Rapid shifts in picoeukaryote community structure in response to ocean acidification

Rapid shifts in picoeukaryote community structure were observed during a CO₂ perturbation experiment in which we followed the development of phytoplankton blooms in nutrient-amended mesocosms under the present day or predicted future atmospheric pCO₂ (750 μatm, seawater pH 7.8). Analysis of rbcL clone libraries (encoding the large subunit of RubisCO) and specific quantitative PCR assays showed that two prasinophytes closely related to Micromonas pusilla and Bathycoccus prasinos were present, but responded very differently to high CO₂/acidification. We found that the abundance of Micromonas-like phylotypes was significantly higher (>20-fold) under elevated CO₂/low pH, whereas the Bathycoccus-like phylotypes were more evenly distributed between treatments and dominated the prasinophyte community under ambient conditions.     

Interactive Effects of Ocean Acidification and Nitrogen-Limitation on the Diatom Phaeodactylum tricornutum

Climate change is expected to bring about alterations in the marine physical and chemical environment that will induce changes in the concentration of dissolved CO₂ and in nutrient availability. These in turn are expected to affect the physiological performance of phytoplankton. In order to learn how phytoplankton respond to the predicted scenario of increased CO₂ and decreased nitrogen in the surface mixed layer, we investigated the diatom Phaeodactylum tricornutum as a model organism. The cells were cultured in both low CO₂ (390 μatm) and high CO₂ (1000 μatm) conditions at limiting (10 μmol L⁻¹) or enriched (110 μmol L⁻¹) nitrate concentrations. Our study shows that nitrogen limitation resulted in significant decreases in cell size, pigmentation, growth rate and effective quantum yield of Phaeodactylum tricornutum, but these parameters were not affected by enhanced dissolved CO₂ and lowered pH. However, increased CO₂ concentration induced higher rETR_max and higher dark respiration rates and decreased the CO₂ or dissolved inorganic carbon (DIC) affinity for electron transfer (shown by higher values for K_{1/2 DIC} or K_{1/2 CO2}). Furthermore, the elemental stoichiometry (carbon to nitrogen ratio) was raised under high CO₂ conditions in both nitrogen limited and nitrogen replete conditions, with the ratio in the high CO₂ and low nitrate grown cells being higher by 45% compared to that in the low CO₂ and nitrate replete grown ones. Our results suggest that while nitrogen limitation had a greater effect than ocean acidification, the combined effects of both factors could act synergistically to affect marine diatoms and related biogeochemical cycles in future oceans.     

Physiological Responses of a Model Marine Diatom to Fast pH Changes: Special Implications of Coastal Water Acidification

Diatoms and other phytoplankton in coastal waters experience rapid pH changes in milieu due to high biological activities and/or upwelled CO₂-rich waters. While CO₂ concentrating mechanisms (CCMs) are employed by all diatoms tested to counter low CO₂ availability in seawater, little is known how this mechanism responds to fast pH changes. In the present study, the model diatom Thalassiosira pseudonana was acclimated for 20 generations to low pH (7.81) at an elevated CO₂ of 1000 μatm (HC) or to high pH (8.18) at ambient CO₂ levels of 390 μatm (LC), then its physiological characteristics were investigated as cells were shifted from HC to LC or vice versa. The maximal electron transport rate (ETR_max) in the HC-acclimated cells was immediately reduced by decreased CO₂ availability, showing much lower values compared to that of the LC-acclimated cells. However, the cells showed a high capacity to regain their photochemical performance regardless of the growth CO₂ levels, with their ETR_max values recovering to initial levels in about 100 min. This result indicates that this diatom might modulate its CCMs quickly to maintain a steady state supply of CO₂, which is required for sustaining photosynthesis. In addition, active uptake of CO₂ could play a fundamental role during the induction of CCMs under CO₂ limitation, since the cells maintained high ETR even when both intracellular and periplasmic carbonic anhydrases were inhibited. It is concluded that efficient regulation of the CCM is one of the key strategies for diatoms to survive in fast changing pH environment, e.g. for the tested species, which is a dominant species in coastal waters where highly fluctuating pH is observed.     

