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The Role of Nitrogen in Prasinophyte Population Dynamics, Physiology and Transcriptional Responses


Marine phytoplankton constitute the base of ocean food webs and are responsible for nearly half of the total annual carbon dioxide uptake on the planet acting as important players in biogeochemical cycles. Nitrogen is an essential nutrient that can limit phytoplankton growth. Eukaryotic phytoplankton within the prasinophytes are widespread in the ocean, found from polar to tropical regions under a wide range of environmental conditions, where nitrogen can be an essential nutrient limiting their growth. However, little is known about prasinophyte population dynamics, distributions, growth and abundance, as well as prasinophyte physiological strategies to cope with fluctuation in nitrogen availability in the ocean. Chapter 2 focuses primarily on phytoplankton population dynamics in offshore mesotrophic sites in the dynamic transition zone of the central California Current System. Based on hierarchical clustering of abiotic factors, two modalities were identified within the mesotrophic sites; each modality with distinct phytoplankton communities (determined by flow cytometry and high resolution 16S V1-V2 region rRNA gene amplicon sequencing) that appear to underlie the differences in phytoplankton growth and biomass production observed (determined by dilution experiments and 14C method). The highest biomass production (38.7 ± 8.1 µg C L-1 D-1 by 14C, 86% from photosynthetic eukaryotes) was found in a mesotrophic high modality (MHM) where nitrate was approximately 4-fold higher than in the MLM (mesotrophic low modality). The pico-prasinophyte Ostreococcus clade OI dominated the MHM, whereas in the MLM higher relative contributions of another prasinophyte (uncultured prasinophyte class VIII) and Pelagomonas (stramenopile) were found. Furthermore, the effects of nitrogen (N) depletion on growth, cellular parameters (Chapter 3) and whole-genome transcriptional responses (Chapter 4) of the pico-prasinophyte Micromonas, were investigated in laboratory experiments. This study showed common responses of three Micromonas species that come from three divergent and ecologically important clades. The main commonalities observed were that nitrogen depletion induced a pronounced decrease in growth rate compared to controls within 1 or 2 days and N-deplete treatments take longer to show signs of growth recovery than stationary phase control cells. One major difference was that M. pusilla under N-depletion exhibited a pronounced increase in mean FALS cell-1 (proxy of cell size). At a transcriptional level (RNA-seq), also several commonalities were observed for both two of the Micromonas species studied under N-depletion, such as a major decrease in the relative transcript abundance of genes related to the photosynthetic machinery, carbon transport and metabolism and fatty acid biosynthesis, and an increase in the relative transcript abundance of genes related to flagella and microtubule formation as well as an increase of a number of transcription factors (TFs). In addition, despite the slower growth recovery of N-deplete treatments after transferring to replete medium, a rapid transcriptional response was observed. The Micromonas species exhibited differences in the transcriptional patterns of genes related to nitrogen transport and assimilation. Under N-depletion, the majority of these genes increased in Micromonas commoda while results were more variable for homologous genes in Micromonas pusilla. Distinct transcriptional patterns were observed for ammonium transporter (AMT) homologous in each Micromonas species. In M. pusilla, one ammonium transporter (AMT1.3) appears to only increase under nitrogen depletion and may be a good marker of nutrient status in the field.

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