Effects of elevated pCO2 on the metabolism of a temperate rhodolith Lithothamnion corallioides grown under different temperatures

Photosynthetic responses of crustose coralline algae recruit from an upwelling area to light intensity, temperature and current flow rate in a mesocosm

The crustose coralline algae (CCA) play an important role building carbonate habitats and enhancing species diversity in rocky shores, rhodolith beds and coral-algal reefs. Though worldwide distributed some taxonomic groups were proved to show light and temperature boundaries. Nevertheless, little is known about the biological limits of CCA recruits and photosynthetic responses in a climate change environment. In this way, experimental studies are essential to identify their optimal conditions development. The aim of the study was to test ideal environmental conditions for CCA recruits from an upwelling area to verify Fv/Fm response patterns as an indicator of photosynthetic performance. Artificial disks were fixed on a rocky shore to provide substrate for CCA recruitments and then moved to be tested in a flow-through system. The CCA tested were species of the Genus Lithophyllum and experiments were performed with CCA exposed to different levels of light, temperature and current flow rates. The photosystem II function, measured by chlorophyll a fluorescence (Fv/Fm), was used as an indicator of potential photosynthetic electron transport. There was an interaction between light intensity, flow rate and temperature. Low light intensity, high temperature and fast flow rate (20 μmol m-2. s-1; 19 and 24 °C; 0.09 m s-1 respectively) provided optimal conditions for CCA recruits.

Whole genome genotyping reveals discrete genetic diversity in north-east Atlantic maerl beds

Maerl beds are vital habitats for a diverse array of marine species across trophic levels, but they are increasingly threatened by human activities and climate change. Furthermore, little is known about the genetic diversity of maerl-forming species and the population structure of maerl beds, both of which are important for understanding the ability of these species to adapt to changing environments and for informing marine reserve planning. In this study, we used a whole genome genotyping approach to explore the population genomics of Phymatolithon calcareum, a maerl-forming red algal species, whose geographical distribution spans the north-east Atlantic, from Norway to Portugal. Our results, using 14,150 genome-wide SNPs (single nucleotide polymorphisms), showed that P. calcareum maerl beds across the north-east Atlantic are generally structured geographically, a pattern likely explained by low dispersal potential and limited connectivity between regions. Additionally, we found that P. calcareum from the Fal Estuary, south-west England, is genetically distinct from all other P. calcareum sampled, even from The Manacles, a site located only 13 km away. Further analysis revealed that this finding is not the result of introgression from two closely related species, Phymatolithon purpureum or Lithothamnion corallioides. Instead, this unique diversity may have been shaped over time by geographical isolation of the Fal Estuary maerl bed and a lack of gene flow with other P. calcareum populations. The genomic data presented in this study suggest that P. calcareum genetic diversity has accumulated over large temporal and spatial scales, the preservation of which will be important for maximizing the resilience of this species to changes in climate and the environment. Moreover, our findings underline the importance of managing the conservation of maerl beds across western Europe as distinct units, at a site-by-site level.

Epiphytes provide micro-scale refuge from ocean acidification

Coralline algae, a major calcifying component of coastal shallow water communities, have been shown to be one of the more vulnerable taxonomic groups to ocean acidification (OA). Under OA, the interaction between corallines and epiphytes was previously described as both positive and negative. We hypothesized that the photosynthetic activity and the complex structure of non-calcifying epiphytic algae that grow on corallines ameliorate the chemical microenvironmental conditions around them, providing protection from OA. Using mesocosm and microsensor experiments, we showed that the widespread coralline Ellisolandia elongata is less susceptible to the detrimental effects of OA when covered with non-calcifying epiphytic algae, and its diffusive boundary layer is thicker than when not covered by epiphytes. By modifying the microenvironmental carbonate chemistry, epiphytes, facilitated by OA, create micro-scale shield (and refuge) with more basic conditions that may allow the persistence of corallines associated with them during acidified conditions. Such ecological refugia could also assist corallines under near-future anthropogenic OA conditions.

