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 ( NH 4 + )‐use, in contrast to nitrate ( NO 3 - )‐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 NH 4 + or NO 3 - 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 NH 4 + ‐grown cells compared to growth with NO 3 - under low CO 2 conditions. NO 3 - ‐grown cells accumulated more of the nine analyzed amino acids, especially under P i ‐limited conditions, compared to cells provided with NH 4 + . This is probably due to a slower protein synthesis in cells provided with NO 3 - . In contrast to our expectations, compared to NH 4 + ‐grown cells NO 3 - ‐grown cells had higher photosynthetic efficiency under P i ‐limitation. In conclusion, growth on the N i ‐source NH 4 + 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.