Background/Question/Methods

Nutrient cycling is fundamental for the functioning of aquatic ecosystems. This cycle is supported by many aquatic organisms such as bacteria, plankton (in the water column) and benthic organisms (in the sediment). Fish species are rarely included in the study of matter flows and nutrient cycle while recent studies suggest that fish communities play an important role in these mechanisms and may create biogeochemical hotspots. For instance, flatfish species are involved in sediment resuspension and zooplanktivorous species promote primary production by exerting a predation pressure. In contrast, fish communities impacts on nutrient cycling through digestion and excretion are less explored. Do all species equally degrade organic matter? Which factor(s) may influence this capacity? To answer these questions, we studied bacterial communities contained in the digestive tract of various fish species sampled in the Patos-Mirim system (Brazil). Indeed, digestion is strongly influenced by commensal bacterial communities living within guts of many animals. For each individual, gut content was collected and divided in two parts: one was cultivated onto Biolog Ecoplates containing carbon sources while the other was used to estimate bacterial ribotypic diversity with DGGE method. Then, the influence of species, diet and environment on the functional and genetic microbial diversity was analyzed using Constrained Analysis of Principal Coordinates (CAP).

Results/Conclusions

A total of 150 individuals, belonging to 22 fish species with varied diet, were sampled. The bacterial community of each individual was characterized by a carbon sources use pattern and a DGGE ribotypic profile. CAP analysis results showed that the host species is the determining factor in the variability of carbon sources used. Fish diet has a secondary impact by setting general guilds. Environment does not influence microbial functional diversity. Ribotypic diversity of each bacterial community confirmed these result.

Although carbon sources available on Ecoplates are not representative of all natural sources found in fish diet, they highlight differential use of nutrients among fish species. Not all species are equivalent in organic matter degradation and diet does not seem an appropriate surrogate for degradation capacity. Besides the impact of foraging behavior and food webs, interactions between fish communities and nutrient cycling may be very important and complex. Further analyses on gut flora and substances excreted are necessary to understand and quantify the role of fish communities in nutrient cycling. However, this innovating work opens new perspectives on the crucial role of fish on matter fluxes in aquatic environments.