Elevated CO2 affects kelp nutrient quality: A case study of Saccharina japonica from CO2 -enriched coastal mesocosm systems.

Kelps provide critical services for coastal food chains and ecosystem, and they are important food source for some segments of human population. Despite their ecological importance, little is known about long-term impacts of elevated CO2 (eCO2 ) on nutrient metabolites in kelps and the underlying regulation mechanisms. In this study, the kelp Saccharina japonica was cultured in CO2 -enriched coastal mesocosm systems for up to 3 months. We found that, although eCO2 significantly increased the growth rate, carbon concentrations, and C/N ratio of S. japonica, and it had no effect on total nitrogen and protein contents at the end of cultivation period. Meanwhile, it decreased the lipid, magnesium, sodium, and calcium content and changed the amino acid and fatty acid composition. Combining the genome-wide transcriptomic and metabolic evidence, we obtained a system-level understanding of metabolic response of S. japonica to eCO2 . The unique ornithine-urea cycle (OUC) and aspartate-argininosuccinate shunt (AAS), coupled with TCA cycle, balanced the carbon and nitrogen metabolism under eCO2 by providing carbon skeleton for amino acid synthesis and reduced power for nitrogen assimilation. This research provides a major advance in the understanding of kelp nutrient metabolic mechanism in the context of global climate change, and such CO2 -induced shifts in nutritional value may induce changes in the structure and stability of marine trophic webs and affect the quality of human nutrition resources.

Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2.

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short-term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long-term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO2 conditions (1,000 µatm). High CO2 -selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO2 (400 µatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO2 -selected cell lines. All of these trait changes persisted when high CO2 -grown populations were moved back to ambient CO2 conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short-term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.

Effects of temperature and nitrogen sources on physiological performance of the coccolithophore Emiliania huxleyi.

Both temperature and nutrient levels are rising in worldwide ocean ecosystems, and they strongly influence biological responses of phytoplankton. However, few studies have addressed the interactive effects of temperature and nitrogen sources on physiological performance of the coccolithophore Emiliania huxleyi. In this study, we evaluated algal growth, photosynthesis and respiration, elemental composition, enzyme activity, and calcification under a matrix of two temperatures gradients (ambient temperature 20 °C and high temperature 24 °C) and two nitrogen sources (nitrate (NO3-) and ammonium (NH4+)). When the algae was cultured with NO3- medium, high temperature reduced algal photosynthesis and nitrate reductase activity, but it did not change other indicators significantly relative to ambient temperature. In addition, E. huxleyi preferred NO3- as the growth medium, whereas NH4+ had negative effects on physiological parameters. In the NH4+ medium, the growth rate, photosynthesis and photosynthetic rate, nitrate reductase activity, and particulate organic carbon and particulate organic nitrogen production rate of the algae decreased as temperature increased. Conversely, high temperature increased cellular particulate organic carbon, cellular particulate organic nitrogen, and particulate inorganic carbon levels. In summary, our findings indicate that the distribution and abundance of microalgae could be greatly affected under warming ocean temperature and different nutrient conditions.

Combined effects of ocean deoxygenation, acidification, and phosphorus limitation on green tide macroalga, Ulva prolifera.

Ocean deoxygenation, acidification, and decreased phosphorus availability are predicted to increase in coastal ecosystems under future climate change. However, little is known regarding the combined effects of such environmental variables on the green tide macroalga Ulva prolifera. Here, we provide quantitative and mechanistic understanding of the acclimation mechanisms of U. prolifera to ocean deoxygenation, acidification, and phosphorus limitation under both laboratory and semi-natural (mesocosms) conditions. We found that there were significant interactions between these global environmental conditions on algal physiological performance. Although algal growth rate and photosynthesis reduced when the nitrogen-to­phosphorus (N/P) ratio increased from 16:1 to 35:1 under ambient CO2 and O2 condition, they remained constant with further increasing N/P ratios of 105:1, 350:1, and 1050:1. However, the increasing alkaline phosphatase activities at high N/P ratios suggests that U. prolifera could use organic P to support its growth under phosphorus limitation. Deoxygenation had no effect on specific growth rate (SGR) but decreased photosynthesis under low N/P ratios of 16:1, 35:1, and 105:1, with reduced activities of several enzymes involved in N assimilation pathway being observed. Elevated CO2 promoted algal growth and alleviated the negative effect of deoxygenation on algal photosynthesis. The patterns of responses to high CO2 and low O2 treatments in in situ experiments were generally consistent with those observed in laboratory experiments. Our results generally found that the strong physiological acclimation capacity to elevated CO2, low O2, and high N/P could contribute to its large-scale blooming in coastal ecosystem.