Environmental stability affects phenotypic evolution in a globally distributed marine picoplankton

Marine phytoplankton can evolve rapidly when confronted with aspects of climate change because of their large population sizes and fast generation times. Despite this, the importance of environment fluctuations, a key feature of climate change, has received little attention—selection experiments with marine phytoplankton are usually carried out in stable environments and use single or few representatives of a species, genus or functional group. Here we investigate whether and by how much environmental fluctuations contribute to changes in ecologically important phytoplankton traits such as C:N ratios and cell size, and test the variability of changes in these traits within the globally distributed species Ostreococcus. We have evolved 16 physiologically distinct lineages of Ostreococcus at stable high CO₂ (1031±87 μatm CO₂, SH) and fluctuating high CO₂ (1012±244 μatm CO₂, FH) for 400 generations. We find that although both fluctuation and high CO₂ drive evolution, FH-evolved lineages are smaller, have reduced C:N ratios and respond more strongly to further increases in CO₂ than do SH-evolved lineages. This indicates that environmental fluctuations are an important factor to consider when predicting how the characteristics of future phytoplankton populations will have an impact on biogeochemical cycles and higher trophic levels in marine food webs.     

Community composition has greater impact on the functioning of marine phytoplankton communities than ocean acidification

Ecosystem functioning is simultaneously affected by changes in community composition and environmental change such as increasing atmospheric carbon dioxide (CO₂) and subsequent ocean acidification. However, it largely remains uncertain how the effects of these factors compare to each other. Addressing this question, we experimentally tested the hypothesis that initial community composition and elevated CO₂ are equally important to the regulation of phytoplankton biomass. We full‐factorially exposed three compositionally different marine phytoplankton communities to two different CO₂ levels and examined the effects and relative importance (ω²) of the two factors and their interaction on phytoplankton biomass at bloom peak. The results showed that initial community composition had a significantly greater impact than elevated CO₂ on phytoplankton biomass, which varied largely among communities. We suggest that the different initial ratios between cyanobacteria, diatoms, and dinoflagellates might be the key for the varying competitive and thus functional outcome among communities. Furthermore, the results showed that depending on initial community composition elevated CO₂ selected for larger sized diatoms, which led to increased total phytoplankton biomass. This study highlights the relevance of initial community composition, which strongly drives the functional outcome, when assessing impacts of climate change on ecosystem functioning. In particular, the increase in phytoplankton biomass driven by the gain of larger sized diatoms in response to elevated CO₂ potentially has strong implications for nutrient cycling and carbon export in future oceans.     

Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO₂ and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO₂ related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO₂. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO₂. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO₂ levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high‐nitrogen batch cultures and lower‐nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO₂ independent of the culture regime. We also find that, despite species‐specific variation in growth response to high CO₂, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO₂. The CO₂‐induced growth response is a good predictor of CO₂‐induced changes in competition (R² > .93) and community composition (R² > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO₂ levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.     

High CO2 concentration and iron availability determine the metabolic inventory in an Emiliania huxleyi-dominated phytoplankton community

Ocean acidification (OA), a consequence of anthropogenic carbon dioxide (CO₂) emissions, strongly impacts marine ecosystems. OA also influences iron (Fe) solubility, affecting biogeochemical and ecological processes. We investigated the interactive effects of CO₂ and Fe availability on the metabolome response of a natural phytoplankton community. Using mesocosms we exposed phytoplankton to ambient (390 μatm) or future CO₂ levels predicted for the year 2100 (900 μatm), combined with ambient (4.5 nM) or high (12 nM) dissolved iron (dFe). By integrating over the whole phytoplankton community, we assigned functional changes based on altered metabolite concentrations. Our study revealed the complexity of phytoplankton metabolism. Metabolic profiles showed three stages in response to treatments and phytoplankton dynamics. Metabolome changes were related to the plankton group contributing respective metabolites, explaining bloom decline and community succession. CO₂ and Fe affected metabolic profiles. Most saccharides, fatty acids, amino acids and many sterols significantly correlated with the high dFe treatment at ambient pCO₂. High CO₂ lowered the abundance of many metabolites irrespective of Fe. However, sugar alcohols accumulated, indicating potential stress. We demonstrate that not only altered species composition but also changes in the metabolic landscape affecting the plankton community may change as a consequence of future high-CO₂ oceans.     

