The influence of the balance of inorganic and organic nitrogen on the trophic dynamics of microbial food webs

A nutrient-manipulation mesocosm study was conducted in Norwegian coastal waters to determine the effect on a nitrogen-limited microbial food web of changes in the relative concentrations of dissolved organic and inorganic nitrogen (N). Four replicated treatments were studied: no N addition, inorganic N, organic N, or a 50:50 mix of inorganic and organic N. Comprehensive abundance, biomass, and rate measurements were made over the 20-d experiment. The form of N available influenced species composition, succession, and the efficiency of carbon (C) incorporation. Inorganic N generated a bloom of the diatom Leptocylindrus danicus, even though silicon concentrations were below 1 μmol L^-1. The γ-proteobacteria initially competed effectively with diatoms for inorganic N, exhibiting high productivity and escaping grazing control. Organic N, when available alone, was utilized more slowly by the microbial community, with diatom growth, fueled by regenerated N, achieving a lower peak abundance. A more diverse diatom species assemblage was also evident. The γ-proteobacteria were less prevalent and net bacterial productivity was initially lower when the N source was organic and, in contrast to the response following inorganic N addition, the bacterial community was initially grazer controlled. A rapid succession of heterotrophic grazers occurred but with differences in species and their contribution to biomass between treatments. The efficiency of C-biomass production was lower when the N source was purely organic. However, when both inorganic and organic N were available, biomass yield was greater than the mean of the two single N source treatments.