Impacts of elevated CO2 on particulate and dissolved organic matter production: microcosm experiments using iron-deficient plankton communities in open subarctic waters

Response of phytoplankton to increasing CO2 in seawater in terms of physiology and ecology is key to predicting changes in marine ecosystems. However, responses of natural plankton communities especially in the open ocean to higher CO2 levels have not been fully examined. We conducted CO2 manipulation experiments in the Bering Sea and the central subarctic Pacific, known as high nutrient and low chlorophyll regions, in summer 2007 to investigate the response of organic matter production in iron-deficient plankton communities to CO2 increases. During the 14-day incubations of surface waters with natural plankton assemblages in microcosms under multiple pCO2 levels, the dynamics of particulate organic carbon (POC) and nitrogen (PN), and dissolved organic carbon (DOC) and phosphorus (DOP) were examined with the plankton community compositions. In the Bering site, net production of POC, PN, and DOP relative to net chlorophyll-a production decreased with increasing pCO2. While net produced POC:PN did not show any CO2-related variations, net produced DOC:DOP increased with increasing pCO2. On the other hand, no apparent trends for these parameters were observed in the Pacific site. The contrasting results observed were probably due to the different plankton community compositions between the two sites, with plankton biomass dominated by large-sized diatoms in the Bering Sea versus ultra-eukaryotes in the Pacific Ocean. We conclude that the quantity and quality of the production of particulate and dissolved organic matter may be altered under future elevated CO2 environments in some iron-deficient ecosystems, while the impacts may be negligible in some systems.

European sea bass, Dicentrarchus labrax, in a changing ocean

Ocean acidification, caused by rising concentrations of carbon dioxide (CO2), is widely considered to be a major global threat to marine ecosystems. To investigate the potential effects of ocean acidification on the early life stages of a commercially important fish species, European sea bass (Dicentrarchus labrax), 12 000 larvae were incubated from hatch through metamorphosis under a matrix of two temperatures (17 and 19 °C) and two seawater pCO2 levels (ambient and 1,000 µatm) and sampled regularly for 42 days. Calculated daily mortality was significantly affected by both temperature and pCO2, with both increased temperature and elevated pCO2 associated with lower daily mortality and a significant interaction between these two factors. There was no significant pCO2 effect noted on larval morphology during this period but larvae raised at 19 °C possessed significantly larger eyes and lower carbon:nitrogen ratios at the end of the study compared to those raised under 17 °C. Similarly, when the incubation was continued to post-metamorphic (juvenile) animals (day 67-69), fish raised under a combination of 19 °C and 1000 µatm pCO2 were significantly heavier. However, juvenile D. labrax raised under this combination of 19 °C and 1000 µatm pCO2 also exhibited lower aerobic scopes than those incubated at 19 °C and ambient pCO2. Most studies investigating the effects of near-future oceanic conditions on the early life stages of marine fish have used incubations of relatively short durations and suggested that these animals are resilient to ocean acidification. Whilst the increased survival and growth observed in this study supports this view, we conclude that more work is required to investigate whether the differences in juvenile physiology observed in this study manifest as negative impacts in adult fish.

Responses of the metabolism of the larvae of Pocillopora damicornis to ocean acidification and warming

