Effects of experimental nutrient enrichment and eutrophication on microbial community structure and function in "marine lakes"

Biogeochemical cycling of nitrogen (N) and carbon (C) are regulated by microbial communities that can respond to altered N and C availability, yet the form, consistency, and magnitude of these responses remain poorly constrained. We enriched and eutrophied distinct microbial communities in "marine lakes" through experimental additions of N (5 mu M NH4Cl), C (100 mu M sucrose), and combined N+C (same forms and concentrations), examining microbial community responses via 16S rRNA sequencing in conjunction with functional responses represented by net community production (NCP) and community respiration (CR) measurements. Individual N or C additions drove significant shifts in microbial community structure only sporadically, and rarely with a corresponding difference in NCP or CR rates. In contrast, the combined addition of N+C elicited strong coincident responses in microbial community structure and function: NCP and CR rates shifted sharply toward heterotrophy and were correlated with multiple microbial networks (r(2) = 0.309-0.599, P < 0.001) that included globally distributed marine bacteria. Across multiple experiments, the consistent response of one network, comprised primarily of gammaproteobacterial heterotrophs (particularly Vibrio and Alteromonas), led initially dissimilar communities to converge toward similar composition. However, the distinct response patterns of other more diverse networks were superimposed on top of this network, indicating that inorganic N and organic C enrichment have multilayered effects on microbial communities. Collectively our results demonstrate that elevated N and C alter microbial community structure and function, selecting for multiple microbial networks that compete for, and rapidly cycle, N and C.