The bacteria–protist link as a main route of dissolved organic matter across contrasting productivity areas on 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 influenced the DOM pool and the transformation of its fluorescent fraction. Dilution experiments were performed during the productive season on the Patagonian Shelf (SW Atlantic Ocean), a region of intense biological activity, with peak productivity observed at the shelf break front. Although phytoplankton biomass was higher than that of bacteria, protists selectively preyed on the faster-growing bacterial population, denoting trophic specificity of grazers. High trophic coupling was suggested by the biomass distribution of protistan consumers and their prey, which predominantly exhibited an inverted trophic pyramid structure. An exception to this pattern was observed at the highly productive shelf break front, where a traditional bottom-heavy pyramid emerged, indicating that most phytoplankton evaded protist predation despite evidence of herbivory. Bacterial consumption of DOM appeared uncoupled from its total amount but was influenced by DOM complexity, while the bacterial production of humic-like substances from protistan plankton precursors observed in most experiments highlighted a potential pathway for carbon sequestration. Protistan grazers also significantly influenced DOM dynamics by scaling their DOM contribution in response to the intensity of grazing on heterotrophic bacteria, regardless of productivity levels. This effect likely arises from reducing the number of active DOM-consuming bacteria and by providing egestion DOM compounds. At the onset of the productive season, high bacterial growth rates stimulate protistan grazing, which serves as a link between bacterial biomass and higher trophic levels. However, as grazing pressure increases, protists can also contribute to the accumulation of a fraction of DOM.