Carbon dioxide‐concentrating mechanism and the development of extracellular carbonic anhydrase in the marine picoeukaryote Micromonas pusilla

Abstract

A range of marine photosynthetic picoeukaryote phytoplankton species grown in culture were screened for the presence of extracellular carbonic anhydrase (CA ext ), a key enzyme in inorganic carbon acquisition under carbonlimiting conditions in some larger marine phytoplankton species. Of the species tested, extracellular carbonic anhydrase was detected only in Micromonas pusilla Butcher. The rapid, light-dependent development of CA ext when cells were transferred from carbon-replete to carbon-limiting conditions was regulated by the available free-CO # concentration and not by total dissolved inorganic carbon. Kinetic studies provided support for a CO # -concentrating mechanism in that the K ! n & [CO # ] (i.e. the CO # concentration required for the half-maximal rate of photosynthesis) was substantially lower than the K m [CO # ] of Rubisco from related taxa, whilst the intracellular carbon pool was at least seven fold greater than the extracellular DIC concentration, for extracellular DIC values 1n0 m. It is proposed that when the flux of CO # into the cell is insufficient to support the photosynthetic rate at an optimum photon irradiance, the development of CA ext increases the availability of CO # at the plasma membrane. This ensures rapid acclimation to environmental change and provides an explanation for the central role of M. pusilla as a carbon sink in oligotrophic environments.Key words : Dissolved inorganic carbon, CO # -concentrating mechanism, extracellular carbonic anhydrase, Micromonas pusilla Butcher. Marine picoplankton are essential components of food webs in the oceans, particularly in oligotrophic environments, where photosynthetic picoplankton are responsible for as much as 50-80 % of the total carbon production (Li et al., 1983 ;Platt, Rao & Irwin, 1983 ; Fogg, 1986 ;Stockner & Antia, 1986). Micromonas pusilla is of typical picoeukaryotic size (0n8-1n0i1n0-2n0 µm), has a single flagellum and occurs in tropical, temperate and Arctic waters (Johnson & Sieburth, 1982 ;Raven, 1986 ; Lee, 1992). Although a major component of the eukaryotic plankton biomass, the mode of photosynthetic carbon utilization in the eukaryotic picoplankton species including M. pusilla has received little * To whom correspondence should be addressed at (present address) : School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK. E-mail : M.D.Iglesias-Rodriguez!bristol.ac.uk attention. The high affinity for inorganic carbon of the majority of marine phytoplankton species suggests the occurrence of a CO # -concentrating mechanism (CCM) in order to increase the CO # concentration at the site of Rubisco (Raven, 1991). This CO # accumulation might be a result of uptake of CO # from the bulk medium, uptake of bicarbonate with subsequent conversion of bicarbonate to CO # , or CO # entry after the catalytic conversion of bicarbonate to CO # outside the plasma membrane via CA ext . The relationship between size of organism and the need for a CCM suggested a decreasing requireme...

Extra and intracelular activities of carbonic anhydrase of the marine microalga Tetraselmis gracilis (Chlorophyta)

“…1b), whereas extracellular activity increased as a means of uptaking the bicarbonate ion, which became the major source of inorganic carbon as a result of alkalization. Similar profiles of extracellular CA activity along ten days of laboratory cultivation were observed for the microalgae Micromonas pusilla and Prorocentrum minimum (15,23,24).…”

The Arctic picoeukaryote <i>Micromonas pusilla</i> benefits from Ocean Acidification under constant and dynamic light

“…In the current study, however, there was no observed increase in POC production (Fig. 2) under higher pCO 2 levels, which could 305 be expected assuming lowered costs due to CCM down-regulation (Iglesias-Rodriguez et al, 1998;Rost et al, 2008). The observed increase in growth rates nonetheless indicates beneficial OA effects, potentially due to re-allocation of energy liberated by eased carbon acquisition.…”

Carbon Concentrating Mechanisms in Eukaryotic Marine Phytoplankton

“…CCMs were described for even tiny picoeukaryotic marine phytoplankton (<2 μm) in which the diffusion of CO 2 is not expected to limit photosynthesis (Iglesias-Rodriguez et al 1998, Giordano et al 2005. A critical finding in this research was that phytoplankton leak substantial CO 2 and DIC to seawater in a light-dependent manner (Tchernov et al 1998(Tchernov et al , 2003, confirming that the operation of CCMs concentrates DIC inside cells in excess of seawater concentrations and that CCM activity is an active, energy-consuming process.…”