The determination of nitrogen status in microalgae

Abstract

Present methods used for the determination of N-status in microalgae typically involve testing for the existence of gross metabolic changes which develop in response to N-stress. Such approaches have 2 problems. First, the experimental tech~uques may be inappropriate for the species present and may perturb the organisms, possibly creating artifacts. Second, these gross changes, such as changes in rates of CO2-fixation and N-source uptake, may be affected by factors other than N-stress. There is a need to develop methods to detect metabolic changes which themselves trigger the genetic response to N-stress rather than to detect the products of that response. Such changes are likely to be in relative proportions of key metabolites of C and N metabolism, as In bacteria. It is suggested that only in the presence of excess NH: are the processes of cellular response to N-stress fully suppressed. As a consequence, microalgae throughout the oceans may show some symptoms of N-stress. The level of derepression of the N-stress responses which corresponds to growth-limitation, and hence is of ecological significance, needs to b e determined.

Changes in carbon and nitrogen physiology during ammonium and nitrate nutrition and nitrogen starvation inIsochrysis galbana

“…It would appear that there is an absolute requirement for products of photosynthesis for amino acid synthesis in such algae. However, during the light phase of growth the Gln/Glu ratio reacts to C-N stress in the usual way with higher ratios of Gln/Glu indicative of higher intracellular ammonium at the point of amino acid synthesis (Flynn, 1990). The fall in the Gln/Glu ratio in nitrate-growing cells after an early peak during the light phase suggests a degree of N-stress, or conversely a surplus of C-skeletons.…”

“…Fixation of lipid, unlike that of proteinaceous bodies, is poor because of the lack of crosslinkage between the fixatives and lipid. If this is a general phenomenon then greater care must be The diurnal cycle of ammonium-grown cells of Isochrysis galbana, with an elevated Gln/Glu ratio during the light phase, is similar to that shown by Dunaliella primolecta (Flynn, 1990). It would appear that there is an absolute requirement for products of photosynthesis for amino acid synthesis in such algae.…”

Ammonium enhancement of dark carbon fixation and nitrogen limitation in symbiotic zooxanthellae: Effects of feeding and starvation of the sea anemone Aiptasia pallida

“…Flynn (1990) reviewed the efficacy of this approach. The enhancement effects vary with algal species, and the technique is sensitive to incubation period and cell density.…”

Carbon and nitrogen sources of planktonic bacteria in Lake Constance studied by the composition and isotope dilution of intracellular amino acids

“…We did not observe such a high mol% of Gln under natural conditions. The Gln : glutamate ratio has been used as an indicator of the N status of marine algae (Flynn 1990) but not, to our knowledge, of bacteria.…”