Molecular ecology and transcriptomics of marine photosynthetic picoeukaryotes

Photosynthetic picoeukaroytes (PPEs), defined here as single celled organisms <3 μm in diameter, are significant contributors to primary production. Until recently, marine PPEs had received relatively little research attention in contrast to the more numerous picocyanobacteria. Molecular studies have now started to reveal the diversity of this group, using both the nuclear 18S rRNA gene and the plastidtargeted 16S rRNA gene as taxonomic markers. The latter marker has the advantage of directly targeting the PPE community, counteracting the problem of heterotrophic sequences dominating clone libraries. As well as PCR based molecular approaches, genomic studies of PPEs are starting to reveal the metabolic capabilities of these organisms. In this thesis, taxonomic information obtained on two flow-sorted PPE populations (Euk-A and Euk-B) showed that pico-prymnesiophytes, largely representing lineages with no close cultured counterpart, dominated the Euk-A and Euk-B libraries (54 and 58%, respectively) in tropical and sub-tropical waters of the Atlantic Ocean. Radiotracer work performed elsewhere had shown these PPE groups contribute up to 19% and 38% (Euk-A and Euk-B, respectively) to total CO2 fixation, demonstrating the importance of these PPE groups in marine carbon cycling. To further assess the taxonomic composition and distribution of these Euk-A and Euk-B PPE populations at the ocean-basin scale, clone libraries were constructed along an Atlantic Meridional Transect (AMT18). Major components of these flow cytometry sorted PPE populations were Prymnesiophyceae and Chrysophyceae using plastid markers, or Prasinophyceae and Dinophyceae (nuclear markers) including several lineages with no cultured counterparts. In surface waters a latitudinal diversity gradient was observed with a peak in PPE diversity found in the equatorial region. Distribution patterns of specific PPE groups and OTUs were subsequently correlated with measured environmental parameters, although most of the variation in PPE diversity was not explained by the measured variables. Attempts were undertaken to obtain into culture novel PPEs, especially those representative of oligotropic regions. However, the majority of isolates obtained were related to Prasinoderma or Chlorella which are cosmopolitan, fast-growing genera. Even so, some isolates more relevant of open ocean environments were obtained, including a clade VIIA prasinophyte and a Pelagomonas sp. Trancriptomics was used to further assess the functional potential of specific PPE populations, firstly in cultures using both an Ochromonas sp. and a prasinophyte as being representative of organisms present along AMT18. This approach revealed a C4 carbon concentrating mechanism in the clade VIIA prasinophyte and enzymes required for a functioning urea cycle in the Ochromonas sp. A pipeline was also developed to undertake a metatranscriptomic approach on a flow cytometrically sorted PPE population from the south Atlantic gyre. This approach revealed a diatom-like C4 carbon concentrating system in the metatranscriptome. Overall, this thesis has given new insights into the diversity of specific PPE groups at the ocean basin-scale, developed a new pipeline for the transcriptomic analysis of PPEs both in culture and in the environment, and in so doing has provided new information on the functional potential of these important photosynthetic organisms.