The Role of Extracellular Carbonic Anhydrase in Biogeochemical Cycling: Recent Advances and Climate Change Responses.

Nur Ili Hamizah Mustaffa,Oliver Wurl,Mohd Talib Latif

doi:10.3390/ijms22147413

Abstract

Climate change has been predicted to influence the marine phytoplankton community and its carbon acquisition strategy. Extracellular carbonic anhydrase (eCA) is a zinc metalloenzyme that catalyses the relatively slow interconversion between HCO3− and CO2. Early results indicated that sub-nanomolar levels of eCA at the sea surface were sufficient to enhance the oceanic uptake rate of CO2 on a global scale by 15%, an addition of 0.37 Pg C year−1. Despite its central role in the marine carbon cycle, only in recent years have new analytical techniques allowed the first quantifications of eCA and its activity in the oceans. This opens up new research areas in the field of marine biogeochemistry and climate change. Light and suitable pH conditions, as well as growth stage, are crucial factors in eCA expression. Previous studies showed that phytoplankton eCA activity and concentrations are affected by environmental stressors such as ocean acidification and UV radiation as well as changing light conditions. For this reason, eCA is suggested as a biochemical indicator in biomonitoring programmes and could be used for future response prediction studies in changing oceans. This review aims to identify the current knowledge and gaps where new research efforts should be focused to better determine the potential feedback of phytoplankton via eCA in the marine carbon cycle in changing oceans.

Highlights

Extracellular carbonic anhydrase is a zinc metalloenzyme that accelerates the slow interconversion between bicarbonate ions (HCO3−) and carbon dioxide (CO2) to the equilibrium concentration at the cell surface [1]. eCA has been widely found in mammals [2], plants and phytoplankton [3], and prokaryotes [4]
Using a fluorescent technique [18], we found that the concentrations of eCA in natural seawater are in the nanomolar range (0.10 nM–0.76 nM) and enriched in the surface microlayer (SML) by a mean of 1.5 ± 0.7 compared to underlying water from 1-metre depth [97]
Our review highlighted the current knowledge and gaps in the knowledge about the role of eCA in the changing ocean. eCA activity and concentrations are affected by environmental stressors such as ocean acidification and UV radiation as well as changing light conditions

Summary

Introduction

Extracellular carbonic anhydrase (eCA) is a zinc metalloenzyme that accelerates the slow interconversion between bicarbonate ions (HCO3−) and carbon dioxide (CO2) to the equilibrium concentration at the cell surface [1]. eCA has been widely found in mammals [2], plants and phytoplankton [3], and prokaryotes [4]. There are seven CA gene classes that have been recognized in photosynthetic organisms, identified as α-, β-, Υ-, δ-, ζ-, θ- [3,5] as well as a recently discovered ι-CA gene class [6]. The δ-CA (TWCA1) [10,11] and ζ-CA (CDCA) [12] classes with the capability to bind with alternative metal cofactors as well as Zn2+, such as cobalt (Co2+) and cadmium (Cd2+), respectively, have been identified in the diatom Thalassiosira weisflogii (T. weisflogii). Jensen, et al [6] discovered a new ι-CA class in Thalassiosira pseudonona (T. pseudonona), which unusually prefers manganese (Mn2+) to Zn2+ as a cofactor. For additional information on the function, physiological relevance, and diverse CA expression in microalgae, we refer to a recent review [14]

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