Spatial variability and distribution of benthic microbial diversity in the Atlantic and Pacific

This thesis focuses on the spatial variability of benthic abyssal microbes, which have important implications for biogeochemical cycling in the deep ocean. Abyssal ecosystems, considered to be between 3,000 to 6,000 m water depth, cover vast areas of the Earth’s surface. The majority of the abyssal seafloor is characterised by fine sediments, and these benthic habitats contain vast numbers of microbes with huge metabolic diversity. Abyssal microbes drive biogeochemical cycles, regulate fluxes of energy and contribute to organic carbon production and remineralization. So far, detailed assessments of the spatial distributions of benthic microbial communities in abyssal regions are still incomplete, and the influence of seafloor topography and heterogeneity on microbial distributions across a range of scales are poorly understood. Therefore, the aim of this thesis is to characterise the spatial variability of benthic microbes, which is essential for understanding their roles in benthic environments and for conducting baseline assessments of areas of the seabed that might be targeted by commercial mining activities. The spatial variation of benthic microbes in the Atlantic and Pacific oceans is the subject of three chapters, one of which focuses on the spatial variation of microbes associated with sediment and polymetallic nodules in the Clarion-Clipperton Fracture Zone (CCFZ) of the Pacific Ocean. The following chapter compares diversity and distribution of sediment microbes across three sites with varying productivity regimes in the Atlantic and Pacific. The final chapter examines the gut microbiome of holothurians, a dominant megafaunal group in the North-East Atlantic. This thesis showed that ammonia-oxidising Archaea (Thaumarchaeota) and nitrogen-cycling microbes were highly abundant in oligotrophic sediments and highlighted that these microbial taxa exhibited a spatial variability across small to large geographic scales, indicative of nîche diversity and resource partitioning. Given that chemolithoautotrophic taxa, such as ammoniaoxidisers and nitrifying microbes may form a significant source of organic carbon through inorganic carbon fixation and as microbes in general form the base of the marine food web, changes to microbial assemblages will likely have cascading impacts to higher trophic levels. Understanding the diversity patterns and functional roles of benthic microbes is vital for the sustainable use and preservation of ecosystems ahead of potential seabed exploration, exploitation and disturbance. Detailed observations are therefore necessary for modelling microbially mediated biogeochemical functions, monitoring biodiversity and ecological assessments for the preservation of ecosystem functioning