[en] 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 43°N, 9°W to 43°N, 15°W). 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.

[gl] A presente investigación examinou a variación vertical da abundancia, actividade, estructura e composición da comunidade microbiana en relación cás propiedades ópticas da materia orgánica (MOD) ó longo dunha sección lonxitudinal da costa galega (NO España, de 43°N, 9°O a 43°N, 15°O). O descenso da actividade microbiana (heterotrófica e autotrófica) coa profundidade coincidiu cun cambio na composición da comunidade microbiana. Thaumarchaeota dominou a comunidade de Arquea na columna de auga (en promedio, 83% do total de Archaea) e a súa abundancia descendeu coa profundidade, mentres a comunidade de Bacteria estivo dominada por Proteobacteria (79% de bacterias totais) na columna de auga. Cianobacteria e Bacteroidetes representaron ~ 10% da comunidade bacteriana epipeláxica, mentres SAR202 representou ~ 5% da comunidade bacteriana profunda. A calidade e cantidade da MOD exhibiu variacións verticais fortemente relacionadas co patrón de distribucion vertical das comunidades de arquea e bacteria, avaliadas por técnicas de pegada dactilar. O fraccionamento molecular da MOD estimulou o crecemento de grupos bacterianos específicos, que estiveron corelacionados con cambios nas propiedades ópticas da MOD. Finalmente, identificamos varios grupos microbianos implicados na fixación escura de carbono inorgánico no océano profundo. Os nosos resultados destacan o forte vínculo entre a cantidad e calidade da MOD con grupos microbianos específicos e o potencial para a fixación oscura de carbono dos diferentes grupos microbianos no océano profundo, indicando que a distribución dos grupos microbianos no océano oscuro está determinada pola preferencia do sustrato dipoñíbel e suxire un gran potencial metabólico dos microbios do océano profundo.