Responses of Two Scleractinian Corals to Cobalt Pollution and Ocean Acidification

The effects of ocean acidification alone or in combination with warming on coral metabolism have been extensively investigated, whereas none of these studies consider that most coral reefs near shore are already impacted by other natural anthropogenic inputs such as metal pollution. It is likely that projected ocean acidification levels will aggravate coral reef health. We first investigated how ocean acidification interacts with one near shore locally abundant metal on the physiology of two major reef-building corals: Stylophora pistillata and Acropora muricata. Two pH levels (pH_T 8.02; pCO₂ 366 μatm and pH_T 7.75; pCO₂ 1140 μatm) and two cobalt concentrations (natural, 0.03 μg L^-1 and polluted, 0.2 μg L^-1) were tested during five weeks in aquaria. We found that, for both species, cobalt input decreased significantly their growth rates by 28% while it stimulated their photosystem II, with higher values of rETR_max (relative Electron Transport Rate). Elevated pCO₂ levels acted differently on the coral rETR_max values and did not affect their growth rates. No consistent interaction was found between pCO₂ levels and cobalt concentrations. We also measured in situ the effect of higher cobalt concentrations (1.06 ± 0.16 μg L^-1) on A. muricata using benthic chamber experiments. At this elevated concentration, cobalt decreased simultaneously coral growth and photosynthetic rates, indicating that the toxic threshold for this pollutant has been reached for both host cells and zooxanthellae. Our results from both aquaria and in situ experiments, suggest that these coral species are not particularly sensitive to high pCO₂ conditions but they are to ecologically relevant cobalt concentrations. Our study reveals that some reefs may be yet subjected to deleterious pollution levels, and even if no interaction between pCO₂ levels and cobalt concentration has been found, it is likely that coral metabolism will be weakened if they are subjected to additional threats such as temperature increase, other heavy metals, and eutrophication.     

Elevated CO2 and Phosphate Limitation Favor Micromonas pusilla through Stimulated Growth and Reduced Viral Impact

Growth and viral infection of the marine picoeukaryote Micromonas pusilla was studied under a future-ocean scenario of elevated partial CO₂ (pCO₂; 750 μatm versus the present-day 370 μatm) and simultaneous limitation of phosphorus (P). Independent of the pCO₂ level, the ratios of M. pusilla cellular carbon (C) to nitrogen (N), C:P and N:P, increased with increasing P stress. Furthermore, in the P-limited chemostats at growth rates of 0.32 and 0.97 of the maximum growth rate (μ_max), the supply of elevated pCO₂ led to an additional rise in cellular C:N and C:P ratios, as well as a 1.4-fold increase in M. pusilla abundance. Viral lysis was not affected by pCO₂, but P limitation led to a 150% prolongation of the latent period (6 to 12 h) and an 80% reduction in viral burst sizes (63 viruses per cell) compared to P-replete conditions (4 to 8 h latent period and burst size of 320). Growth at 0.32 μ_max further prolonged the latent period by another 150% (12 to 18 h). Thus, enhanced P stress due to climate change-induced strengthened vertical stratification can be expected to lead to reduced and delayed virus production in picoeukaryotes. This effect is tempered, but likely not counteracted, by the increase in cell abundance under elevated pCO₂. Although the influence of potential P-limitation-relieving factors, such as the uptake of organic P and P utilization during infection, is unclear, our current results suggest that when P limitation prevails in future oceans, picoeukaryotes and grazing will be favored over larger-sized phytoplankton and viral lysis, with increased matter and nutrient flow to higher trophic levels.     

Ocean acidification and temperature increase impact mussel shell shape and thickness: problematic for protection?

Ocean acidification threatens organisms that produce calcium carbonate shells by potentially generating an under‐saturated carbonate environment. Resultant reduced calcification and growth, and subsequent dissolution of exoskeletons, would raise concerns over the ability of the shell to provide protection for the marine organism under ocean acidification and increased temperatures. We examined the impact of combined ocean acidification and temperature increase on shell formation of the economically important edible mussel Mytilus edulis. Shell growth and thickness along with a shell thickness index and shape analysis were determined. The ability of M. edulis to produce a functional protective shell after 9 months of experimental culture under ocean acidification and increasing temperatures (380, 550, 750, 1000 μatm pCO ₂, and 750, 1000 μatm pCO ₂ + 2°C) was assessed. Mussel shells grown under ocean acidification conditions displayed significant reductions in shell aragonite thickness, shell thickness index, and changes to shell shape (750, 1000 μatm pCO ₂) compared to those shells grown under ambient conditions (380 μatm pCO ₂). Ocean acidification resulted in rounder, flatter mussel shells with thinner aragonite layers likely to be more vulnerable to fracture under changing environments and predation. The changes in shape presented here could present a compensatory mechanism to enhance protection against predators and changing environments under ocean acidification when mussels are unable to grow thicker shells. Here, we present the first assessment of mussel shell shape to determine implications for functional protection under ocean acidification.     