Temperature amplifies the effect of high CO2 on the photosynthesis, respiration, and calcification of the coralline algae Phymatolithon lusitanicum

The combination of ocean acidification (OA) and global warming is expected to have a significant effect on the diversity and functioning of marine ecosystems, particularly on calcifying algae such as rhodoliths (maërl) that form extensive beds worldwide, from polar to tropical regions. In addition, the increasing frequency of extreme events, such as heat waves, threatens coastal ecosystems and may affect their capacity to fix blue carbon. The few studies where the simultaneous effects of both temperature and CO2 were investigated have revealed contradictory results. To assess the effect that high temperature spells can have on the maërl beds under OA, we tested the short-time effects of temperature and CO2 on the net photosynthesis, respiration, and calcification of the recently described species Phymatolithon lusitanicum, the most common maërl species of southern Portugal. Photosynthesis, calcification, and respiration increased with temperature, and the differences among treatments were enhanced under high CO2. We found that in the short term, the metabolic rates of Phymatolithon lusitanicum will increase with CO2 and temperature as will the coupling between calcification and photosynthesis. However, under high CO2, this coupling will favor photosynthesis over calcification, which, in the long term, can have a negative effect on the blue carbon fixing capacity of the maërl beds from southern Portugal.

Rhodolith primary and carbonate production in a changing ocean: The interplay of warming and nutrients

Rhodolith beds, like many other marine ecosystems, are affected by climate change that is causing an increase in the magnitude and frequency of extreme high temperature events (heat waves). Unfortunately, this does not represent the sole peril for these communities, as coastal urbanization in conjunction with altered precipitation patterns can increase terrestrial-derived nutrient input. In Brazil, rhodolith beds are among the most extensive coastal benthic ecosystems, but despite their vast distribution and great ecological and economic importance, studies on the productivity of these communities and the impact of changing environmental conditions are almost non-existent. This study addressed the individual and combined effects of increases in temperature and nutrient concentration on the physiological performance of two widely distributed rhodolith species, Lithothamnion crispatum and Melyvonnea erubescens. The results showed species-specific responses in net photosynthetic performance, with no response in L. crispatum, while M. erubescens responded negatively to both increase in temperature and nutrients. In contrast, calcification in both species showed a significant decline at high temperature. No interactive effects were found between temperature and nutrients, yet their combined negative effects were additive, resulting in negative daily-integrated net productivity and a large decline in daily carbonate production in both species. This has strong implications for rhodolith bed primary productivity and carbonate production, as heat waves may potentially cause a strong decline in carbonate production (ca. 50% loss), accompanied by a severe drop in primary productivity that will be even more pronounced under high-nutrient conditions. Also, the species-specific responses to changes in temperature and nutrient concentration suggest that the magnitude of impact of these factors on rhodolith bed productivity will depend on the species dominating the community and may finally result in changes in rhodolith community composition.

The role of irradiance and C-use strategies in tropical macroalgae photosynthetic response to ocean acidification

Fleshy macroalgae may increase photosynthesis with greater CO2 availability under ocean acidification (OA) and outcompete calcifying macroalgae important for tropical reef accretion. Macroalgae use energy-dependent carbon concentrating mechanisms (CCMs) to take up HCO3-, the dominant inorganic carbon for marine photosynthesis, but carbon-use strategies may depend on the pCO2, pH and irradiance. We examined photosynthesis in eight tropical macroalgae across a range of irradiances (0-1200 μmol photon m-2 s-1), pH levels (7.5-8.5) and CO2 concentrations (3-43 μmol kg-1). Species-specific CCM strategies were assessed using inhibitors and δ13C isotope signatures. Our results indicate that the log of irradiance is a predictor of the photosynthetic response to elevated pCO2 (R2 > 0.95). All species utilized HCO3-, exhibited diverse C-use pathways and demonstrated facultative HCO3- use. All fleshy species had positive photosynthetic responses to OA, in contrast to a split amongst calcifiers. We suggest that shifts in photosynthetically-driven tropical macroalgal changes due to OA will most likely occur in moderate to high-irradiance environments when CCMs are ineffective at meeting the C-demands of photosynthesis. Further, facultative use of HCO3- allows greater access to CO2 for photosynthesis under OA conditions, particularly amongst fleshy macroalgae, which could contribute to enhance fleshy species dominance over calcifiers.

