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.

Seawater carbonate chemistry, respiration, maximum photochemical efficiency and mortality of brooded larvae of Pocillopora//damicornis in a laboratory experiment

To evaluate the effects of temperature and pCO2 on coral larvae, brooded larvae of Pocillopora damicornis from Nanwan Bay, Taiwan (21°56.179' N, 120°44.85' E), were exposed to ambient (419-470 µatm) and high (604-742 µatm) pCO2 at ~25 and ~29 °C in two experiments conducted in March 2010 and March 2012. Larvae were sampled from four consecutive lunar days (LD) synchronized with spawning following the new moon, incubated in treatments for 24 h, and measured for respiration, maximum photochemical efficiency of PSII (F v/F m), and mortality. The most striking outcome was a strong effect of time (i.e., LD) on larvae performance: respiration was affected by an LD × temperature interaction in 2010 and 2012, as well as an LD × pCO2 × temperature interaction in 2012; F v/F m was affected by LD in 2010 (but not 2012); and mortality was affected by an LD × pCO2 interaction in 2010, and an LD × temperature interaction in 2012. There were no main effects of pCO2 in 2010, but in 2012, high pCO2 depressed metabolic rate and reduced mortality. Therefore, differences in larval performance depended on day of release and resulted in varying susceptibility to future predicted environmental conditions. These results underscore the importance of considering larval brood variation across days when designing experiments. Subtle differences in experimental outcomes between years suggest that transgenerational plasticity in combination with unique histories of exposure to physical conditions can modulate the response of brooded coral larvae to climate change and ocean acidification.

Offspring sensitivity to ocean acidification changes seasonally in a coastal marine fish

Experimental assessments of species vulnerabilities to ocean acidification are rapidly increasing in number, yet the potential for short- and long-term adaptation to high CO2 by contemporary marine organisms remains poorly understood. We used a novel experimental approach that combined bi-weekly sampling of a wild, spawning fish population (Atlantic silverside Menidia menidia) with standardized offspring CO2 exposure experiments and parallel pH monitoring of a coastal ecosystem. We assessed whether offspring produced at different times of the spawning season (April to July) would be similarly susceptible to elevated (1100 µatm, pHNIST = 7.77) and high CO2 levels (2300 µatm, pHNIST = 7.47). Early in the season (April), high CO2 levels significantly (p < 0.05) reduced fish survival by 54% (2012) and 33% (2013) and reduced 1 to 10 d post-hatch growth by 17% relative to ambient conditions. However, offspring from parents collected later in the season became increasingly CO2-tolerant until, by mid-May, offspring survival was equally high at all CO2 levels. This interannually consistent plasticity coincided with the rapid annual pH decline in the species' spawning habitat (mean pH: 1 April/31 May = 8.05/7.67). It suggests that parents can condition their offspring to seasonally acidifying environments, either via changes in maternal provisioning and/or epigenetic transgenerational plasticity (TGP). TGP to increasing CO2 has been shown in the laboratory but never before in a wild population. Our novel findings of direct CO2-related survival reductions in wild fish offspring and seasonally plastic responses imply that realistic assessments of species CO2-sensitivities must control for parental environments that are seasonally variable in coastal habitats.

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.

Uranium in larval shells as a barometer of molluscan ocean acidification exposure

As the ocean undergoes acidification, marine organisms will become increasingly exposed to reduced pH, yet variability in many coastal settings complicates our ability to accurately estimate pH exposure for those organisms that are difficult to track. Here we present shell-based geochemical proxies that reflect pH exposure from laboratory and field settings in larvae of the mussels Mytilus californianus and M. galloprovincialis. Laboratory-based proxies were generated from shells precipitated at pH 7.51 to 8.04. U/Ca, Sr/Ca, and multielemental signatures represented as principal components varied with pH for both species. Of these, U/Ca was the best predictor of pH and did not vary with larval size, with semidiurnal pH fluctuations, or with oxygen concentration. Field applications of U/Ca were tested with mussel larvae reared in situ at both known and unknown pH conditions. Larval shells precipitated in a region of greater upwelling had higher U/Ca, and these U/Ca values corresponded well with the laboratory-derived U/Ca-pH proxy. Retention of the larval shell after settlement in molluscs allows use of this geochemical proxy to assess ocean acidification effects on marine populations.