Giant Clams and Rising CO2: Light May Ameliorate Effects of Ocean Acidification on a Solar-Powered Animal

Global climate change and ocean acidification pose a serious threat to marine life. Marine invertebrates are particularly susceptible to ocean acidification, especially highly calcareous taxa such as molluscs, echinoderms and corals. The largest of all bivalve molluscs, giant clams, are already threatened by a variety of local pressures, including overharvesting, and are in decline worldwide. Several giant clam species are listed as ‘Vulnerable’ on the IUCN Red List of Threatened Species and now climate change and ocean acidification pose an additional threat to their conservation. Unlike most other molluscs, giant clams are ‘solar-powered’ animals containing photosynthetic algal symbionts suggesting that light could influence the effects of ocean acidification on these vulnerable animals. In this study, juvenile fluted giant clams Tridacna squamosa were exposed to three levels of carbon dioxide (CO₂) (control ~400, mid ~650 and high ~950 μatm) and light (photosynthetically active radiation 35, 65 and 304 μmol photons m^-2 s^-1). Elevated CO₂ projected for the end of this century (~650 and ~950 μatm) reduced giant clam survival and growth at mid-light levels. However, effects of CO₂ on survival were absent at high-light, with 100% survival across all CO₂ levels. Effects of CO₂ on growth of surviving clams were lessened, but not removed, at high-light levels. Shell growth and total animal mass gain were still reduced at high-CO₂. This study demonstrates the potential for light to alleviate effects of ocean acidification on survival and growth in a threatened calcareous marine invertebrate. Managing water quality (e.g. turbidity and sedimentation) in coastal areas to maintain water clarity may help ameliorate some negative effects of ocean acidification on giant clams and potentially other solar-powered calcifiers, such as hard corals.     

The response of a natural phytoplankton community from the Godavari River Estuary to increasing CO2 concentration during the pre-monsoon period

This paper reports for the first time upon the effects of increasing CO₂ concentrations on a natural phytoplankton assemblage in a tropical estuary (the Godavari River Estuary in India). Two short-term (5-day) bottle experiments were conducted (with and without nutrient addition) during the pre-monsoon season when the partial pressure of CO₂ in the surface water is quite low. The results reveal that the concentrations of total chlorophyll, the phytoplankton growth rate, the concentrations of particulate organic matter, the photosynthetic oxygen evolution rates, and the total bacterial count were higher under elevated CO₂ treatments, as compared to ambient conditions (control). δ¹³C of particulate organic matter (POM) varied inversely with respect to CO₂, indicating a clear signature of higher CO₂ influx under the elevated CO₂ levels. Whereas, δ¹³C_POM in the controls indicated the existence of an active bicarbonate transport system under limited CO₂ supply. A considerable change in phytoplankton community structure was noticed, with marker pigment analysis by HPLC revealing that cyanobacteria were dominant over diatoms as CO₂ concentrations increased. A mass balance calculation indicated that insufficient nutrients (N, P and Si) might have inhibited diatom growth compared to cyanobacteria, regardless of increased CO₂ supply. The present study suggests that CO₂ concentration and nutrient supply could have significant effects on phytoplankton physiology and community composition for natural phytoplankton communities in this region. However, this work was conducted during a non-discharge period (nutrient-limited conditions) and the responses of phytoplankton to increasing CO₂ might not necessarily be the same during other seasons with high physicochemical variability. Further investigation is therefore needed.     

