Enrichment of Extracellular Carbonic Anhydrase in the Sea Surface Microlayer and Its Effect on Air‐Sea CO₂ Exchange

Abstract

This paper describes the quantification of extracellular carbonic anhydrase (eCA) concentrations in the sea surface microlayer (SML), the boundary layer between the ocean and the atmosphere of the Indo-West Pacific. We demonstrated that the SML is enriched with eCA by 1.5 ± 0.7 compared to the mixed underlying water. Enrichment remains up to a wind speed of 7 m s À1 (i.e., under typical oceanic conditions). As eCA catalyzes the interconversion of HCO 3 À and CO 2 , it has been hypothesized that its enrichment in the SML enhances the air-sea CO 2 exchange. We detected concentrations in the range of 0.12 to 0.76 nM, which can enhance the exchange by up to 15% based on the model approach described in the literature.

The carbonic anhydrase activity of sinking and suspended particles in the North Pacific Ocean

“…Two recent studies reported total CA concentrations—not activities—on the order of 0.2 nM eCA in the surface waters of the Mediterranean (Mustaffa et al ) and Equatorial Pacific (Mustaffa et al ). These concentrations are > 1000 times higher than the effective BCA concentrations we measured in the subpolar gyre (~ 0.15 pM, Fig.…”

“…Using the 100× scaling factor described above based on CA/Chl a ratios, our measured CA activity would represent 10–40% of the Zn quota. A scaling factor of 1000×, based on the data of Mustaffa et al (), would imply that CA is responsible for 100–400% of the Zn quota, which is unreasonable given the many other biological demands for Zn (Sunda ; Twining and Baines ). Therefore, we suggest that CA may account for up to 10–40% of the Zn quota in marine plankton assemblages, although this estimate is still speculative.…”

“…Marine diatoms, model organisms for studying the role of CA in photosynthesis, are known to express several extracellular CA (eCA) isoforms (Samukawa et al ), and eCA has now been identified in the surface ocean (Mustaffa et al ). At the sea surface, CA activities have been shown to scale with seawater p CO 2 and chlorophyll a (Chl a ) concentration (Tortell et al ).…”

“…Most CA isoforms require either Zn or Cd in their active site, implicating the cycling of bioactive trace metals such as Zn and Cd in the expression of CA (Christianson and Fierke ; Xu et al ; Twining and Baines ). Despite a few measurements of CA in the surface ocean (Tortell et al ; Mustaffa et al , b ), little is known about the production of marine CA, its fate in material sinking out of the surface ocean, or if it is active in other marine environments such as on particles or in ocean sediments.…”

“…This transport mechanism relies on CA activity in the cellular diffusive boundary layer surrounding diatoms (Martin and Tortell 2008;Hopkinson et al 2011Hopkinson et al , 2013, and within their cytoplasm and chloroplasts (Hopkinson et al 2011). Marine diatoms, model organisms for studying the role of CA in photosynthesis, are known to express several extracellular CA (eCA) isoforms (Samukawa et al 2014), and eCA has now been identified in the surface ocean (Mustaffa et al 2017a). At the sea *Correspondence: asubhas@whoi.edu surface, CA activities have been shown to scale with seawater pCO 2 and chlorophyll a (Chl a) concentration (Tortell et al 2006).…”