Experiment: Marine fungi may benefit from ocean acidification

Recent studies have discussed the consequences of ocean acidification for bacterial processes and diversity. However, the decomposition of complex substrates in marine environments, a key part of the flow of energy in ecosystems, is largely mediated by marine fungi. Although marine fungi have frequently been reported to prefer low pH levels, this group has been neglected in ocean acidification research. We present the first investigation of direct pH effects on marine fungal abundance and community structure. In microcosm experiments repeated in 2 consecutive years, we incubated natural North Sea water for 4 wk at in situ seawater pH (8.10 and 8.26), pH 7.82 and pH 7.67. Fungal abundance was determined by colony forming unit (cfu) counts, and fungal community structure was investigated by the culture-independent fingerprint method Fungal Automated Ribosomal Intergenic Spacer Analysis (F-ARISA). Furthermore, pH at the study site was determined over a yearly cycle. Fungal cfu were on average 9 times higher at pH 7.82 and 34 times higher at pH 7.67 compared to in situ seawater pH, and we observed fungal community shifts predominantly at pH 7.67. Currently, surface seawater pH at Helgoland Roads remains >8.0 throughout the year; thus we cannot exclude that fungal responses may differ in regions regularly experiencing lower pH values. However, our results suggest that under realistic levels of ocean acidification, marine fungi will reach greater importance in marine biogeochemical cycles. The rise of this group of organisms will affect a variety of biotic interactions in the sea.

Larval development of the barnacle Amphibalanus improvisus responds variably but robustly to near-future ocean acidification

Increasing atmospheric CO2 decreases seawater pH in a phenomenon known as ocean acidification. In two separate experiments we found that larval development of the barnacle Amphibalanus (Balanus) improvisus was not significantly affected by the level of reduced pH that has been projected for the next 150 years. After 3 and 6 days of incubation, we found no consistent effects of reduced pH on developmental speed or larval size at pH 7.8 compared with the control pH of 8.1. After 10 days of incubation, there were no net changes in survival or overall development of larvae raised at pH 7.8 or 7.6 compared with the control pH of 8.0. In all cases, however, there was significant variation in responses between replicate batches (parental genotypes) of larvae, with some batches responding positively to reduced pH. Our results suggest that the non-calcifying larval stages of A. improvisus are generally tolerant to near-future levels of ocean acidification. This result is in line with findings for other barnacle species and suggests that barnacles do not show the greater sensitivity to ocean acidification in early life history reported for other invertebrate species. Substantial genetic variability in response to low pH may confer adaptive benefits under future ocean acidification.

Elevated CO2 affects embryonic development and larval phototaxis in a temperate marine fish

As an effect of anthropogenic CO2 emissions, the chemistry of the world's oceans is changing. Understanding how this will affect marine organisms and ecosystems are critical in predicting the impacts of this ongoing ocean acidification. Work on coral reef fishes has revealed dramatic effects of elevated oceanic CO2 on sensory responses and behavior. Such effects may be widespread but have almost exclusively been tested on tropical reef fishes. Here we test the effects elevated CO2 has on the reproduction and early life history stages of a temperate coastal goby with paternal care by allowing goby pairs to reproduce naturally in an aquarium with either elevated (ca 1400 µatm) CO2 or control seawater (ca 370 µatm CO2). Elevated CO2 did not affect the occurrence of spawning nor clutch size, but increased embryonic abnormalities and egg loss. Moreover, we found that elevated CO2 significantly affected the phototactic response of newly hatched larvae. Phototaxis is a vision-related fundamental behavior of many marine fishes, but has never before been tested in the context of ocean acidification. Our findings suggest that ocean acidification affects embryonic development and sensory responses in temperate fishes, with potentially important implications for fish recruitment.

