The nature of the CO₂‐concentrating mechanisms in a marine diatom, Thalassiosira pseudonana

Abstract

SummaryDiatoms are widespread in aquatic ecosystems where they may be limited by the supply of inorganic carbon. Their carbon dioxide-concentrating mechanisms (CCMs) involving transporters and carbonic anhydrases (CAs) are well known, but the contribution of a biochemical CCM involving C 4 metabolism is contentious.The CCM(s) present in the marine-centric diatom, Thalassiosira pseudonana, were studied in cells exposed to high or low concentrations of CO 2 , using a range of approaches.At low CO 2 , cells possessed a CCM based on active uptake of CO 2 (70% contribution) and bicarbonate, while at high CO 2 , cells were restricted to CO 2 . CA was highly and rapidly activated on transfer to low CO 2 and played a key role because inhibition of external CA produced uptake kinetics similar to cells grown at high CO 2 . The activities of phosphoenolpyruvate (PEP) carboxylase (PEPC) and the PEP-regenerating enzyme, pyruvate phosphate dikinase (PPDK), were lower in cells grown at low than at high CO 2 . The ratios of PEPC and PPDK to ribulose bisphosphate carboxylase were substantially lower than 1, even at low CO 2 .Our data suggest that the kinetic properties of this species results from a biophysical CCM and not from C 4 type metabolism.

Carbonic Anhydrases Function in Anther Cell Differentiation Downstream of the Receptor-Like Kinase EMS1

“…For example, the membrane-localized CA IX, an established marker for tumor hypoxia and aggressive cancers, plays a critical role in tumor cell proliferation by regulating pH (Alterio et al, 2009;Swietach et al, 2009). Besides their photosynthetic functions (Hatch and Burnell, 1990;Badger and Price, 1994;Smith and Griffiths, 2000;Studer et al, 2014;Clement et al, 2016;Ho et al, 2016;Ludwig, 2016), recent studies found that bCAs are critical for CO 2 -induced stomatal development and movement (Hu et al, 2010;Engineer et al, 2014;Hu et al, 2015;Matrosova et al, 2015;Wang et al, 2016a) as well as for amino acid biosynthesis (DiMario et al, 2016).…”

Metatranscriptome analysis reveals environmental and diel regulation of a Heterosigma akashiwo (raphidophyceae) bloom

“…Carbon dioxide-concentrating mechanism (CCM) in H. akashiwo Many groups of phytoplankton possess CCMs to elevate CO 2 concentrations around Rubisco and overcome CO 2limitation in their habitats (Clement et al, 2016). CA exists in five forms in phytoplankton, three common (a, b and g) and two unusual (d and f), and play critical roles in CCMs.…”

John A. Raven

“…Clements et al . () conclude that, under their conditions, C 4 biochemistry made a negligible contribution to photosynthesis. While these papers make very significant advances, further investigation is needed.…”

“…Finally Clements et al . () addressed the vexed question of the acquisition and assimilation of inorganic carbon by the diatom Thalassiosira pseudonana , and particularly the possible contribution of C 4 photosynthetic biochemistry. Clements et al .…”