There is evidence that the CO2-concentrating mechanism in the marine diatom Thalassiosira weissflogii operates as a type of single-cell C 4 photosynthesis. In quantitative-PCR assays, we observed 2- to 4-fold up-regulation of two phosphoenolpyruvate carboxylase (PEPC) gene transcripts in Thalassiosira pseudonana cells adapted to low pCO2, but did not detect such regulation in Phaeodactylum tricornutum grown under similar conditions. Transcripts encoding phosphoenolpyruvate carboxykinase did not appear to be regulated by pCO2 in either diatom. In T. pseudonana and T. weissflogii, net CO2 fixation was blocked by 3, 3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (a specific inhibitor of PEPC), but was restored by about 50% and 80%, respectively, by addition of millimolar concentrations of KHCO3. In T. pseudonana, T. weissflogii, and P. tricornutum, rates of net O2 evolution were reduced by an average of 67%, 55%, and 62%, respectively, in the presence of 20 μM quercetin, also an inhibitor of PEPC. Quercetin promoted net CO2 leakage from inhibited cells to levels in excess of the equilibrium CO 2 concentration, suggesting that a fraction of the HCO 3- taken up is fated to leak back into the medium as CO2 when PEPC activity is blocked. In parallel to these experiments, in vitro assays on crude extracts of T. pseudonana demonstrated mean inhibition of 65% of PEPC activity by quercetin. In the presence of 5 mM 3-mercaptopicolinic acid (3-MPA), a classic inhibitor of phosphoenolpyruvate carboxykinase, photosynthetic O2 evolution was reduced by 90% in T. pseudonana. In T. weissflogii and P. tricornutum, 5 mM 3-MPA totally inhibited net CO2 fixation and O2 evolution. Neither quercetin nor 3-MPA had a significant inhibitory effect on photosynthetic O2 evolution or CO2 uptake in the marine chlorophyte isolates Chlamydomonas sp. or Dunaliella tertiolecta. Our evidence supports the idea that C4-based CO2-concentrating mechanisms are generally distributed in diatoms. This conclusion is discussed within the context of the evolutionary success of diatoms.
All Science Journal Classification (ASJC) codes
- Plant Science