Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

Ocean acidification (OA) and its associated decline in calcium carbonate saturation states is one of the major threats that tropical coral reefs face this century. Previous studies of the effect of OA on coral reef calcifiers have described a wide variety of outcomes for studies using comparable partial pressure of CO2 (pCO2) ranges, suggesting that key questions remain unresolved. One unresolved hypothesis posits that heterogeneity in the response of reef calcifiers to high pCO2 is a result of regional-scale variation in the responses to OA. To test this hypothesis, we incubated two coral taxa (Pocillopora damicornis and massive Porites) and two calcified algae (Porolithon onkodes and Halimeda macroloba) under 400, 700 and 1000 µatm pCO2 levels in experiments in Moorea (French Polynesia), Hawaii (USA) and Okinawa (Japan), where environmental conditions differ. Both corals and H. macroloba were insensitive to OA at all three locations, while the effects of OA on P. onkodes were location-specific. In Moorea and Hawaii, calcification of P. onkodes was depressed by high pCO2, but for specimens in Okinawa, there was no effect of OA. Using a study of large geographical scale, we show that resistance to OA of some reef species is a constitutive character expressed across the Pacific.

Effects of feeding and light intensity on the response of the coral Porites rus to ocean acidification

Recently, it has been suggested that there are conditions under which some coral species appear to be resistant to the effects of ocean acidification. To test if such resistance can be explained by environmental factors such as light and food availability, the present study investigated the effect of 3 feeding regimes crossed with 2 light levels on the response of the coral Porites rus to 2 levels of pCO2 at 28 °C. After 1, 2, and 3 weeks of incubation under experimental conditions, none of the factors-including pCO2-significantly affected area-normalized calcification and biomass-normalized calcification. Biomass also was unaffected during the first 2 weeks, but after 3 weeks, corals that were fed had more biomass per unit area than starved corals. These results suggest that P. rus is resistant to short-term exposure to high pCO2, regardless of food availability and light intensity. P. rus might therefore represent a model system for exploring the genetic basis of tolerance to OA.

Seawater carbonate chemistry and carbon allocation, growth and morphology of the coccolithophore Emiliania huxleyi (calcifying strain CCMP 371) during experiments, 2011

Coccolithophores are unicellular phytoplankton that produce calcium carbonate coccoliths as an exoskeleton. Emiliania huxleyi, the most abundant coccolithophore in the world's ocean, plays a major role in the global carbon cycle by regulating the exchange of CO2 across the ocean-atmosphere interface through photosynthesis and calcium carbonate precipitation. As CO2 concentration is rising in the atmosphere, the ocean is acidifying and ammonium (NH4) concentration of future ocean water is expected to rise. The latter is attributed to increasing anthropogenic nitrogen (N) deposition, increasing rates of cyanobacterial N2 fixation due to warmer and more stratified oceans, and decreased rates of nitrification due to ocean acidification. Thus future global climate change will cause oceanic phytoplankton to experience changes in multiple environmental parameters including CO2, pH, temperature and nitrogen source. This study reports on the combined effect of elevated pCO2 and increased NH4 to nitrate (NO3) ratio (NH4/NO3) on E. huxleyi, maintained in continuous cultures for more than 200 generations under two pCO2 levels and two different N sources. Here we show that NH4 assimilation under N-replete conditions depresses calcification at both low and high pCO2, alters coccolith morphology, and increases primary production. We observed that N source and pCO2 synergistically drive growth rates, cell size and the ratio of inorganic to organic carbon. These responses to N source suggest that, compared to increasing CO2 alone, a greater disruption of the organic carbon pump could be expected in response to the combined effect of increased NH4/NO3 ratio and CO2 level in the future acidified ocean. Additional experiments conducted under lower nutrient conditions are needed prior to extrapolating our findings to the global oceans. Nonetheless, our results emphasize the need to assess combined effects of multiple environmental parameters on phytoplankton biology in order to develop accurate predictions of phytoplankton responses to ocean acidification.