THE GENETIC DIVERSITY BETWEEN SUBLINEAGES OF UCYN-A AND THEIR ALGAL HOST PLASTIDS AND THE DYNAMICS OF NITROGEN FIXING MICROBES IN THE BEDFORD BASIN

Abstract

The availability of fixed nitrogen, which is imperative to the productivity of the ocean, is largely controlled by microbial diazotrophs. These nitrogen-fixing organisms are diverse and widespread, and studying their ecology and genomics gives us important information on the workings of the marine nitrogen cycle. This thesis focuses on the study of marine diazotrophs in the Northwest Atlantic Ocean. Chapter 2 presents findings from metagenomic analyses of mixed cultures containing the globally important diazotroph UCYN-A, one sublineage of which has recently been presented as a nitrogen fixing organelle, or “nitroplast” within the haptophyte alga Braarudosphaera bigelowii. Results of these analyses produced metagenome assembled genomes of UCYN-A4, and the Braarudosphaera bigelowii host plastid associated with it. Next the pangenome of UCYN-A was examined; the results showed that UCYN-A4 has similar average nucleotide identity to UCYN-A1 and UCYN-A2 as they do to each other, and the genome of UCYN-A4 is missing many of the same genes as the other UCYN-A genomes. The co-occurring microbes within the mixed cultures were also examined and compared to natural co-occurrences and prey of Braarudosphaera bigelowii. Chapter 3 examines the diazotrophic community within the Bedford Basin. Data from the Bedford Basin time series were used as the basis of an analysis of community composition of the nifH amplicon gene sequences and more than doubled the time frame of the data analysed previously. Temporal patterns revealed high cyanobacterial abundances in the summer and fall in the surface waters which contrasted with little seasonal variation in the deeper waters. The environmental drivers of variation in nifH community composition were analysed for the two depths and found temperature driving more variation in the surface and oxygen concentration driving more variation at depth. Using the sequences from Chapter 2, dynamics of UCYN-A and B. bigelowii were studied by sublineage and genotype. Overall, this thesis provides new knowledge on UCYN-A and Braarudosphaera bigelowii and shows the value of high-resolution time series data in monitoring diazotroph community dynamics.