Estudio de las interacciones microbianas de comunidades planctónicas en gradientes ambientales

Microbial communities play a primordial role in the biogeochemical cycles in nature. Phytoplankton and bacterioplankton are at the base of the microbial food web in aquatic systems, being the main producer and decomposer of organic carbon, respectively. In aquatic systems, these communities are controlled by the characteristic dynamism and heterogeneity of the environmental variables in pelagic ecosystems. One of the best ways to understand how communities are affected by environmental variables is by studying ecological gradients, which are progressive spatiotemporal changes of biotic and/or abiotic characteristics within an ecosystem. Many ecosystems present remarkable environmental gradients in which the interactions between the microbial communities and the effect these strong abiotic gradients have on them are poorly known. Without this information it is difficult to predict how the structure of microbial community, its functionality and in a greater extent the ecosystem itself, might respond to future environmental changes. Here we show how the microbial communities and their interactions in the environmental gradients studied here, a longitudinal transect in a tropical estuary and a vertical gradient in an acid reservoir, are strongly affected by the changes in the environmental conditions in space and time. In the tropical estuary, a marked zonation in terms of productivity was observed from the head to the mouth of the estuary, being daily net production only positive in the middle of the estuarine gradient. The strong dissolved organic carbon inflows to the tropical estuary during the rainy season could potentially change the ecosystem to a system driven in a higher degree by allochthonous carbon leading to a higher importance of bacterioplankton activity. In contrast, during the dry season, remarkable direct (by dissolved organic carbon) and indirect (by competition) coupling between phyto- and bacterioplankton groups were revealed from the co-occurrence patterns. In the acid reservoir, during stratification, a strong coupling between the phytoplankton that forms a deep chlorophyll maximum in the metalimnion and the bacterial carbon dioxide production in the hypolimnion was demonstrated by a 1-D reactive transport model, indicating that inorganic carbon can be the limiting nutrient for phytoplankton community in acid lakes. The taxonomic composition and single-cell traits of bacterioplankton changed in parallel between stratification and mixing seasons suggesting that both structural levels of this community are linked each other and are affected by the same environmental variables. Overall, our results demonstrate how the environmental gradients affect the microbial communities at different levels, including single cell characteristics, physiology, taxonomic structure, spatiotemporal distributions of different populations and ecosystem level properties like net ecosystem production and metabolism. This multilevel approach is essential to understand the ecological function of the microbial community in complex ecosystems and predict future changes in response to climate and anthropogenic forcing.