The analysis of photosynthesis in air and water of Ascophyllum nodosum (L.) Le Jol

Acquisition and metabolism of carbon in the Ochrophyta other than diatoms

The acquisition and assimilation of inorganic C have been investigated in several of the 15 clades of the Ochrophyta other than diatoms, with biochemical, physiological and genomic data indicating significant mechanistic variation. Form ID Rubiscos in the Ochrophyta are characterized by a broad range of kinetics values. In spite of relatively high K0.5CO2 and low CO2 : O2 selectivity, diffusive entry of CO2 occurs in the Chrysophyceae and Synurophyceae. Eustigmatophyceae and Phaeophyceae, on the contrary, have CO2 concentrating mechanisms, usually involving the direct or indirect use of [Formula: see text] This variability is possibly due to the ecological contexts of the organism. In brown algae, C fixation generally takes place through a classical C3 metabolism, but there are some hints of the occurrence of C4 metabolism and low amplitude CAM in a few members of the Fucales. Genomic data show the presence of a number of potential C4 and CAM genes in Ochrophyta other than diatoms, but the other core functions of many of these genes give a very limited diagnostic value to their presence and are insufficient to conclude that C4 photosynthesis is present in these algae.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Separate and combined effects of copper and freshwater on the biodiversity and functioning of fouling assemblages

Increased levels of anthropogenic stressors in the marine environment are leading to complex changes in its diversity and functioning. Many marine ecosystems are exposed simultaneously to multiple stressors, and their combined effects are difficult to predict. Climate change will intensify the input of terrestrial pollutants and increase the flow of water to coastal systems through increased precipitation. Subtidal assemblages were subjected to factorial combinations of copper and freshwater to test effects on their structure and on ecosystem processes. Assemblages were also subjected to seawater to separate the intended effects of water flow and salinity. For the first three months, no effects of copper or freshwater were found. After three months, copper significantly reduced the taxon richness and the percentage cover of taxa, and reduced the rate of community respiration and gross primary production. The flow of water also reduced community respiration, regardless of whether the water was fresh or saline.

Large-scale variation in combined impacts of canopy loss and disturbance on community structure and ecosystem functioning

Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.

Climate change and ocean acidification effects on seagrasses and marine macroalgae

Although seagrasses and marine macroalgae (macro-autotrophs) play critical ecological roles in reef, lagoon, coastal and open-water ecosystems, their response to ocean acidification (OA) and climate change is not well understood. In this review, we examine marine macro-autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [DIC], carbon dioxide [CO2 ], and lower carbonate [CO3 (2-) ] and pH. We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [CO2 ]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro-autotroph photosynthesis is overwhelmingly C3 (≥ 85%) with most species capable of utilizing HCO3 (-) ; however, most are not saturated at current ocean [DIC]. These results, and the presence of CO2 -only users, lead us to conclude that photosynthetic and growth rates of marine macro-autotrophs are likely to increase under elevated [CO2 ] similar to terrestrial C3 species. In the tropics, many species live close to their thermal limits and will have to up-regulate stress-response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [CO2 ] effects on thermal acclimation are unknown. Fundamental linkages between elevated [CO2 ] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [CO2 ] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H(+) and DIC. These fluxes control micro-environments that promote calcification over dissolution and may be more important than CaCO3 mineralogy in predicting macroalgal responses to OA. Calcareous macroalgae are highly vulnerable to OA, and it is likely that fleshy macroalgae will dominate in a higher CO2 ocean; therefore, it is critical to elucidate the research gaps identified in this review.

Chlorophyll a fluorescence responses of temperate Phaeophyceae under submersion and emersion regimes: a comparison of rapid and steady-state light curves

The potential of algae to acclimate to environmental stress is commonly assessed using chlorophyll a fluorescence, with changes in parameters of photosynthesis versus irradiance (P/E) curves measured either as rapid light curves (RLC) or steady-state light curves (LC). Here, effects of emersion on primary photosynthesis of four brown macroalgae (Ascophyllum nodosum, Fucus serratus, Sargassum muticum, Laminaria digitata) were compared by applying both RLC and LC. When LC were used, photosynthetic performance was enhanced during emersion in A. nodosum and F. serratus as shown by increases in q(P), rETR(max) and E(k). By contrast, emersion had no impact on photosynthetic parameters of S. muticum and L. digitata. Relative changes in the NPQ-rETR relationship were reduced in A. nodosum, F. serratus and S. muticum, but remained unaffected in L. digitata. As none of the species developed their potential NPQ(max), corresponding values could not be determined from RLC. Using RLC, observed photosynthetic performance of F. serratus and L. digitata was reduced upon emersion, whilst values for NPQ(max) were enhanced. Only results derived from LC provide evidence for a potential physiological adaptation of brown macroalgae to their natural habitat; it is recommended using the LC protocol to detect environmental impacts on photosynthesis.

