Zuoxi Ruan
Shantou University
China
Title: The influence of the N source on C allocation in the cyanobacterium Synechococcus sp. depends on energy availability
Biography
Biography: Zuoxi Ruan
Abstract
The assimilation of N-NO3- by algae requires more energy than that of N-NH4+. Therefore, the availability of either N-source may have appreciable consequences for the energetic budget of cells. This becomes relevant especially when energy (i.e. light) is in short supply. Consequently, the impact of N assimilation on the overall energy budget of phytoplankton may vary appreciably depending on the season, the time of the day and the position of cells in the water column. Furthermore, C:N stoichiometry has physiological and structural constraints and a change in the C:N ratio is reflected by the cell organic composition, with obvious consequences for trophic webs. In this work, we used the cyanobacterium Synechococcus sp. UTEX 2380 to investigate if and to what extent N availability and chemical form influenced growth, elemental stoichiometry and carbon allocation, under N or light limitation, in the presence of either NO3- or NH4+ as the N-source. When energy was limiting, Synechococcus grew faster in NH4+ than in NO3-, and had higher C (20%), N (38%), S (30%) cell quotas (a similar trend was also found for Fe, Zn, and Cr). Also, more C was allocated to protein, with a decrease of the carbohydrate/protein ratio, whereas the lipid/protein ratio did not change. Energy limitation also led to higher (129%) biomass productivity. We interpreted these results as an indication that, under energy limitation, the use of the least expensive N source allows a greater investment into growth and altered cell organic composition.
