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Seabird guano enters coastal waters providing bioavailable substrates for microbial plankton, but their role in marine ecosystem functioning remains poorly understood. Two concentrations of the water soluble fraction (WSF) of gull guano were added to different natural microbial communities collected in surface waters from the Ría de Vigo (NW Spain) in spring, summer, and winter. Samples were incubated with or without antibiotics (to block bacterial activity) to test whether gull guano stimulated phytoplankton and bacterial growth, caused changes in taxonomic composition, and altered phytoplankton–bacteria interactions. Alteromonadales, Sphingobacteriales, Verrucomicrobia and diatoms were generally stimulated by guano. Chlorophyll a (Chl a) concentration and bacterial abundance significantly increased after additions independently of the initial ambient nutrient concentrations. Our study demonstrates, for the first time, that the addition of guano altered the phytoplankton–bacteria interaction index from neutral (i.e. phytoplankton growth was not affected by bacterial activity) to positive (i.e. phytoplankton growth was stimulated by bacterial activity) in the low-nutrient environment occurring in spring. In contrast, when environmental nutrient concentrations were high, the interaction index changed from positive to neutral after guano additions, suggesting the presence of some secondary metabolite in the guano that is needed for phytoplankton growth, which would otherwise be supplied by bacteria

Subterranean estuaries (STEs) modulate the chemical composition of continental groundwater before it reaches the coast, but their microbial community is poorly known. Here, we explored the microbial ecology of two neighbouring, yet contrasting STEs (Panxón and Ladeira STEs; Ría de Vigo, NW Iberian Peninsula). We investigated microbial composition (16S rRNA gene sequencing), abundance, heterotrophic production and their geochemical drivers. A total of 10,150 OTUs and 59 phyla were retrieved from porewater sampled during four surveys covering each STE seepage face. In both STEs, we find a very diverse microbial community composed by abundant cosmopolitans and locally restricted rare taxa. Porewater oxygen and dissolved organic matter are the main environmental predictors of microbial community composition. More importantly, the high variety of benthic microbiota links to biogeochemical processes of different elements in STEs. The oxygen-rich Panxón beach showed strong associations of the ammonium oxidizing archaea Nitrosopumilales with the heterotrophic community, thus acting as a net source of nitrogen to the coast. On the other hand, the prevailing anoxic conditions of Ladeira beach promoted the dominance of anaerobic heterotrophs related to the degradation of complex and aromatic compounds, such as Dehalococcoidia and Desulfatiglans, and the co-occurrence of methane oxidizers and methanogens

Anthropogenic alterations of global biogeochemical cycles are changing not only the magnitude but also the nature of matter entering into the ocean, thus increasing the C/P and N/P ratios. Allochthonous inputs are particularly significant in coastal systems and even may equal autochthonous sources. The interactions between phytoplankton and bacteria likely play a major role in the response of primary producers to anthropogenic matter inputs. However, the magnitude and nature of such interactions, as well as the factors controlling their intensity and sign are largely unknown in marine planktonic ecosystems. A set of 18 microcosms (2 L) experiments were conducted to test the hypothesis that the nature of the nutrient inputs modulates the strength and sign of the interaction between phytoplankton and bacteria. The experiments consisted in the addition of nutrients (nitrate, ammonium an phosphate), nutrients and dissolved organic matter (DOM) (glucose and amino acids), or riverine water (10% final concentration) containing different DOM:P proportions to intact microbial communities and microbial communities where bacteria were selectively blocked with a previously tested combination of antibiotics. Sampling was conducted in different seasons in the Ría de Vigo, a highly productive coastal ecosystem located NW Spain. An interaction index was calculated as the quotient between the change in chlorophyll-a concentration after nutrient amendment in the intact communities and the corresponding change in the same community but treated with antibiotics. An interaction index >1 thus indicates a positive effect of bacteria on phytoplankton, while an interaction index <1 indicates an overall negative impact. The interaction index ranged from 0.51 to 1.24. The index was significantly higher in autumn (1.01±0.03) than in spring (0.91±0.02), and winter (0.90±0.03). Overall, treatments with a high DOM:P ratio promoted a strong negative impact of bacteria on phytoplankton (0.84±0.06)