Extracellular carbonic anhydrase: Method development and its enrichment in the sea surface microlayer

Carbonic anhydrase (CA) is a zinc-containing enzyme that catalyzed the slow interconversion between HCO₃^- and CO₂ at the cell surface. Due to the disequilibrium towards HCO₃^-, extracellular CA is an important pathway for cellular CO₂ acquisition used by phytoplankton. To date, studies of CA were only focused on culture solutions but not in natural seawater due to the insensitivity of available analytical techniques. Assessment of concentration levels in natural communities will provide new insights into the marine carbon cycle. For example, hypothetical enrichment of CA in the sea surface microlayer (SML), proposed 20 years ago, could possibly create sufficient CO₂ catalysis and thus influence the air-sea CO₂ exchange. In combination with a treatment to separate the extracellular CA from the cell membranes, we developed a new fluorometric technique to quantify the concentration of CA in seawater. We applied this technique to samples collected from the Baltic Sea including the SML and underlying water (ULW). The results show that the concentration of CA ranged from 0.54 to 1.48 nM (mean ± sd: 1.10 ± 0.32 nM) and 0.54 to 1.95 nM (1.02 ± 0.40 nM) in SML and ULW, respectively. The CA enrichment factor, obtained from the ratio of the concentration in the SML to that the ULW, ranged from 0.38 to 2.15 (1.19 ± 0.45).The concentration levels in the SML were not significant higher than in ULW, even though we observed occasionally enrichment factors of up to 2. However, the nanomolar levels of CA in the SML are likely sufficient to influence the exchange of CO₂ across the microlayer based on supplementary data from the literature. We discuss the enrichment and possible role of CA in the air-sea exchange of CO₂.