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(i)) sources whose incorporation varies in biochemical energy demand. We hypothesized that due to the lesser energy requirement of ammonium ([Formula: see text])‐use, in contrast to nitrate ([Formula: see text])‐use, more energy remains for other metabolic processes, especially under CO(2)‐ and phosphorus (P(i)) limiting conditions. Therefore, we tested differences in cell characteristics of the green alga Chlamydomonas acidophila grown on [Formula: see text] or [Formula: see text] under covariation of CO(2) and P(i)‐supply in order to determine limitations, in a full‐factorial design. As expected, results revealed higher carbon fixation rates for [Formula: see text] ‐grown cells compared to growth with [Formula: see text] under low CO(2) conditions. [Formula: see text] ‐grown cells accumulated more of the nine analyzed amino acids, especially under P(i)‐limited conditions, compared to cells provided with [Formula: see text]. This is probably due to a slower protein synthesis in cells provided with [Formula: see text]. In contrast to our expectations, compared to [Formula: see text] ‐grown cells [Formula: see text] ‐grown cells had higher photosynthetic efficiency under P(i)‐limitation. In conclusion, growth on the N(i)‐source [Formula: see text] did not result in a clearly enhanced C(i)‐assimilation, as it was highly dependent on P(i) and CO(2) conditions (replete or limited). Results are potentially connected to the fact that C. acidophila is able to use only CO(2) as its inorganic carbon (C(i)) source.