Ocean acidification and warming are expected to threaten the persistence of tropical coral reef ecosystems. As coral reefs face multiple stressors, the distribution and abundance of corals will depend on the successful dispersal and settlement of coral larvae under changing environmental conditions. To explore this scenario, we used metabolic rate, at holobiont and molecular levels, as an index for assessing the physiological plasticity of Pocillopora damicornis larvae from this site to conditions of ocean acidity and warming. Larvae were incubated for 6 hours in seawater containing combinations of CO2 concentration (450 and 950 µatm) and temperature (28 and 30°C). Rates of larval oxygen consumption were higher at elevated temperatures. In contrast, high CO2 levels elicited depressed metabolic rates, especially for larvae released later in the spawning period. Rates of citrate synthase, a rate-limiting enzyme in aerobic metabolism, suggested a biochemical limit for increasing oxidative capacity in coral larvae in a warming, acidifying ocean. Biological responses were also compared between larvae released from adult colonies on the same day (cohorts). The metabolic physiology of Pocillopora damicornis larvae varied significantly by day of release. Additionally, we used environmental data collected on a reef in Moorea, French Polynesia to provide information about what adult corals and larvae may currently experience in the field. An autonomous pH sensor provided a continuous time series of pH on the natal fringing reef. In February/March, 2011, pH values averaged 8.075±0.023. Our results suggest that without adaptation or acclimatization, only a portion of naïve Pocillopora damicornis larvae may have suitable metabolic phenotypes for maintaining function and fitness in an end-of-the century ocean.

Effects of ultraviolet radiation and CO2 increase on winter phytoplankton assemblages in a temperate coastal lagoon

Increases in ultraviolet radiation (UVR) and CO2 affect phytoplankton growth and mortality in a variety of different ways. However, in situ responses of natural phytoplankton communities to climate change, as well as its effects on phytoplankton annual cycles, are still largely unknown. Although temperature and UVR have been increasing in temperate latitudes during winter, this season is still particularly neglected in climate change studies, being considered a non-active season regarding phytoplankton growth and production. Additionally, coastal lagoons are highly productive ecosystems and very vulnerable to climate change. This study aims, therefore, to evaluate the short-term effects of increased UVR and CO2 on the composition and growth of winter phytoplankton assemblages in a temperate coastal lagoon. During winter 2012, microcosm experiments were used to evaluate the isolated and combined effects of UVR and CO2, under ambient and high CO2 treatments, exposed to ambient UV levels and photosynthetically active radiation (PAR), or to PAR only. Phytoplankton composition, abundance, biomass and photosynthetic parameters were evaluated during the experiments. Significant changes were observed in the growth of specific phytoplankton groups, leading to changes in community composition. The cyanobacterium Synechococcus was dominant at the beginning of the experiment, but it was negatively affected by UVR and CO2. Diatoms clearly benefited from high CO2 and UVR, particularly Thalassiosira. Despite the changes observed in specific phytoplankton groups, growth and production of the whole phytoplankton community did not show significant responses to UVR and/or CO2.

Trichodesmium's strategies to alleviate phosphorus limitation in the future acidified oceans

Global warming may exacerbate inorganic nutrient limitation, including phosphorus (P), in the surface-waters of tropical oceans that are home to extensive blooms of the marine diazotrophic cyanobacterium, Trichodesmium. We examined the combined effects of P limitation and pCO2, forecast under ocean acidification scenarios, on Trichodesmium erythraeum IMS101 cultures. We measured nitrogen acquisition, glutamine synthetase activity, C uptake rates, intracellular Adenosine Triphosphate (ATP) concentration and the pool sizes of related key proteins. Here, we present data supporting the idea that cellular energy re-allocation enables the higher growth and N2 fixation rates detected in Trichodesmium cultured under high pCO2. This is reflected in altered protein abundance and metabolic pools. Also modified are particulate organic carbon and nitrogen production rates, enzymatic activities, and cellular ATP concentrations. We suggest that adjusting these cellular pathways to changing environmental conditions enables Trichodesmium to compensate for low P availability and to thrive in acidified oceans. Moreover, elevated pCO2 could provide Trichodesmium with a competitive dominance that would extend its niche, particularly in P-limited regions of the tropical and subtropical oceans.

