Carbon dioxide concentrating mechanisms in marine diatoms: genetics, physiology, and diversity
MetadataShow full item record
CO2 concentrating mechanisms (CCMs) have been well studied in model cyanobacteria and in the model green eukaryotic alga, Chlamydomonas reinhardtii, while CCMs in marine diatoms are less clearly understood. Given the ecological significance of diatoms, thorough knowledge of diatom CCMs is desired to better understand its role in carbon assimilation and its expected response to future environmental change. In this dissertation, the genetics, physiology and diversity of diatom CCMs are explored using a range of approaches: genetic transformation techniques, Membrane Inlet Mass spectrometry (MIMS), and bioinformatics analyses. First, using genetic transformation techniques, a putative δ-carbonic anhydrase (CA) was localized to the chloroplast periplastidal compartment and three putative bicarbonate transporters were also successfully localized in the diatom Thalassiosira pseudonana, one to the plasma membrane and two to the chloroplast. Physiological assessments were performed to demonstrate the activity and role of an SLC4 (solute carrier 4) HCO3- transporter found in the plasma membrane of diatom Phaeodactylum tricornutum. The genetic and physiological characterization of CCM components conducted here helps create a more integrated description of CCMs in model diatoms. Secondly, the importance of extracellular carbonic anhydrase (eCA) in supporting CO2 uptake in centric diatoms was demonstrated by quantitative measurement of eCA activity using MIMS. Assessment of centric diatoms spanning a large size range showed that eCA activity increased with cell radius to support the greater demand of CO2 for photosynthesis in larger centric diatoms. Photosynthesis was reduced when eCA was inhibited but there was no overall relationship between the effect of eCA-inhibited photosynthesis and cell size. Thirdly, the diversity of marine diatom CCMs was evaluated based on the distribution of CCM components (CA and bicarbonate transporter) among 34 marine diatom strains whose genome or transcriptome sequences were available. Diatom CCM structure was quite diverse, driven primarily by extensive variation in α-CAs and δ-CAs, indicating radiation of these genes within diatom lineages. Comparing relationships among diatom CCMs (using hierarchical clustering) with diatom species phylogeny (based on an18S rDNA phylogenetic tree) suggests that CCM structure is somewhat congruent with diatom phylogeny.