Abstract
Abstract. Marine calcification is an important component of the global carbon cycle. The mechanism by which some organisms take up inorganic carbon for the production of their shells or skeletons, however, remains only partly known. Although foraminifera are responsible for a large part of the global calcium carbonate production, the process by which they concentrate inorganic carbon is debated. Some evidence suggests that seawater is taken up by vacuolization and participates relatively unaltered in the process of calcification, whereas other results suggest the involvement of transmembrane transport and the activity of enzymes like carbonic anhydrase. Here, we tested whether inorganic-carbon uptake relies on the activity of carbonic anhydrase using incubation experiments with the perforate, large benthic, symbiont-bearing foraminifer Amphistegina lessonii. Calcification rates, determined by the alkalinity anomaly method, showed that inhibition of carbonic anhydrase by acetazolamide (AZ) stopped most of the calcification process. Inhibition of photosynthesis either by 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU) or by incubating the foraminifera in the dark also decreased calcification rates but to a lesser degree than with AZ. Results from this study show that carbonic anhydrase plays a key role in biomineralization of Amphistegina lessonii and indicates that calcification of those perforate, large benthic foraminifera might, to a certain extent, benefit from the extra dissolved inorganic carbon (DIC), which causes ocean acidification.
Highlights
Fossil fuel burning and land use changes have been steadily increasing atmospheric CO2 levels
total alkalinity (TA) decreased on average by 53 μmol kg−1 and dissolved inorganic carbon (DIC) by 38 μmol L−1 during the incubation (Table 1). This corresponds to 2.74 g L−1 of precipitated calcite
When comparing the changes in TA and DIC between treatments, calcification is minimized by the AZ, and net respiration slightly increases (Fig. 3)
Summary
Fossil fuel burning and land use changes have been steadily increasing atmospheric CO2 levels. About one-third of the added carbon has been taken up by the ocean (Sabine and Tanhua, 2010), and the resulting increase in seawater dissolved carbon dioxide and associated acidification are lowering the saturation state of seawater with respect to calcite and likely affect marine calcifiers. Foraminifera are responsible for almost 25 % of the total marine calcium carbonate production (Langer, 2008), and their response to ongoing acidification is important to predict future marine inorganic-carbon cycling. Despite its relevance for future CO2 scenarios, it is still unclear how increased pCO2 in seawater will affect foraminiferal calcification. Previous research has shown discrepancies in their results: in some cases higher pCO2 increased the growth rate of benthic foraminifera, while in other cases calcification decreased or halted (Haynert et al, 2014; Hikami et al, 2011)
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