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

Abstract

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 ( ${\text{NH}}_4^{+}$ )-use, in contrast to nitrate ( ${\text{NO}}_3^{-}$ )-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 ${\text{NH}}_4^{+}$ or ${\text{NO}}_3^{-}$ 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 ${\text{NH}}_4^{+}$ -grown cells compared to growth with ${\text{NO}}_3^{-}$ under low CO₂ conditions. ${\text{NO}}_3^{-}$ -grown cells accumulated more of the nine analyzed amino acids, especially under P_i-limited conditions, compared to cells provided with ${\text{NH}}_4^{+}$ . This is probably due to a slower protein synthesis in cells provided with ${\text{NO}}_3^{-}$ . In contrast to our expectations, compared to ${\text{NH}}_4^{+}$ -grown cells ${\text{NO}}_3^{-}$ -grown cells had higher photosynthetic efficiency under P_i-limitation. In conclusion, growth on the N_i-source ${\text{NH}}_4^{+}$ 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.

Keywords: CO2 conditions; amino acids; carbon uptake kinetics; nitrogen; phosphorus limitation.