The bacteria-protist link as a main route of dissolved organic matter across contrasting productivity areas in the Patagonian Shelf

Abstract. While the sources of dissolved organic matter (DOM) in the open ocean are relatively well identified, its fate due to microbial activity is still evolving. Here, we explored how microbial community structure, growth, and grazing of phytoplankton and heterotrophic bacteria influence the DOM pool and the transformation of its fluorescent fraction (FDOM) during dilution experiments in the Patagonian Shelf (SW Atlantic Ocean). This area constitutes a global hotspot of carbon sequestration due to intense biological productivity which peaks at the shelf break front. The productive stations at the shelf break front featured a food web primarily based on phytoplankton and heterotrophic bacteria, while less productive mid-shelf stations showed greater dependence of protistan predators on bacterial biomass. Although phytoplankton biomass was higher than that of bacteria, protists selectively preyed on the latter, which exhibited faster growth rates, denoting high trophic specificity of grazers. Trophic efficiency and omnivory favored a bottom-heavy biomass distribution, characterized by consumer biomass dominance over producers, except in highly productive stations influenced by nutrient-rich upwelling waters, where a typical pyramid structure was observed. Our results showed that in addition to the commonly accepted factors such as phytoplankton growth stage and bacterial community composition, DOM accumulation versus consumption is also linked to bacterial grazing. Intense grazing on heterotrophic bacteria promoted DOM accumulation, likely by reducing the number of active, DOM-consuming bacteria and by providing egestion compounds to the DOM pool. Moreover, bacterial consumption of DOM appeared uncoupled from its total amount but was influenced by FDOM properties. These findings suggest that under high bacterial growth rate that follows the onset of the productive season, protistan grazers act as a link between bacterial biomass and higher trophic levels, partially diverting DOM lysate production by virus.