Release of nitrite by marine dinoflagellates: development of a mathematical simulation

The dinoflagellates Scrippsiella trochoidea (Stein) and Alexandrium minutum (Halim) were grown in a light-dark cycle with nitrate or nitrate plus ammonium under three different nutrient-supply regimes (dilution with fresh media in dark phase only or during the entire light-dark cycle at the same daily dilution rate, or with a faster continuous dilution). When supplied with nitrate + ammonium, A. minutum released a proportion (as much as 100% from dark-fed cells) of the nitrate taken up during the dark phase as nitrite, reflecting a rate-limiting step at nitrite reduction and poor regulation of inorganic-N uptake and assimilation. S. trochoidea released much smaller amounts of nitrite, if any. Nitrate and ammonium were not accumulated to any extent by either species in darkness, and the transient increases in the size of the free amino acid pool were too small to explain the fate of the newly assimilated N. Thus uptake through to incorporation of N into macromolecules appeared to be coupled in these species, even in darkness when increasing glutamine:glutamate (Gln:Glu) ratios suggested rising C-stress. A mechanistic model was developed from an earlier ammonium-nitrate interaction model (ANIM) by the inclusion of an internal nitrite pool, with control over the supply of reductant for nitrite reduction linked to photosynthetic and respiratory components. The model can reproduce the release of nitrite seen in the experiments, and also the release of nitrite in response to nitrate-feeding of N-stressed cells reported elsewhere.