Saiyed I. Ahmed

Species differences in accumulation of nitrogen pools in phytoplankton

The changes in the intracellular concentrations of nitrate, ammonium, free amino acids, protein, DNA, RNA and total nitrogen were measured in batch cultures of seven species of marine phytoplankton as they progressed from being nitrogen sufficient to being nitrogen starved. After several days of nitrogen starvation, either nitrate or ammonium was added to the cultures, and the measurements were continued for 10 to 36 h. By this means it was possible to assess the long-term and short-term changes in cellular nitrogen compounds and how they relate to phytoplankton nitrogen uptake and growth. Considerable species differences were observed in the amounts and kinds of nitrogen compounds which were stored and the degree to which the utilization of these compounds could support growth if the external nitrogen supply is low or variable. Despite the species variation, the results suggest that the concentrations or ratios of a number of intracellular nitrogen compounds can be used to assess the nitrogen deficiency and/or growth rate of natural phytoplankton populations.

Resolution by high pressure liquid chromatography of intracellular and extracellular free amino acids of a nitrogen deficient marine diatom, Skeletonema costatum (Grev.) Cleve, pulsed with nitrate or ammonium

Abstract Nitrate and ammonium were added to nitrogen (N) deficient, batch cultures of Skeletonema costatum (Grev.) Cleve. The range of total internal free amino acids (IN FAAs) after N perturbations was 13.0–28.5 mM, while the total external FAAs (EX FAAs) varied from 3.9–17.5 μM. The total basic IN FAAs (and the total basic IN FAA N) predominated over the acidic and neutral IN FAAs (and acidic and neutral IN FAA N) after 50 h of N deficiency and after the N perturbations. Ornithine, a non-protein basic amino acid, was the most abundant individual IN FAA in N deficient cells, accounting for 43% of the total IN FAAs. All internal FAAs increased more quickly after the NH4+ addition than after the NO3− addition. During the assimilation of either N addition, ornithine increased to a temporary peak concentration more rapidly than any other observed IN FAAs, and contributed up to 80% of the total IN FAA pool. After most of the respective N additions had been assimilated, and as free ornithine began to decrease, glutamine and glutamate increased to form a combined contribution of 30% of the total IN FAA pool. The increases of internal free glutamine and glutamate occurred more rapidly with the addition of NH4+ than with NO3−, and these increases did not occur until the internal NH4+ was depleted. The EX FAA did not undergo a distinct cycling in response to the NO3− or NH4+ additions, as was observed with the IN FAAs. In contrast to the composition of the IN FAAs the most abundant EX FAAs were histidine (relatively sparse IN FAA), glutamate, aspartate, ornithine, and lysine.

The effect of short-term fluctuations in pH on NO−3 uptake and intracellular constituents in Skeletonemacostatum (Grev.) Cleve☆

Abstract Measurements of uptake rates, intracellular nitrogen pools, and other key intracellular constituents were made during exponential growth in Skeletonema costatum (Grev.) Cleve under varying pH levels. An understanding of the overall effects of extracellular pH on the above mentioned cellular parameters is crucial in order to ascertain the degree to which pH must be regulated and monitored in laboratory experiments with marine phytoplankton. It was found that uptake rates and intracellular pool sizes of NO − 3 were directly influenced by the extracellular pH level, whereas, other cellular compounds remained relatively unchanged. Therefore, nitrogen uptake and intracellular nitrogen storage are dependent on key H + and OH − ion transport mechanisms that are associated with phytoplankton metabolism. These findings reiterate the fact that investigators examining nitrogen uptake and assimilatory mechanisms in marine phytoplankton must be conscious of cellular H + and OH − fluxes that contribute to intracellular pH regulation and changes in extracellular pH levels, both of which interact to affect phytoplankton metabolic processes.