Ecophysiology of photosynthesis in macroalgae

Macroalgae occur in the marine benthos from the upper intertidal to depths of more than 200 m, contributing up to 1 Pg C per year to global primary productivity. Freshwater macroalgae are mainly green (Chlorophyta) with some red (Rhodophyta) and a small contribution of brown (Phaeophyceae) algae, while in the ocean all three higher taxa are important. Attempts to relate the depth distribution of three higher taxa of marine macroalgae to their photosynthetic light use through their pigmentation in relation to variations in spectral quality of photosynthetically active radiation (PAR) with depth (complementary chromatic adaptation) and optical thickness (package effect) have been relatively unsuccessful. The presence (Chlorophyta, Phaeophyceae) or absence (Rhodophyta) of a xanthophyll cycle is also not well correlated with depth distribution of marine algae. The relative absence of freshwater brown algae does not seem to be related to their photosynthetic light use. Photosynthetic inorganic carbon acquisition in some red and a few green macroalgae involves entry of CO(2) by diffusion. Other red and green macroalgae, and brown macroalgae, have CO(2) concentrating mechanisms; these frequently involve acid and alkaline zones on the surface of the alga with CO(2) (produced from HCO(3) (-)) entering in the acid zones, while some macroalgae have CCMs based on active influx of HCO(3) (-). These various mechanisms of carbon acquisition have different responses to the thickness of the diffusion boundary layer, which is determined by macroalgal morphology and water velocity. Energetic predictions that macroalgae growing at or near the lower limit of PAR for growth should rely on diffusive CO(2) entry without acid and alkaline zones, and on NH(4) (+) rather than NO(3) (-) as nitrogen source, are only partially borne out by observation. The impact of global environmental change on marine macroalgae mainly relates to ocean acidification and warming with shoaling of the thermocline and decreased nutrient flux to the upper mixed layer. Predictions of the impact on macroalgae requires further experiments on interactions among increased inorganic carbon, increased temperature and decreased nitrogen and phosphorus supply, and, when possible, studies of genetic adaptation to environmental change.

Algal and aquatic plant carbon concentrating mechanisms in relation to environmental change

Carbon dioxide concentrating mechanisms (also known as inorganic carbon concentrating mechanisms; both abbreviated as CCMs) presumably evolved under conditions of low CO(2) availability. However, the timing of their origin is unclear since there are no sound estimates from molecular clocks, and even if there were, there are no proxies for the functioning of CCMs. Accordingly, we cannot use previous episodes of high CO(2) (e.g. the Palaeocene-Eocene Thermal Maximum) to indicate how organisms with CCMs responded. Present and predicted environmental change in terms of increased CO(2) and temperature are leading to increased CO(2) and HCO(3)(-) and decreased CO(3)(2-) and pH in surface seawater, as well as decreasing the depth of the upper mixed layer and increasing the degree of isolation of this layer with respect to nutrient flux from deeper waters. The outcome of these forcing factors is to increase the availability of inorganic carbon, photosynthetic active radiation (PAR) and ultraviolet B radiation (UVB) to aquatic photolithotrophs and to decrease the supply of the nutrients (combined) nitrogen and phosphorus and of any non-aeolian iron. The influence of these variations on CCM expression has been examined to varying degrees as acclimation by extant organisms. Increased PAR increases CCM expression in terms of CO(2) affinity, whilst increased UVB has a range of effects in the organisms examined; little relevant information is available on increased temperature. Decreased combined nitrogen supply generally increases CO(2) affinity, decreased iron availability increases CO(2) affinity, and decreased phosphorus supply has varying effects on the organisms examined. There are few data sets showing interactions amongst the observed changes, and even less information on genetic (adaptation) changes in response to the forcing factors. In freshwaters, changes in phytoplankton species composition may alter with environmental change with consequences for frequency of species with or without CCMs. The information available permits less predictive power as to the effect of the forcing factors on CCM expression than for their overall effects on growth. CCMs are currently not part of models as to how global environmental change has altered, and is likely to further alter, algal and aquatic plant primary productivity.

BENTHIC METABOLISM ON A SHELTERED ROCKY SHORE: ROLE OF THE CANOPY IN THE CARBON BUDGET(1)

While the importance of canopy-forming algae in structuring ecosystems is recognized, their role in the carbon budget is still not well understood. To our knowledge, no measurements of rocky shores primary production and respiration under emersion periods have been carried out in situ. A benthic chamber coupled to a CO2 -infrared gas analyzer was used to measure gross primary production and respiration on the Ascophyllum nodosum (L.) Le Jol. zone of a sheltered rocky shore in Brittany, France. Over a year of monthly measurements on the zone with and without the A. nodosum canopy showed fairly high production and respiration values for the global community as well as carbon fluxes due to the canopy that largely dominated the benthic metabolism of the zone. The strong canopy respiration relative to the primary production also suggested a high metabolic activity by microscopic heterotrophs on the surface of the alga. Both the canopy and the understory annual primary production and respiration were under the control of light and temperature seasonal variations. Finally, the range of the amount of carbon produced on the A. nodosum zone during diurnal emersions was estimated. Additional measures accounting for the day-night cycles and seasonal light variations over an entire tidal cycle are, however, necessary to establish an annual carbon budget. Such measures using the benthic chamber together with complementary techniques would allow a better understanding of the functioning of sheltered rocky shores.