Surface water CO2 concentration influences phytoplankton production but not community composition across boreal lakes

Recent experimental evidence suggests that changes in the partial pressure of CO₂ (pCO₂), in concert with nutrient fertilisation, may result in increased primary production and shifted phytoplankton community composition that favours species lacking adaptations to low CO₂ environments. It is not clear whether these results apply in ambient freshwaters, which are already often supersaturated in CO₂, and where phytoplankton structure and activity are under complex control of diverse local and regional factors. Here, we use a large‐scale comparative study of 69 boreal lakes to explore the influence of existing CO₂ gradients (c. 50–2300 μatm) on phytoplankton community composition and biomass production. While community composition did not respond to pCO₂ gradients, gross primary production was enhanced, but only in lakes already supersaturated in CO₂, demonstrating that environmental context is key in determining pCO₂–phytoplankton interactions. We further argue that increased atmospheric CO₂ is unlikely to influence phytoplanktonic composition and production in northern lakes.     

Effects of ocean acidification on Antarctic marine organisms: A meta‐analysis

Southern Ocean waters are among the most vulnerable to ocean acidification. The projected increase in the CO₂ level will cause changes in carbonate chemistry that are likely to be damaging to organisms inhabiting these waters. A meta‐analysis was undertaken to examine the vulnerability of Antarctic marine biota occupying waters south of 60°S to ocean acidification. This meta‐analysis showed that ocean acidification negatively affects autotrophic organisms, mainly phytoplankton, at CO₂ levels above 1,000 μatm and invertebrates above 1,500 μatm, but positively affects bacterial abundance. The sensitivity of phytoplankton to ocean acidification was influenced by the experimental procedure used. Natural, mixed communities were more sensitive than single species in culture and showed a decline in chlorophyll a concentration, productivity, and photosynthetic health, as well as a shift in community composition at CO₂ levels above 1,000 μatm. Invertebrates showed reduced fertilization rates and increased occurrence of larval abnormalities, as well as decreased calcification rates and increased shell dissolution with any increase in CO₂ level above 1,500 μatm. Assessment of the vulnerability of fish and macroalgae to ocean acidification was limited by the number of studies available. Overall, this analysis indicates that many marine organisms in the Southern Ocean are likely to be susceptible to ocean acidification and thereby likely to change their contribution to ecosystem services in the future. Further studies are required to address the poor spatial coverage, lack of community or ecosystem‐level studies, and the largely unknown potential for organisms to acclimate and/or adapt to the changing conditions.     

Effect of carbon dioxide on nitrate accumulation and nitrate reductase induction in corn seedlings

Exposure of the leaf canopy of corn seedlings (Zea mays L.) to atmospheric CO₂ levels ranging from 100 to 800 μl/l decreased nitrate accumulation and nitrate reductase activity. Plants pretreated with CO₂ in the dark and maintained in an atmosphere containing 100 μl/l CO₂ accumulated 7-fold more nitrate and had 2-fold more nitrate reductase activity than plants exposed to 600 μl/l CO₂, after 5 hours of illumination. Induction of nitrate reductase activity in leaves of intact corn seedlings was related to nitrate content. Changes in soluble protein were related to in vitro nitrate reductase activity suggesting that in vitro nitrate reductase activity was a measure of in situ nitrate reduction. In longer experiments, levels of nitrate reductase and accumulation of reduced N supported the concept that less nitrate was being absorbed, translocated, and assimilated when CO₂ was high. Plants exposed to increasing CO₂ levels for 3 to 4 hours in the light had increased concentrations of malate and decreased concentrations of nitrate in the leaf tissue. Malate and nitrate concentrations in the leaf tissue of seven of eight corn genotypes grown under comparable and normal (300 μl/l CO₂) environments, were negatively correlated. Exposure of roots to increasing concentrations of potassium carbonate with or without potassium sulfate caused a progressive increase in malate concentrations in the roots. When these roots were subsequently transferred to a nitrate medium, the accumulation of nitrate was inversely related to the initial malate concentrations. These data suggest that the concentration of malate in the tissue seem to be related to the accumulation of nitrate.     