High CO2 decreases the long-term resilience of the free-living coralline algae Phymatolithon lusitanicum

Mäerl/rhodolith beds are protected habitats that may be affected by ocean acidification (OA), but it is still unclear how the availability of CO 2 will affect the metabolism of these organisms. Some of the inconsistencies found among OA experimental studies may be related to experimental exposure time and synergetic effects with other stressors. Here, we investigated the long-term (up to 20 months) effects of OA on the production and calcification of the most common mäerl species of southern Portugal, Phymatolithon lusitanicum. Both the photosynthetic and calcification rates increased with CO 2 after the first 11 months of the experiment, whereas respiration slightly decreased with CO 2. After 20 months, the pattern was reversed. Acidified algae showed lower photosynthetic and calcification rates, as well as lower accumulated growth than control algae, suggesting that a metabolic threshold was exceeded. Our results indicate that long-term exposure to high CO 2 will decrease the resilience of Phymatolithon lusitanicum. Our results also show that shallow communities of these rhodoliths may be particularly at risk, while deeper rhodolith beds may become ocean acidification refuges for this biological community.

Integration of chlorophyll a fluorescence and photorespirometry techniques to understand production dynamics in macroaglal communities

Global declines of macroalgal beds in coastal waters have prompted a plethora of studies attempting to understand the drivers of change within dynamic nearshore ecosystems. Photosynthetic measurements are good tools for assessing the consequences of numerous stressors of macroalgae, but there is somewhat of a disconnection between studies that focus on organism-specific ecophysiological responses and those that address causes and consequences of shifts in macroalgal productivity. Our goal is to highlight the applications of two complementary tools for measuring photosynthesis-variable chlorophyll a fluorescence and photorespirometry-and provide guidance for the integration of physiology and ecology to understand the drivers of change in macroalgal communities. Photorespirometry can provide an integrated measure of whole-community metabolism, including an estimate of the physiological costs associated with stressors, while fluorescence-based techniques provide point measures of the efficiency of the photosynthetic apparatus within communities. Variable chlorophyll a fluorescence does not provide an estimate of carbon balance or integrated photosynthesis across either whole plants or whole communities but can be used to estimate the contribution of individual community components in the dynamic subcanopy environment to help us understand the mechanisms underlying observed responses. We highlight the importance of the highly dynamic light environment within macroalgal communities and call for better integration of physiological techniques in an ecological context to enhance our understanding of the responses of whole communities to local and global stressors.

A direct CO2 control system for ocean acidification experiments: testing effects on the coralline red algae Phymatolithon lusitanicum

Most ocean acidification (OA) experimental systems rely on pH as an indirect way to control CO₂. However, accurate pH measurements are difficult to obtain and shifts in temperature and/or salinity alter the relationship between pH and pCO₂. Here we describe a system in which the target pCO₂ is controlled via direct analysis of pCO₂ in seawater. This direct type of control accommodates potential temperature and salinity shifts, as the target variable is directly measured instead of being estimated. Water in a header tank is permanently re-circulated through an air-water equilibrator. The equilibrated air is then routed to an infrared gas analyzer (IRGA) that measures pCO₂ and conveys this value to a Proportional-Integral-Derivative (PID) controller. The controller commands a solenoid valve that opens and closes the CO₂ flush that is bubbled into the header tank. This low-cost control system allows the maintenance of stabilized levels of pCO₂ for extended periods of time ensuring accurate experimental conditions. This system was used to study the long term effect of OA on the coralline red algae Phymatolithon lusitanicum. We found that after 11 months of high CO₂ exposure, photosynthesis increased with CO₂ as opposed to respiration, which was positively affected by temperature. Results showed that this system is adequate to run long-term OA experiments and can be easily adapted to test other relevant variables simultaneously with CO₂, such as temperature, irradiance and nutrients.

Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs

This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO₃ cm^-2 yr^-1) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO₃ accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO₃ accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA extremely susceptible changes in ocean water pH, emphasizing the far-reaching threat that ocean acidification poses to the ecological function and persistence of coral reefs worldwide.

Coralline algae (Rhodophyta) in a changing world: integrating ecological, physiological, and geochemical responses to global change

Coralline algae are globally distributed benthic primary producers that secrete calcium carbonate skeletons. In the context of ocean acidification, they have received much recent attention due to the potential vulnerability of their high-Mg calcite skeletons and their many important ecological roles. Herein, we summarize what is known about coralline algal ecology and physiology, providing context to understand their responses to global climate change. We review the impacts of these changes, including ocean acidification, rising temperatures, and pollution, on coralline algal growth and calcification. We also assess the ongoing use of coralline algae as marine climate proxies via calibration of skeletal morphology and geochemistry to environmental conditions. Finally, we indicate critical gaps in our understanding of coralline algal calcification and physiology and highlight key areas for future research. These include analytical areas that recently have become more accessible, such as resolving phylogenetic relationships at all taxonomic ranks, elucidating the genes regulating algal photosynthesis and calcification, and calibrating skeletal geochemical metrics, as well as research directions that are broadly applicable to global change ecology, such as the importance of community-scale and long-term experiments in stress response.