Physiological responses and scope for growth upon medium-term exposure to the combined effects of ocean acidification and temperature in a subtidal scavenger Nassarius conoidalis

Physiological responses (ingestion rate, absorption rate and efficiency, respiration, rate, excretion rate) and scope for growth of a subtidal scavenging gastropod Nassarius conoidalis under the combined effects of ocean acidification (pCO2 levels: 380, 950, 1250 µatm) and temperature (15, 30 °C) were investigated for 31 days. There was a significant reduction in all the physiological rates and scope for growth following short-term exposure (1-3 days) to elevated pCO2 except absorption efficiency at 15 °C and 30 °C, and respiration rate and excretion rate at 15 °C. The percentage change in the physiological rates ranged from 0% to 90% at 15 °C and from 0% to 73% at 30 °C when pCO2 was increased from 380 µatm to 1250 µatm. The effect of pCO2 on the physiological rates was enhanced at high temperature for ingestion, absorption, respiration and excretion. When the exposure period was extended to 31 days, the effect of pCO2 was significant on the ingestion rate only. All the physiological rates remained unchanged when temperature increased from 24 °C to 30 °C but the rates at 15 °C were significantly lower, irrespective of the duration of exposure. Our data suggested that a medium-term exposure to ocean acidification has no effect on the energetics of N. conoidalis. Nevertheless, the situation may be complicated by a longer term of exposure and/or a reduction in salinity in a warming world.

The effects of thermal and high-CO2 stresses on the metabolism and surrounding microenvironment of the coral Galaxea fascicularis

The effects of elevated temperature and high pCO2 on the metabolism of Galaxea fascicularis were studied with oxygen and pH microsensors. Photosynthesis and respiration rates were evaluated from the oxygen fluxes from and to the coral polyps. High-temperature alone lowered both photosynthetic and respiration rates. High pCO2 alone did not significantly affect either photosynthesis or respiration rates. Under a combination of high-temperature and high-CO2, the photosynthetic rate increased to values close to those of the controls. The same pH in the diffusion boundary layer was observed under light in both (400 and 750 ppm) CO2 treatments, but decreased significantly in the dark as a result of increased CO2. The ATP contents decreased with increasing temperature. The effects of temperature on the metabolism of corals were stronger than the effects of increased CO2. The effects of acidification were minimal without combined temperature stress. However, acidification combined with higher temperature may affect coral metabolism due to the amplification of diel variations in the microenvironment surrounding the coral and the decrease in ATP contents.

Ocean acidification rapidly reduces dinitrogen fixation associated with the hermatypic coral Seriatopora hystrix

Since productivity and growth of coral-associated dinoflagellate algae is nitrogen (N)-limited, dinitrogen (N2) fixation by coral-associated microbes is likely crucial for maintaining the coral-dinoflagellate symbiosis. It is thus essential to understand the effects future climate change will have on N2 fixation by the coral holobiont. This laboratory study is the first to investigate short-term effects of ocean acidification on N2 fixation activity associated with the tropical, hermatypic coral Seriatopora hystrix using the acetylene reduction assay in combination with calcification measurements. Findings reveal that simulated ocean acidification ( pCO2 1080 µatm) caused a rapid and significant decrease (53%) in N2 fixation rates associated with S. hystrix compared to the present day scenario ( pCO2 486 µatm). In addition, N2 fixation associated with the coral holobiont showed a positive exponential relationship with its calcification rates. This suggests that even small declines in calcification rates of hermatypic corals under high CO2 conditions may result in decreased N2 fixation activity, since these 2 processes may compete for energy in the coral holobiont. Ultimately, an intensified N limitation in combination with a decline in skeletal growth may trigger a negative feedback loop on coral productivity exacerbating the negative long-term effects of ocean acidification.