Increasing pCO2 correlates with low concentrations of intracellular dimethylsulfoniopropionate in the sea anemone Anemonia viridis

Marine anthozoans maintain a mutualistic symbiosis with dinoflagellates that are prolific producers of the algal secondary metabolite dimethylsulfoniopropionate (DMSP), the precursor of the climate-cooling trace gas dimethyl sulfide (DMS). Surprisingly, little is known about the physiological role of DMSP in anthozoans and the environmental factors that regulate its production. Here, we assessed the potential functional role of DMSP as an antioxidant and determined how future increases in seawater pCO₂ may affect DMSP concentrations in the anemone Anemonia viridis along a natural pCO₂ gradient at the island of Vulcano, Italy. There was no significant difference in zooxanthellae genotype and characteristics (density of zooxanthellae, and chlorophyll a) as well as protein concentrations between anemones from three stations along the gradient, V1 (3232 μatm CO₂), V2 (682 μatm) and control (463 μatm), which indicated that A. viridis can acclimate to various seawater pCO₂. In contrast, DMSP concentrations in anemones from stations V1 (33.23 ± 8.30 fmol cell⁻¹) and V2 (34.78 ± 8.69 fmol cell⁻¹) were about 35% lower than concentrations in tentacles from the control station (51.85 ± 12.96 fmol cell⁻¹). Furthermore, low tissue concentrations of DMSP coincided with low activities of the antioxidant enzyme superoxide dismutase (SOD). Superoxide dismutase activity for both host (7.84 ± 1.37 U·mg⁻¹ protein) and zooxanthellae (2.84 ± 0.41 U·mg⁻¹ protein) at V1 was 40% lower than at the control station (host: 13.19 ± 1.42; zooxanthellae: 4.72 ± 0.57 U·mg⁻¹ protein). Our results provide insight into coastal DMSP production under predicted environmental change and support the function of DMSP as an antioxidant in symbiotic anthozoans.     

Warming and Ocean Acidification Effects on Phytoplankton—From Species Shifts to Size Shifts within Species in a Mesocosm Experiment

While the isolated responses of marine phytoplankton to climate warming and to ocean acidification have been studied intensively, studies on the combined effect of both aspects of Global Change are still scarce. Therefore, we performed a mesocosm experiment with a factorial combination of temperature (9 and 15°C) and pCO₂ (means: 439 ppm and 1040 ppm) with a natural autumn plankton community from the western Baltic Sea. Temporal trajectories of total biomass and of the biomass of the most important higher taxa followed similar patterns in all treatments. When averaging over the entire time course, phytoplankton biomass decreased with warming and increased with CO₂ under warm conditions. The contribution of the two dominant higher phytoplankton taxa (diatoms and cryptophytes) and of the 4 most important species (3 diatoms, 1 cryptophyte) did not respond to the experimental treatments. Taxonomic composition of phytoplankton showed only responses at the level of subdominant and rare species. Phytoplankton cell sizes increased with CO₂ addition and decreased with warming. Both effects were stronger for larger species. Warming effects were stronger than CO₂ effects and tended to counteract each other. Phytoplankton communities without calcifying species and exposed to short-term variation of CO₂ seem to be rather resistant to ocean acidification.     

Transition from Anoxygenic to Oxygenic Photosynthesis in a Microcoleus chthonoplastes Cyanobacterial Mat

Benthic cyanobacterial mats with the filamentous Microcoleus chthonoplastes as the dominant phototroph grow in oxic hypersaline environments such as Solar Lake, Sinai. The cyanobacteria are in situ exposed to chemical variations between 200 μmol of sulfide liter⁻¹ at night and 1 atm pO₂ during the day. During experimental H₂S to O₂ transitions the microbial community was shown to shift from anoxygenic photosynthesis, with H₂S as the electron donor, to oxygenic photosynthesis. Microcoleus filaments could carry out both types of photosynthesis concurrently. Anoxygenic photosynthesis dominated at high sulfide levels, 500 μmol liter⁻¹, while the oxygenic reaction became dominant when the sulfide level was reduced below 100 to 300 μmol liter⁻¹ (25 to 75 μmol of H₂S liter⁻¹). An increasing inhibition of the oxygenic photosynthesis was observed upon transition to oxic conditions from increasing sulfide concentrations. Oxygen built up within the Microcoleus layer of the mat even under 5 mmol of sulfide liter⁻¹ (500 μmol of H₂S liter⁻¹) in the overlying water. The implications of such a localized O₂ production in a highly reducing environment are discussed in relation to the evolution of oxygenic photosynthesis during the Proterozoic era.