Agricultural land-use affects the nutritional quality of stream microbial communities.

We investigated how the lipid composition (fatty acids and sterols) of benthic microbial mats, which represent an important basal food resource for stream food webs, differs between tropical streams located in protected pristine and agricultural Cerrado savannah areas. The total microbial biomass and lipid composition differed significantly between pristine and agricultural streams in parallel with differences in water quality and hydrodynamic characteristics. Agricultural streams exhibited lower total biomass of benthic microbial mats than pristine streams. However, the higher concentrations of essential polyunsaturated fatty acids, such as linoleic acid (LIN, 18:2ω6), α-linolenic acid (ALA, 18:3ω3), and eicosapentaenoic acid (EPA, 20:5ω3), that were observed in agricultural streams suggest enhanced lipid complexity and a higher nutritional quality of the microbial community relative to pristine streams. Meanwhile, pristine stream microbial communities had higher total concentrations of saturated fatty acids and cholesterol than those of agricultural streams, reflecting their heterotrophic microbial communities. Moreover, stream morphotype and associated differences in the hydrodynamic characteristics affected the community composition and thereby also the lipid composition of microbial mats. Land-use-induced changes in the total biomass and lipid composition of microbial communities may affect the trophic transfer of energy in stream food webs, leading to changes in the composition and productivity of primary consumers and their predators, and thereby affecting stream ecosystem functioning.