Abstract. Ocean acidification (decreases in carbonate ion concentration and pH) in response to rising atmospheric pCO 2 is generally expected to reduce rates of calcification by reef calcifying organisms, with potentially severe implications for coral reef ecosystems. Large, algal symbiontbearing benthic foraminifers, which are important primary and carbonate producers in coral reefs, produce high-Mg calcite shells, whose solubility can exceed that of aragonite produced by corals, making them the "first responder" in coral reefs to the decreasing carbonate saturation state of seawater. Here we report results of culture experiments performed to assess the effects of ongoing ocean acidification on the calcification of symbiont-bearing reef foraminifers using a high-precision pCO 2 control system. Living clone individuals of three foraminiferal species (Baculogypsina sphaerulata, Calcarina gaudichaudii, and Amphisorus hemprichii) were subjected to seawater at five pCO 2 levels from 260 to 970 µatm. Cultured individuals were maintained for about 12 weeks in an indoor flow-through system under constant water temperature, light intensity, and photoperiod. After the experiments, the shell diameter and weight of each cultured specimen were measured. Net calcification of B. sphaerulata and C. gaudichaudii, which secrete a hyaline shell and host diatom symbionts, increased under intermediate levels of pCO 2 (580 and/or 770 µatm) and decreased at a higher pCO 2 level (970 µatm). Net calcification of A. hemprichii, which secretes a porcelaneous shell and hosts dinoflagellate symbionts, tended to decrease at elevated pCO 2 . Observed different responses between hyaline and porcelaneous species are possibly caused by the relative importance of Correspondence to: K. Fujita (fujitaka@sci.u-ryukyu.ac.jp) elevated pCO 2 , which induces CO 2 fertilization effects by algal symbionts, versus associated changes in seawater carbonate chemistry, which decreases a carbonate concentration. Our findings suggest that ongoing ocean acidification might favor symbiont-bearing reef foraminifers with hyaline shells at intermediate pCO 2 levels (580 to 770 µatm) but be unfavorable to those with either hyaline or porcelaneous shells at higher pCO 2 levels (near 1000 µatm).

Ocean acidification, which like global warming is an outcome of anthropogenic CO2 emissions, severely impacts marine calcifying organisms, especially those living in coral reef ecosystems. However, knowledge about the responses of reef calcifiers to ocean acidification is quite limited, although coral responses are known to be generally negative. In a culture experiment with two algal symbiont‐bearing, reef‐dwelling foraminifers, Amphisorus kudakajimensis and Calcarina gaudichaudii, in seawater under five different pCO2 conditions, 245, 375, 588, 763 and 907 μatm, maintained with a precise pCO2‐controlling technique, net calcification of A. kudakajimensis was reduced under higher pCO2, whereas calcification of C. gaudichaudii generally increased with increased pCO2. In another culture experiment conducted in seawater in which bicarbonate ion concentrations were varied under a constant carbonate ion concentration, calcification was not significantly different between treatments in Amphisorus hemprichii, a species closely related to A. kudakajimensis, or in C. gaudichaudii. From these results, we concluded that carbonate ion and CO2 were the carbonate species that most affected growth of Amphisorus and Calcarina, respectively. The opposite responses of these two foraminifer genera probably reflect different sensitivities to these carbonate species, which may be due to their different symbiotic algae.

Ocean acidification (decreases in carbonate ion concentration and pH) in response to rising atmospheric <i>p</i>CO₂ is generally expected to reduce rates of calcification by reef calcifying organisms, with potentially severe implications for coral reef ecosystems. Large, algal symbiont-bearing benthic foraminifers, which are important primary and carbonate producers in coral reefs, produce high-Mg calcite shells, whose solubility can exceed that of aragonite produced by corals, making them the "first responder" in coral reefs to the decreasing carbonate saturation state of seawater. Here we report results of culture experiments performed to assess the effects of ongoing ocean acidification on the calcification of symbiont-bearing reef foraminifers using a high-precision <i>p</i>CO₂ control system. Living clone individuals of three foraminiferal species (<i>Baculogypsina sphaerulata</i>, <i>Calcarina gaudichaudii</i>, and <i>Amphisorus hemprichii</i>) were subjected to seawater at five <i>p</i>CO₂ levels from 260 to 970 μatm. Cultured individuals were maintained for about 12 weeks in an indoor flow-through system under constant water temperature, light intensity, and photoperiod. After the experiments, the shell diameter and weight of each cultured specimen were measured. Net calcification of <i>Baculogypsina</i> and <i>Calcarina</i>, which secrete a hyaline shell and host diatom symbionts, increased under intermediate levels of <i>p</i>CO₂ (580 and/or 770 μatm) and decreased at a higher <i>p</i>CO₂ level (970 μatm). Net calcification of <i>Amphisorus</i>, which secretes a porcelaneous shell and hosts dinoflagellate symbionts, tended to decrease at elevated <i>p</i>CO₂. These different responses among the three species are possibly due to differences in calcification mechanisms (in particular, the specific carbonate species used for calcification) between hyaline and porcelaneous taxa, and to links between calcification by the foraminiferal hosts and photosynthesis by the algal endosymbionts. Our findings suggest that ongoing ocean acidification might favor symbiont-bearing reef foraminifers with hyaline shells at intermediate <i>p</i>CO₂ levels (580 to 770 μatm) but be unfavorable to those with either hyaline or porcelaneous shells at higher <i>p</i>CO₂ levels (near 1000 μatm)

No abstract