Shell condition and survival of Puget Sound pteropods are impaired by ocean acidification conditions

We tested whether the thecosome pteropod Limacina helicina from Puget Sound, an urbanized estuary in the northwest continental US, experiences shell dissolution and altered mortality rates when exposed to the high CO2, low aragonite saturation state (Omega a) conditions that occur in Puget Sound and the northeast Pacific Ocean. Five, week-long experiments were conducted in which we incubated pteropods collected from Puget Sound in four carbon chemistry conditions: current summer surface (460-500 µatm CO2, Omega a=1.59), current deep water or surface conditions during upwelling (760 and 1600-1700 µatm CO2, Omega a=1.17 and 0.56), and future deep water or surface conditions during upwelling (2800-3400 µatm CO2, Omega a=0.28). We measured shell condition using a scoring regime of five shell characteristics that capture different aspects of shell dissolution. We characterized carbon chemistry conditions in statistical analyses with Omega a, and conducted analyses considering Omega a both as a continuous dataset and as discrete treatments. Shell dissolution increased linearly as aragonite saturation state decreased. Discrete treatment comparisons indicate that shell dissolution was greater in undersaturated treatments compared to oversaturated treatments. Survival increased linearly with aragonite saturation state, though discrete treatment comparisons indicated that survival was similar in all but the lowest saturation state treatment. These results indicate that, under starvation conditions, pteropod survival may not be greatly affected by current and expected near-future aragonite saturation state in the NE Pacific, but shell dissolution may. Given that subsurface waters in Puget Sound's main basin are undersaturated with respect to aragonite in the winter and can be undersaturated in the summer, the condition and persistence of the species in this estuary warrants further study.

The potential of ocean acidifi cation on suppressing larval development in the Pacifi c oyster Crassostrea gigas and blood cockle Arca infl ata Reeve

We evaluated the effect of pH on larval development in larval Pacific oyster (Crassostrea gigas) and blood cockle ( Arca inflata Reeve). The larvae were reared at pH 8.2 (control), 7.9, 7.6, or 7.3 beginning 30 min or 24 h post fertilization. Exposure to lower pH during early embryonic development inhibited larval shell formation in both species. Compared with the control, larvae took longer to reach the D-veliger stage when reared under pH 7.6 and 7.3. Exposure to lower pH immediately after fertilization resulted in significantly delayed shell formation in the Pacific oyster larvae at pH 7.3 and blood cockle larvae at pH 7.6 and 7.3. However, when exposure was delayed until 24 h post fertilization, shell formation was only inhibited in blood cockle larvae reared at pH 7.3. Thus, the early embryonic stages were more sensitive to acidified conditions. Our results suggest that ocean acidification will have an adverse effect on embryonic development in bivalves. Although the effects appear subtle, they may accumulate and lead to subsequent issues during later larval development.

Combined metabolome and proteome analysis of the mantle tissue from Pacific oyster Crassostrea gigas exposed to elevated pCO2

Ocean acidification (OA) has been found to affect an array of normal physiological processes in mollusks, especially posing a significant threat to the fabrication process of mollusk shell. In the current study, the impact of exposure to elevated pCO2 condition was investigated in mantle tissue of Crassostrea gigas by an integrated metabolomic and proteomic approach. Analysis of metabolome and proteome revealed that elevated pCO2 could affect energy metabolism in oyster C. gigas, marked by differentially altered ATP, succinate, MDH, PEPCK and ALDH levels. Moreover, the up-regulated calponin-2, tropomyosins and myosin light chains indicated that elevated pCO2 probably caused disturbances in cytoskeleton structure in mantle tissue of oyster C. gigas. This work demonstrated that a combination of proteomics and metabolomics could provide important insights into the effects of OA at molecular levels.

Detection of a variable intracellular acid-labile carbon pool in Thalassiosira weissflogii (Heterokontophyta) and Emiliania huxleyi (Haptophyta) in response to changes in the seawater carbon system

