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 ()‐use, in contrast to nitrate ()‐use, more energy remains for other metabolic processes, especially under CO₂‐ and phosphorus (P_i) limiting conditions. Therefore, we tested differences in cell characteristics of the green alga Chlamydomonas acidophila grown on or under covariation of CO₂ and P_i‐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 CO₂ conditions. ‐grown cells accumulated more of the nine analyzed amino acids, especially under P_i‐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_i‐limitation. In conclusion, growth on the N_i‐source did not result in a clearly enhanced C_i‐assimilation, as it was highly dependent on P_i and CO₂ conditions (replete or limited). Results are potentially connected to the fact that C. acidophila is able to use only CO₂ as its inorganic carbon (C_i) source.