Response of a natural phytoplankton community from the Qingdao coast (Yellow Sea, China) to variable CO2 levels over a short-term incubation experiment

Since marine phytoplankton play a vital role in stabilizing earth's climate by removing significant amount of atmospheric CO2, their responses to increasing CO2 levels are indeed vital to address. The responses of a natural phytoplankton community from the Qingdao coast (NW Yellow Sea, China) was studied under different CO2 levels in microcosms. HPLC pigment analysis revealed the presence of diatoms as a dominant microalgal group; however, members of chlorophytes, prasinophytes, cryptophytes and cyanophytes were also present. delta 13CPOM values indicated that the phytoplankton community probably utilized bicarbonate ions as dissolved inorganic carbon source through a carbon concentration mechanism (CCM) under low CO2 levels, and diffusive CO2 uptake increased upon the increase of external CO2 levels. Although, considerable increase in phytoplankton biomass was noticed in all CO2 treatments, CO2-induced effects were absent. Higher net nitrogen uptake under low CO2 levels could be related to the synthesis of CCM components. Flow cytometry analysis showed slight reduction in the abundance of Synechococcus and pico-eukaryotes under the high CO2 treatments. Diatoms did not show any negative impact in response to increasing CO2 levels; however, chlorophytes revealed a reverse tend. Heterotrophic bacterial count enhanced with increasing CO2 levels and indicated higher abundance of labile organic carbon. Thus, the present study indicates that any change in dissolved CO2 concentrations in this area may affect phytoplankton physiology and community structure and needs further long-term study.

Microsensor studies on Padina from a natural CO2 seep: implications of morphology on acclimation to low pH

Low seawater pH can be harmful to many calcifying marine organisms, but the calcifying macroalgae Padina spp. flourish at natural submarine carbon dioxide seeps where seawater pH is low. We show that the microenvironment created by the rolled thallus margin of Padina australis facilitates supersaturation of CaCO3 and calcifi-cation via photosynthesis-induced elevated pH. Using microsensors to investigate oxygen and pH dynamics in the microenvironment of P. australis at a shallow CO2 seep, we found that, under saturating light, the pH inside the microenvironment (pHME) was higher than the external seawater (pHSW) at all pHSW levels investigated, and the difference (i.e., pHME-pHSW) increased with decreasing pHSW (0.9 units at pHSW 7.0). Gross photosynthesis (Pg) inside the microenvironment increased with decreasing pHSW, but algae from the control site reached a threshold at pH 6.5. Seep algae showed no pH threshold with respect to Pg within the pHSW range investigated. The external carbonic anhydrase (CA) inhibitor, acetazolamide, strongly inhibited Pg of P. australis at pHSW 8.2, but the effect was diminished under low pHSW (6.4-7.5), suggesting a greater dependence on membrane-bound CA for the dehydration of HCO3- ions during dissolved inorganic carbon uptake at the higher pHSW. In comparison, a calcifying green alga, Halimeda cuneata f. digitata, was not inhibited by AZ, suggesting efficient bicarbonate transport. The ability of P. australis to elevate pHME at the site of calcification and its strong dependence on CA may explain why it can thrive at low pHSW.

