ALGAL CARBON NITROGEN-METABOLISM - A BIOCHEMICAL BASIS FOR MODELING THE INTERACTIONS BETWEEN NITRATE AND AMMONIUM UPTAKE

It is hypothesized that the regulation of the response by phytoplankton to nitrogen-stress centres directly or indirectly on the intracellular proportions of key metabolites of nitrogen and carbon metabolism, such as glutamine and 2-oxoglutarate, as has been demonstrated for bacteria. Many of the interactions between ammonium and nitrate assimilations with respiration, CO2-fixation and nitrogen-stress, may be explained by such a mechanism. This mode of regulation allows a progressive response to stress by (in)activation and (de)repression of transport and assimilatory processes, with growth on excess ammonion causing maximum repression of the capacity to use other sources of nitrogen, such as nitrate. At the concentration of ammonium in marine waters, however, a measure of derepression is probably the norm enabling a simultaneous uptake of several sources of nitrogen.