Ecological, Physiological and Systematic Insights into a Model Non-Cyanobacterial Diazotroph: Thalassolituus haligoni sp. nov. BB40

Abstract

Marine non-cyanobacterial diazotrophs (NCDs) are recognized as globally important for the marine nitrogen (N) cycle, however, their distribution remains uncertain, and their physiology remains largely unknown due to the lack of cultured representatives. The goal of this thesis was to examine the ecological and physiological characteristics of an isolated NCD, Thalassoltiuus haligoni sp. nov., BB40, to better understand its dinitrogen (N2) fixation capability, metabolism, regulation and global significance. In chapter 2, I examined the distribution and phylogenomic relationships of T. haligoni as well as the N2 fixation capability of the isolate under prolonged nitrate exposure. Results from this chapter show that T. haligoni is a globally distributed NCD, belongs within the order Oceanospirillales and can fix N2 in the presence of various NO3- concentrations, with minor changes to the proteome. Chapter 3 examines changes in growth rate (induced by changes in substrate and temperature), cell morphology, and N2 fixation rates in various N (NH3, NO3-, N2) and oxygen (O2) (hypoxic and oxic) conditions. Results from chapter 3 demonstrated the isolate requires hypoxic conditions for active diazotrophy (i.e., N deprivation) but is also able to fix N2 at a basal rate in fixed N exposure with a range of 9.34 × 10-6 to 2.9 ×10-4 fmol N cell-1 day-1. Experiments with BIOLOG plates also determined T. haligoni can grow on an array of C, N and aromatic compounds and has a wide temperature range (4 oC – 40 oC), consistent with its global detection. In Chapter 4, I integrate transcriptomic and proteomic data obtained from T. haligoni grown with either fixed N or N2 in oxic and hypoxic environments. Results reveal distinct metabolic trade-offs between ribosomal protein allocation, N2 fixation and stress response mechanisms. Data from this chapter also suggests that hypoxic conditions provide a competitive advantage for T. haligoni irrespective of fixed N sources through extensive nifH transcript priming. Collectively the data generated from this thesis provides a comprehensive overview of the impacts of fixed N and O2 on a globally distributed marine NCD belonging within the Oceanospirillales family. Data generated from chapter 4 can be used to further investigate N2 fixation regulation at the transcriptomic and proteomic level, while data generated from chapter 3 can be used to guide future cultivation efforts of NCDs. The findings from this thesis advance our understanding of NCD physiology, informs models of N2 fixation regulation, and supports predictions of the ecological role and cultivation potential of closely related uncultured Thalassolituus lineages.