Ocean acidification exacerbates the inhibition of fluctuating light on the productivity of Ulva prolifera.

Ulva prolifera, a common species of green macroalgae, is often harmful-algal-bloom causative and significantly impacts local marine ecosystems. Previous studies on the physiological characteristics of U. prolifera have been conducted under constant light (CL). However, light in the natural environment continually changes, and little is known about fluctuating light (FL). Ocean acidification (OA) has been proposed to interact with dynamic surrounding environments to affect the physiological performance of macroalgae. Therefore, we investigated the combined effects of FL (80/300, alternating between 80 µmol photons m-2 s-1 for 2.5 h and 300 µmol photons m-2 s-1 for 1.5 h, with an average light intensity of 160 µmol photons m-2 s-1 and OA (1000 ppm CO2) on U. prolifera. The results clearly showed that FL had no significant effect on the relative growth rate (RGR), whereas OA obviously improved RGR. However, under FL-OA combination conditions, RGR was inhibited significantly, accompanied by a concomitant downgraded photosynthetic performance, while the photoprotective abilities were enhanced. The results would help us accurately predict the primary productivity of macroalgae in coastal waters under future OA conditions with irradiance fluctuations.

Ocean acidification stimulation of phytoplankton growth depends on the extent of departure from the optimal growth temperature.

Ocean acidification and warming are two major environmental stressors; however, the generality of how warming will alter growth responses of phytoplankton to ocean acidification is less known. Here, enhancement of growth by high CO2 (HC) in Phaeodactylum tricornutum and Thalassiosira weissflogii was most prominent at optimum temperature. The extent to which growth rates in HC cultures were raised compared to low CO2 (LC) cultures tended to decrease with increasing or decreasing temperature, compared to the optimum. Further mechanistic studies in P. tricornutum revealed that cellular carbon and nitrogen content, superoxide dismutase activity, and respiration were generally higher in HC than those in LC at high and low temperatures, whereas PSII photochemical parameters were generally lower in HC than in LC at high and low temperatures. These results indicate that HC-grown cells needed to invest more energy and materials to maintain intracellular homeostasis and repair damage induced by the unsuitable temperatures.

Influence of ocean acidification on thermal reaction norms of carbon metabolism in the marine diatom Phaeodactylum tricornutum.

Under the present CO2 condition, the efficiency of biological pump mediating carbon sequestration is predicted to decline in the future because respiration tends to be more sensitive to rising temperature than is photosynthesis. However, it remains unknown whether the impacts of global warming on metabolic rates of phytoplankton can be modulated by elevated CO2 induced ocean acidification. Here we show that in the model diatom species Phaeodactylum tricornutum, Ea (activation energy) of photosynthesis (~0.5 eV) was significantly lower than that of respiration (1.8 eV), while CO2 concentration had no effect on the Ea value. Eh (deactivation energy) of respiration was increased to 2.5 eV, that was equivalent to Eh of photosynthesis in high CO2-grown cells and 28.4% higher than that in low CO2-grown ones. The respiration to photosynthesis ratio (R/P) was consistently higher in high CO2 condition, which increased with temperature at the beginning and subsequently decreased in both CO2 conditions. The ratio of R/P in high CO2 to R/P in low CO2 gradually increased with temperature above the optimal temperature. Our results imply that ocean acidification will aggravate the negative impacts or offset the alleviating effects of warming on the R/P ratio depending on the temperature range in Phaeodactylum tricornutum.

Integrating Transcriptomics and Metabolomics to Characterize Metabolic Regulation to Elevated CO2 in Chlamydomonas Reinhardtii.