The future of the northeast Atlantic benthic flora in a high CO2 world

Seaweed and seagrass communities in the northeast Atlantic have been profoundly impacted by humans, and the rate of change is accelerating rapidly due to runaway CO2 emissions and mounting pressures on coastlines associated with human population growth and increased consumption of finite resources. Here, we predict how rapid warming and acidification are likely to affect benthic flora and coastal ecosystems of the northeast Atlantic in this century, based on global evidence from the literature as interpreted by the collective knowledge of the authorship. We predict that warming will kill off kelp forests in the south and that ocean acidification will remove maerl habitat in the north. Seagrasses will proliferate, and associated epiphytes switch from calcified algae to diatoms and filamentous species. Invasive species will thrive in niches liberated by loss of native species and spread via exponential development of artificial marine structures. Combined impacts of seawater warming, ocean acidification, and increased storminess may replace structurally diverse seaweed canopies, with associated calcified and noncalcified flora, with simple habitats dominated by noncalcified, turf-forming seaweeds.

Field study of growth and calcification rates of three species of articulated coralline algae in British Columbia, Canada

Ocean acidification caused by rising atmospheric CO₂ is predicted to negatively impact growth and calcification rates of coralline algae. Decreases in coralline abundance may have cascading effects on marine ecosystems and on carbon sequestration worldwide. In this study, we measured growth and calcification rates of three common species of articulated coralline algae (Bossiella plumosa, Calliarthron tuberculosum, and Corallina vancouveriensis) at an intertidal field site in British Columbia. Linear growth rates measured in the field were slow, although Bossiella grew significantly faster (0.22 cm mon⁻¹) than Calliarthron and Corallina (0.17 and 0.15 cm mon⁻¹, respectively). Growth rates in the field were generally slower than growth rates in the laboratory, suggesting that data generated in the laboratory may not be representative of natural field conditions. Growth rates did not decrease as fronds approached their maximum observed size, suggesting that maximum frond size might be determined not by intrinsic factors but by external factors such as wave-induced drag forces. Using growth data, we estimate that the largest observed Bossiella frond (20 cm²) and Calliarthron frond (40 cm²) were about 4- and 11-years-old, respectively, and had deposited approximately 1 and 6 g CaCO₃ in that time. Given the great abundance of coralline algae along the coast of British Columbia, deposition rates of CaCO₃ are expected to play a significant but poorly characterized role in carbon sequestration.

Does encapsulation protect embryos from the effects of ocean acidification? The example of Crepidula fornicata

Early life history stages of marine organisms are generally thought to be more sensitive to environmental stress than adults. Although most marine invertebrates are broadcast spawners, some species are brooders and/or protect their embryos in egg or capsules. Brooding and encapsulation strategies are typically assumed to confer greater safety and protection to embryos, although little is known about the physico-chemical conditions within egg capsules. In the context of ocean acidification, the protective role of encapsulation remains to be investigated. To address this issue, we conducted experiments on the gastropod Crepidula fornicata. This species broods its embryos within capsules located under the female and veliger larvae are released directly into the water column. C. fornicata adults were reared at the current level of CO₂ partial pressure (pCO₂) (390 μatm) and at elevated levels (750 and 1400 μatm) before and after fertilization and until larval release, such that larval development occurred entirely at a given pCO₂. The pCO₂ effects on shell morphology, the frequency of abnormalities and mineralization level were investigated on released larvae. Shell length decreased by 6% and shell surface area by 11% at elevated pCO₂ (1400 μatm). The percentage of abnormalities was 1.5- to 4-fold higher at 750 μatm and 1400 μatm pCO₂, respectively, than at 390 μatm. The intensity of birefringence, used as a proxy for the mineralization level of the larval shell, also decreased with increasing pCO₂. These negative results are likely explained by increased intracapsular acidosis due to elevated pCO₂ in extracapsular seawater. The encapsulation of C. fornicata embryos did not protect them against the deleterious effects of a predicted pCO₂ increase. Nevertheless, C. fornicata larvae seemed less affected than other mollusk species. Further studies are needed to identify the critical points of the life cycle in this species in light of future ocean acidification.