, and proteins, as well as on some antioxidant and proteolytic enzymes of Trachydiscus minutus (Bourr.) H. Ettl, were investigated. The optimum growth temperature and light intensity were 25°C and 2 × 132 μmol photons · m(-2 ) · s(-1) , respectively. Under these conditions, proteins were the main biomass components (33.45% dry weight [dwt]), with high levels of carbohydrates (29% dwt) and lipids (21.77% dwt). T. minutus tolerated temperatures between 20°C and 32°C, with only moderate changes in cell growth and biochemical composition. Extremely low (15°C) and high (40°C) temperatures decreased chl and RUBISCO contents and inhibited cell growth. The biochemical response of the alga to both unfavorable conditions was an increase in lipid content (up to 35.19% dwt) and a decrease in carbohydrates (down to 13.64% dwt) with much less of a change in total protein content (in the range of 30.51%-38.13% dwt). At the same time, the defense system of T. minutus was regulated differently in response to heat or cold treatments. Generally, at 40°C, the activities of superoxide dismutase (SOD), catalase (CAT), and proteases were drastically elevated, and three polypeptides were overexpressed, whereas the glutathione reductase (GR) and peroxidase (POD) activities were reduced. In contrast, at 15°C, all these enzymes except GR were suppressed. The effect of light was to enhance or decrease the temperature stress responses, depending on intensity. Our studies demonstrate the broad temperature adaptability of T. minutus as well as the potential for the production of valuable algal biomass.

Algal chemodiversity and bioactivity: sources of natural variability and implications for commercial application

There has been significant recent interest in the commercial utilisation of algae based on their valuable chemical constituents many of which exhibit multiple bioactivities with applications in the food, cosmetic, agri- and horticultural sectors and in human health. Compounds of particular commercial interest include pigments, lipids and fatty acids, proteins, polysaccharides and phenolics which all display considerable diversity between and within taxa. The chemical composition of natural algal populations is further influenced by spatial and temporal changes in environmental parameters including light, temperature, nutrients and salinity, as well as biotic interactions. As reported bioactivities are closely linked to specific compounds it is important to understand, and be able to quantify, existing chemical diversity and variability. This review outlines the taxonomic, ecological and chemical diversity between, and within, different algal groups and the implications for commercial utilisation of algae from natural populations. The biochemical diversity and complexity of commercially important types of compounds and their environmental and developmental control are addressed. Such knowledge is likely to help achieve higher and more consistent levels of bioactivity in natural samples and may allow selective harvesting according to algal species and local environmental conditions for different groups of compounds.