Biodiversity and microbial activity in the epi-, meso- and bathypelagic realms of the ocean

Abstract

This research thesis examined the vertical variation in microbial abundance, activity, community composition and structure, together with optical properties of the dissolved organic matter (DOM), along a longitudinal transect off the Galician coast (NW Spain, from 43ºN, 9ºW to 43ºN, 15ºW). The decrease of microbial activity (heterotrophic and autotrophic) with depth coincided with a change in the microbial community composition. Thaumarchaeota was the dominant archaeal (on average 83% of the total Archaea) group throughout the water column and decreased with depth, whereas the bacterial community was dominated by Proteobacteria (79% of the total Bacteria) throughout the water column. However, Cyanobacteria and Bacteriodetes contributed ~ 15% to the bacterial community in the epipelagic, and SAR202 accounted for ~ 5% in the deep waters. The DOM quality and quantity exhibited pronounced vertical variations strongly connected to the depth patterns of archaeal and bacterial community composition, assessed by fingerprinting analysis. Moreover, different size fractions of DOM stimulated the growth of specific bacterial groups, which were linked to changes in the optical indexes of the DOM. Finally, we identified several microbial groups involved in dark inorganic carbon fixation rates in the deep ocean. Taken together, our results highlight the strong link between DOM quality and quantity and specific microbial groups, and the potential for autotrophic carbon fixation by different groups of deep ocean microbes, indicating that the distribution of microbial groups in the ocean is determined by the availability of their preferred substrates and suggest a large metabolic potential of deep ocean microbes.