Microenvironmental changes support evidence of photosynthesis and calcification inhibition in Halimeda under ocean acidification and warming

The effects of elevated CO2 and temperature on photosynthesis and calcification of two important calcifying reef algae (Halimeda macroloba and Halimeda cylindracea) were investigated with O2 microsensors and chlorophyll a fluorometry through a combination of two pCO2 (400 and 1,200 µatm) and two temperature treatments (28 and 32 °C) equivalent to the present and predicted conditions during the 2100 austral summer. Combined exposure to pCO2 and elevated temperature impaired calcification and photosynthesis in the two Halimeda species due to changes in the microenvironment around the algal segments and a reduction in physiological performance. There were no significant changes in controls over the 5-week experiment, but there was a 50-70 % decrease in photochemical efficiency (maximum quantum yield), a 70-80 % decrease in O2 production and a threefold reduction in calcification rate in the elevated CO2 and high temperature treatment. Calcification in these species is closely coupled with photosynthesis, such that a decrease in photosynthetic efficiency leads to a decrease in calcification. Although pH seems to be the main factor affecting Halimeda species, heat stress also has an impact on their photosystem II photochemical efficiency. There was a strong combined effect of elevated CO2 and temperature in both species, where exposure to elevated CO2 or temperature alone decreased photosynthesis and calcification, but exposure to both elevated CO2 and temperature caused a greater decline in photosynthesis and calcification than in each stress individually. Our study shows that ocean acidification and ocean warming are drivers of calcification and photosynthesis inhibition in Halimeda. Predicted climate change scenarios for 2100 would therefore severely affect the fitness of Halimeda, which can result in a strongly reduced production of carbonate sediments on coral reefs under such changed climate conditions.

Calcification of the Arctic coralline red algae Lithothamnion glaciale in response to elevated CO2

Rising atmospheric CO2 concentrations could cause a calcium carbonate subsaturation of Arctic surface waters in the next 20 yr, making these waters corrosive for calcareous organisms. It is presently unknown what effects this will have on Arctic calcifying organisms and the ecosystems of which they are integral components. So far, acidification effects on crustose coralline red algae (CCA) have only been studied in tropical and Mediterranean species. In this work, we investigated calcification rates of the CCA Lithothamnion glaciale collected in northwest Svalbard in laboratory experiments under future atmospheric CO2 concentrations. The algae were exposed to simulated Arctic summer and winter light conditions in 2 separate experiments at optimum growth temperatures. We found a significant negative effect of increased CO2 levels on the net calcification rates of L. glaciale in both experiments. Annual mean net dissolution of L. glaciale was estimated to start at an aragonite saturation state between 1.1 and 0.9 which is projected to occur in parts of the Arctic surface ocean between 2030 and 2050 if emissions follow 'business as usual' scenarios (SRES A2; IPCC 2007). The massive skeleton of CCA, which consist of more than 80% calcium carbonate, is considered crucial to withstanding natural stresses such as water movement, overgrowth or grazing. The observed strong negative response of this Arctic CCA to increased CO2 levels suggests severe threats of the projected ocean acidification for an important habitat provider in the Arctic coastal ocean.

Experiment: Dissecting the impact of CO2 and pH on the mechanisms of photosynthesis and calcification in the coccolithophore Emiliania huxleyi

Coccolithophores are important calcifying phytoplankton predicted to be impacted by changes in ocean carbonate chemistry caused by the absorption of anthropogenic CO2. However, it is difficult to disentangle the effects of the simultaneously changing carbonate system parameters (CO2, bicarbonate, carbonate and protons) on the physiological responses to elevated CO2. Here, we adopted a multifactorial approach at constant pH or CO2 whilst varying dissolved inorganic carbon (DIC) to determine physiological and transcriptional responses to individual carbonate system parameters. We show that Emiliania huxleyi is sensitive to low CO2 (growth and photosynthesis) and low bicarbonate (calcification) as well as low pH beyond a limited tolerance range, but is much less sensitive to elevated CO2 and bicarbonate. Multiple up-regulated genes at low DIC bear the hallmarks of a carbon-concentrating mechanism (CCM) that is responsive to CO2 and bicarbonate but not to pH. Emiliania huxleyi appears to have evolved mechanisms to respond to limiting rather than elevated CO2. Calcification does not function as a CCM, but is inhibited at low DIC to allow the redistribution of DIC from calcification to photosynthesis. The presented data provides a significant step in understanding how E. huxleyi will respond to changing carbonate chemistry at a cellular level