With atmospheric CO2 increasing, a large amount of CO2 is absorbed by oceans and lakes, which changes the carbonate system and affects the survival of aquatic plants, especially microalgae. The main aim of our study was to explore the responses of Chlamydomonas reinhardtii (Chlorophyceae) to elevated CO2 by combined transcriptome and metabolome analysis under three different scenarios: control (CK, 400 ppm), short-term elevated CO2 (ST, 1000 ppm), and long-term elevated CO2 (LT, 1000 ppm). The transcriptomic data showed moderate changes between ST and CK. However, metabolic analysis indicated that fatty acids (FAs) and partial amino acids (AAs) were increased under ST. There was a global downregulation of genes involved in photosynthesis, glycolysis, lipid metabolism, and nitrogen metabolism but increase in the TCA cycle and ß-oxidation under LT. Integrated transcriptome and metabolome analyses demonstrated that the nutritional constituents (FAs, AAs) under LT were poor compared with CK, and most genes and metabolites involved in C and N metabolism were significantly downregulated. However, the growth and photosynthesis of cells under LT increased significantly. Thus, C. reinhardtii could form a specific adaptive evolution to elevated CO2, affecting future biogeochemical cycles.

Acclimation and adaptation to elevated pCO2 increase arsenic resilience in marine diatoms.

Arsenic pollution is a widespread threat to marine life, but the ongoing rise pCO2 levels is predicted to decrease bio-toxicity of arsenic. However, the effects of arsenic toxicity on marine primary producers under elevated pCO2 are not well characterized. Here, we studied the effects of arsenic toxicity in three globally distributed diatom species (Phaeodactylum tricornutum, Thalassiosira pseudonana, and Chaetoceros mulleri) after short-term acclimation (ST, 30 days), medium-term exposure (MT, 750 days), and long-term (LT, 1460 days) selection under ambient (400 µatm) and elevated (1000 and 2000 µatm) pCO2. We found that elevated pCO2 alleviated arsenic toxicity even after short acclimation times but the magnitude of the response decreased after mid and long-term adaptation. When fed with these elevated pCO2 selected diatoms, the scallop Patinopecten yessoensis had significantly lower arsenic content (3.26-52.83%). Transcriptomic and biochemical analysis indicated that the diatoms rapidly developed arsenic detoxification strategies, which included upregulation of transporters associated with shuttling harmful compounds out of the cell to reduce arsenic accumulation, and upregulation of proteins involved in synthesizing glutathione (GSH) to chelate intracellular arsenic to reduce arsenic toxicity. Thus, our results will expand our knowledge to fully understand the ecological risk of trace metal pollution under increasing human activity induced ocean acidification.

The acclimation process of phytoplankton biomass, carbon fixation and respiration to the combined effects of elevated temperature and pCO2 in the northern South China Sea.

We conducted shipboard microcosm experiments at both off-shore (SEATS) and near-shore (D001) stations in the northern South China Sea (NSCS) under three treatments, low temperature and low pCO2 (LTLC), high temperature and low pCO2 (HTLC), and high temperature and high pCO2 (HTHC). Biomass of phytoplankton at both stations were enhanced by HT. HTHC did not affect phytoplankton biomass at station D001 but decreased it at station SEATS. HT alone increased net primary productivity by 234% at station SEATS and by 67% at station D001 but the stimulating effect disappeared when HC was combined. HT also increased respiration rate by 236% at station SEATS and by 87% at station D001 whereas HTHC reduced it by 61% at station SEATS and did not affect it at station D001. Overall, our findings indicate that the positive effect of ocean warming on phytoplankton assemblages in NSCS could be damped or offset by ocean acidification.

The fatty acid content of plankton is changing in subtropical coastal waters as a result of OA: Results from a mesocosm study.

