The role of external carbonic anhydrase in photosynthesis during growth of the marine diatom Chaetoceros muelleri

Abstract

Carbon dioxide concentrating mechanisms (CCMs) act to improve the supply of CO₂ at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase. There is substantial evidence that in some microalgal species CCMs involve an external carbonic anhydrase (CA_ext ) and that CA_ext activity is induced by low CO₂ concentrations in the growth medium. However, much of this work has been conducted on cells adapted to air-equilibrium concentrations of CO₂ , rather than to changing CO₂ conditions caused by growing microalgal populations. We investigated the role of CA_ext in inorganic carbon (C_i ) acquisition and photosynthesis at three sampling points during the growth cycle of the cosmopolitan marine diatom Chaetoceros muelleri. We observed that CA_ext activity increased with decreasing C_i , particularly CO₂ , concentration, supporting the idea that CA_ext is modulated by external CO₂ concentration. Additionally, we found that the contribution of CA_ext activity to carbon acquisition for photosynthesis varies over time, increasing between the first and second sampling points before decreasing at the last sampling point, where external pH was high. Lastly, decreases in maximum quantum yield of photosystem II (F_v /F_m ), chlorophyll, maximum relative electron transport rate, light harvesting efficiency (α) and maximum rates of C_i - saturated photosynthesis (V_max ) were observed over time. Despite this decrease in photosynthetic capacity an up-regulation of CCM activity, indicated by a decreasing half-saturation constant for CO₂ (K_0.5 CO₂ ), occurred over time. The flexibility of the CCM during the course of growth in C. muelleri may contribute to the reported dominance and persistence of this species in phytoplankton blooms.

Keywords: Chaetoceros muelleri; carbon dioxide-concentrating mechanisms; diatoms; external carbonic anhydrase; photosynthetic O2 evolution.