RELEASE OF NITRITE BY MARINE DINOFLAGELLATES - DEVELOPMENT OF A MATHEMATICAL SIMULATION

The dinoflagellates Scrippsiella trochoidea (Stein) and Alexandrium mi nutum (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 c ycle at the same daily dilution rate, or with a faster continuous dilu tion). When supplied with nitrate + ammonium, A. minutum released a pr oportion (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 assimi lation. S. trochoidea released much smaller amounts of nitrite, if any . Nitrate and ammonium were not accumulated to any extent by either sp ecies 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 assim ilated N. Thus uptake through to incorporation of N into macromolecule s appeared to be coupled in these species, even in darkness when incre asing glutamine:glutamate (Gln:Glu) ratios suggested rising C-stress. A mechanistic model was developed from an earlier ammonium-nitrate int eraction model (ANIM) by the inclusion of an internal nitrite pool, wi th control over the supply of reductant for nitrite reduction linked t o photosynthetic and respiratory components. The model can reproduce t he release of nitrite seen in the experiments, and also the release of nitrite in response to nitrate-feeding of N-stressed cells reported e lsewhere.