Ocean Acidification (OA) effects on marine plankton are most often considered in terms of inorganic carbon chemistry, but decreasing pH may influence other aspects of cellular metabolism. Here we present the effects of OA on the fatty acid (FA) content and composition of an artificial phytoplankton community (Phaeodactylum tricornutum, Thalassiosira weissflogii, and Emiliania huxleyi) in a fully replicated, ∼4 m3 mesocosm study in subtropical coastal waters (Wuyuan Bay, China, 24.52°N, 117.18°E) at present day (400 µatm) and elevated (1000 µatm) pCO2 concentrations. Phytoplankton growth occurred in three phases during the 33-day experiment: an initial exponential growth leading to senescence and a subsequent decline phase. Phytoplankton sampled from these mesocosms were fed to mesozooplankton collected by net haul from Wuyuan Bay. Concentrations of saturated fatty acids (SFA) in both phytoplankton and mesozooplankton remained high under acidified and non-acidified conditions. However, polyunsaturated fatty acids (PUFA) and monounsaturated fatty acids (MUFA) increased significantly more under elevated pCO2 during the late exponential phase (Day 13), indicating increased nutritional value for zooplankton and higher trophic levels. Indeed, uptake rates of the essential FA docosahexaenoic acid (C20:5n3, DHA) increased in mesozooplankton under acidified conditions. However, mesozooplankton grazing rates decreased overall with elevated pCO2. Our findings show that these selected phytoplankton species have a relatively high tolerance to acidification in terms of FA production, and local mesozooplankton in these subtropical coastal waters can maintain their FA composition under end of century ocean acidification conditions.

Carbon assimilation and losses during an ocean acidification mesocosm experiment, with special reference to algal blooms.

A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China, to investigate the effects of elevated pCO2 on bloom formation by phytoplankton species previously studied in laboratory-based ocean acidification experiments, to determine if the indoor-grown species performed similarly in mesocosms under more realistic environmental conditions. We measured biomass, primary productivity and particulate organic carbon (POC) as well as particulate organic nitrogen (PON). Phaeodactylum tricornutum outcompeted Thalassiosira weissflogii and Emiliania huxleyi, comprising more than 99% of the final biomass. Mainly through a capacity to tolerate nutrient-limited situations, P. tricornutum showed a powerful sustained presence during the plateau phase of growth. Significant differences between high and low CO2 treatments were found in cell concentration, cumulative primary productivity and POC in the plateau phase but not during the exponential phase of growth. Compared to the low pCO2 (LC) treatment, POC increased by 45.8-101.9% in the high pCO2 (HC) treated cells during the bloom period. Furthermore, respiratory carbon losses of gross primary productivity were found to comprise 39-64% for the LC and 31-41% for the HC mesocosms (daytime C fixation) in phase II. Our results suggest that the duration and characteristics of a diatom bloom can be affected by elevated pCO2. Effects of elevated pCO2 observed in the laboratory cannot be reliably extrapolated to large scale mesocosms with multiple influencing factors, especially during intense algal blooms.

Contrasting Photophysiological Characteristics of Phytoplankton Assemblages in the Northern South China Sea.

The growth of phytoplankton and thus marine primary productivity depend on photophysiological performance of phytoplankton cells that respond to changing environmental conditions. The South China Sea (SCS) is the largest marginal sea of the western Pacific and plays important roles in modulating regional climate and carbon budget. However, little has been documented on photophysiological characteristics of phytoplankton in the SCS. For the first time, we investigated photophysiological characteristics of phytoplankton assemblages in the northern South China Sea (NSCS) using a real-time in-situ active chlorophyll a fluorometry, covering 4.0 × 105 km2. The functional absorption cross section of photosystem II (PSII) in darkness (σPSII) or under ambient light (σPSII') (A2 quanta-1) increased from the surface to deeper waters at all the stations during the survey period (29 July to 23 August 2012). While the maximum (Fv/Fm, measured in darkness) or effective (Fq'/Fm', measured under ambient light) photochemical efficiency of PSII appeared to increase with increasing depth at most stations, it showed inverse relationship with depth in river plume areas. The functional absorption cross section of PSII changes could be attributed to light-adapted genotypic feature due to niche-partition and the alteration of photochemical efficiency of PSII could be attributed to photo-acclimation. The chlorophyll a fluorometry can be taken as an analog to estimate primary productivity, since areas of higher photochemical efficiency of PSII coincided with those of higher primary productivity reported previously in the NSCS.