Nitrate or ammonium: Influences of nitrogen source on the physiology of a green alga.

In freshwaters, algal species are exposed to different inorganic nitrogen (N) sources whose incorporation varies in biochemical energy demand. We hypothesized that due to the lesser energy requirement of ammonium ( )-use, in contrast to nitrate ( )-use, more energy remains for other metabolic processes, especially under CO- and phosphorus (P) limiting conditions. Therefore, we tested differences in cell characteristics of the green alga grown on or under covariation of CO and P-supply in order to determine limitations, in a full-factorial design. As expected, results revealed higher carbon fixation rates for -grown cells compared to growth with under low CO conditions. -grown cells accumulated more of the nine analyzed amino acids, especially under P-limited conditions, compared to cells provided with . This is probably due to a slower protein synthesis in cells provided with . In contrast to our expectations, compared to -grown cells -grown cells had higher photosynthetic efficiency under P-limitation. In conclusion, growth on the N-source did not result in a clearly enhanced C-assimilation, as it was highly dependent on P and CO conditions (replete or limited). Results are potentially connected to the fact that is able to use only CO as its inorganic carbon (C) source.