Seasonal acclimatization of antioxidants and photosynthesis in Chondrus crispus and Mastocarpus stellatus, two co-occurring red algae with differing stress tolerances

Mastocarpus stellatus and Chondrus crispus are red macroalgae that co-dominate the lower rocky intertidal zones of the northern Atlantic coast. M. stellatus is more tolerant than C. crispus of environmental stresses, particularly those experienced during winter. This difference in tolerance has been attributed, in part, to greater contents or activities of certain antioxidants in M. stellatus. We compared the photosynthetic capacities and activities of three antioxidant enzymes--superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR)--as well as the contents of ascorbate from fronds of M. stellatus and C. crispus collected over a year. Photosynthetic capacity increased in winter, but did not differ between species in any season. The activities of the three antioxidant enzymes and the contents of ascorbate were significantly greater in tissues collected during months with mean air and water temperatures below 7.5 degrees C ("cold" months; December, February, March, April) than in months with mean air temperatures above 11 degrees C ("warm" months; June, July, August, October). Overall, C. crispus had significantly greater SOD and APX activities, while M. stellatus had higher ascorbate contents. Species-specific differences in GR activity depended upon mean monthly temperatures at the time of tissue collection; C. crispus had higher activities during cold months, whereas M. stellatus had higher activities during warm months. Taken together, these data indicate that increased ROS scavenging capacity is a part of winter acclimatization; however, only trends in ascorbate content support the hypothesis that greater levels of antioxidants underlie the relatively greater winter tolerance of M. stellatus in comparison to C. crispus.

Patterns of allocation and growth in aquatic Sphagnum species

The hypothesis that aquatic plants have greater allocation to photosynthetic tissues was tested using a common garden experiment with three pairs of ecologically contrasting Sphagnum species (S. portoricense and S. papillosum; S. trinitense and S. recurvum; S. macrophyllum var. floridanum and S. strictum). When submerged, aquatic taxa (the former in each pair) all had greater relative growth rates and greater allocation to photosynthetic tissues, as evidenced by higher whole plant chlorophyll content. The evolution of greater photosynthetic allocation appears to result from changes in two independently evolving systems: allocation to photosynthetic cells rather than nonphotosynthetic hyaline cells, and a shift in biochemical partitioning within photosynthetic cells favoring light-reaction proteins. The former was assessed by comparing photosynthetic cell volume in leaves, and the latter was estimated from chlorophyll to nitrogen ratios. The mechanism whereby greater photosynthetic allocation was achieved differed in each aquatic species. In a second experiment, variation in growth and allocation was assessed for three populations each of S. trinitense and S. recurvum. Nested analysis of variance verified that the differences between S. trinitense and S. recurvum observed in relative growth rate and chlorophyll content in the species-level test were robust given the magnitude of population-level variation. It is concluded that (i) aquatic habitats do select for features that lead to greater relative growth rate in submerged environments, (ii) relative growth rate is functionally correlated with allocation to photosynthetic tissues, (iii) allocations at the cellular and biochemical level evolve independently, and (iv) patterns of allocation represent adaptations to environmental conditions present in the aquatic environment. Key words: Sphagnum, relative growth rate, chlorophyll content, allocation, aquatic plants, adaptation.

Transport and assimilation of inorganic carbon by Lichina pygmaea under emersed and submersed conditions

Photosynthetic O₂ evolution by the upper littoral lichen, Lichina pygmaea (Lightf.) C.Ag., under light-saturated conditions at 5 °C is saturated by the 2 mol m^-3 inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m^-3 inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO₃ ^- . It is concluded that HCO₃ ^- is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO₂ enters the photobiont (Calothrix) cells even during HCO₃ use. pH drift experiments support the notion of HCO₃ ^- use. Emersed photosynthesis at 5 °C is more than half-saturated by 35 Pa (normal atmospheric) CO₂ ; the light- and CO₂ -saturated emersed photosynthetic rate is not significantly different from the light and inorganic C-saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO₃ ^- fluxes to supplement CO₂ movement. The CO₂ compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark ¹⁴ C-inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM-like variation in intracellular titratable acidity. The high value (-11.5%) of δ¹³ C, the low CO₂ compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free-living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.