Accumulation of an intracellular pool of carbon (C(i) pool) is one strategy by which marine algae overcome the low abundance of dissolved CO2 (CO2 (aq) ) in modern seawater. To identify the environmental conditions under which algae accumulate an acid-labile C(i) pool, we applied a (14) C pulse-chase method, used originally in dinoflagellates, to two new classes of algae, coccolithophorids and diatoms. This method measures the carbon accumulation inside the cells without altering the medium carbon chemistry or culture cell density. We found that the diatom Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle] and a calcifying strain of the coccolithophorid Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler] develop significant acid-labile C(i) pools. C(i) pools are measureable in cells cultured in media with 2-30 µmol/l CO2 (aq), corresponding to a medium pH of 8.6-7.9. The absolute C(i) pool was greater for the larger celled diatoms. For both algal classes, the C(i) pool became a negligible contributor to photosynthesis once CO2 (aq) exceeded 30 µmol/l. Combining the (14) C pulse-chase method and (14) C disequilibrium method enabled us to assess whether E. huxleyi and T. weissflogii exhibited thresholds for foregoing accumulation of DIC or reduced the reliance on bicarbonate uptake with increasing CO2 (aq) . We showed that the C(i) pool decreases with higher CO2 :HCO3 (-) uptake rates.

Mixed effects of elevated pCO2 on fertilisation, larval and juvenile development and adult responses in the mobile subtidal scallop Mimachlamys asperrima (Lamarck, 1819)

Ocean acidification is predicted to have severe consequences for calcifying marine organisms especially molluscs. Recent studies, however, have found that molluscs in marine environments with naturally elevated or fluctuating CO2 or with an active, high metabolic rate lifestyle may have a capacity to acclimate and be resilient to exposures of elevated environmental pCO2. The aim of this study was to determine the effects of near future concentrations of elevated pCO2 on the larval and adult stages of the mobile doughboy scallop, Mimachlamys asperrima from a subtidal and stable physio-chemical environment. It was found that fertilisation and the shell length of early larval stages of M. asperrima decreased as pCO2 increased, however, there were less pronounced effects of elevated pCO2 on the shell length of later larval stages, with high pCO2 enhancing growth in some instances. Byssal attachment and condition index of adult M. asperrima decreased with elevated pCO2, while in contrast there was no effect on standard metabolic rate or pHe. The responses of larval and adult M. asperrima to elevated pCO2 measured in this study were more moderate than responses previously reported for intertidal oysters and mussels. Even this more moderate set of responses are still likely to reduce the abundance of M. asperrima and potentially other scallop species in the world's oceans at predicted future pCO2 levels.

Ocean acidification outweighs nutrient effects in structuring seagrass epiphyte communities

1. Developing a framework for assessing interactions between multiple anthropogenic stressors remains an important goal in environmental research. In coastal ecosystems, the relative effects of aspects of global climate change (e.g. CO2 concentrations) and localized stressors (e.g. eutrophication), in combination, have received limited attention. 2. Using a long-term (11 month) field experiment, we examine how epiphyte assemblages in a tropical seagrass meadow respond to factorial manipulations of dissolved carbon dioxide (CO2(aq)) and nutrient enrichment. In situ CO2(aq) manipulations were conducted using clear, open-top chambers, which replicated carbonate parameter forecasts for the year 2100. Nutrient enrichment consisted of monthly additions of slow-release fertilizer, nitrogen (N) and phosphorus (P), to the sediments at rates equivalent to theoretical maximum rates of anthropogenic loading within the region (1.54 g N/m**2/d and 0.24 g P m**2/d). 3. Epiphyte community structure was assessed on a seasonal basis and revealed declines in the abundance of coralline algae, along with increases in filamentous algae under elevated CO2(aq). Surprisingly, nutrient enrichment had no effect on epiphyte community structure or overall epiphyte loading. Interactions between CO2(aq) and nutrient enrichment were not detected. Furthermore, CO2(aq)-mediated responses in the epiphyte community displayed strong seasonality, suggesting that climate change studies in variable environments should be conducted over extended time-scales. 4. Synthesis. The observed responses indicate that for certain locations, global stressors such as ocean acidification may take precedence over local eutrophication in altering the community structure of seagrass epiphyte assemblages. Given that nutrient-driven algal overgrowth is commonly cited as a widespread cause of seagrass decline, our findings highlight that alternate climate change forces may exert proximate control over epiphyte community structure.