Eurgain H John

Growth dynamics and toxicity of Alexandrium fundyense (Dinophyceae): the effect of changing N:P supply ratios on internal toxin and nutrient levels

The effect of N- and P-limitation on the growth and paralytic shellfish toxin content of Alexandrium fundyense was studied in a series of batch cultures in which the N:P supply mass ratio was varied from less than 1 up to 160 (cf. Redfield ≡ 7.2). N- and P-limitation of growth was observed at the lower and higher ratios respectively, with dual limitation between. Cellular parameters were similar during the exponential phase of growth, regardless of external N:P ratio. Upon nutrient exhaustion cell yields and internal pools were affected, although these also varied according to the N-source (nitrate vs. ammonium). Generally N-limitation alone led to a higher C:N ratio, lower C:P and N:P ratios and a decreased toxin content in stationary-phase cells. P-limitation led to increased cell size and cell quotas, lower C:N ratios, and higher C:P and N:P ratios (to maxima of 350 and 35 respectively). Complex internal P-fractionation schemes were found unnecessary to determine the P-status of cells and extraction using potassium persulphate, cold water and cold HCl alone were adequate. Severe P-limitation led to increased toxin content but only when N was also limiting, suggesting a synergistic effect of N and P availability on toxin synthesis and turnover. A positive relationship was found between toxin content and the intracellular concentration of arginine, but this varied with the nutrient status (in particular P-status) of the cells.

Modelling phosphate transport and assimilation in microalgae; how much complexity is warranted?

The operation of a three compartment phosphate model of microalgal growth (phosphate interaction model, PIM), containing internal P pools for polyphosphate (PolyP), soluble inorganic P (SIP) and structural and other organic P (SOP), is compared with that of a conventional quota model which contains only a single internal nutrient pool. PIM enables a simulation of the decoupling of phosphate transport and assimilation and is thus more suitable for studies of short-term transient events and of transport kinetics. For general use, however, the quota model appears satisfactory provided that it has been parameterised using a full data set including cells grown under P-replete conditions in which PolyP can accumulate. This may not be the case for studies undertaken using steady-state P-limited chemostats, which will thus underestimate the value of the maximum value of the P:C quota. From the simulations it appears unwarranted to use a more complex model of P than the three pool PIM. Simulations using PIM indicate that, in addition to the well documented advantage in accumulating P over that required for immediate use, the accumulation of PolyP enables a better decoupling of transport from assimilation and hence the maintenance of high transport rates for longer. This would be of importance to organisms living in a transient nutrient regime and it may be important to simulate competition for P using such a model in ecosystem simulations.

Prey selectivity and the influence of prey carbon : nitrogen ratio on microflagellate grazing

We investigated the influence of prey species and nutritional value, in terms of carbon:nitrogen (C:N) ratio, on prey selection by the predatory microflagellate Paraphysomonas vestita. Experiments were conducted with two phytoplankton prey species of similar diameter to remove size-specific grazing effects. Live cells of both low and high C:N ratio (ranging from 4.8 to 14; N-replete and N-deplete, respectively) were offered to the predator either individually or in combination. By utilising analytical flow cytometry, we were able to enumerate the two prey species and, hence, study selective predation in the mixed-prey assemblage. In single prey experiments, the maximum observed ingestion rates were found to be higher, at all prey C:N ratios, when Isochrysis galbana was the prey item when compared to Pavlova lutheri, whilst maximum specific predator division rates were similar for both prey. Ingestion rates were influenced by prey nutrient status, higher values being observed with N-replete than N-deplete prey. When the two prey species were presented to P. vestita as a mixture, I. galbana was ingested more rapidly than P. lutheri, although ingestion was found to be suppressed when compared to when this was the sole prey species. Conversely, the presence of I. galbana did not influence the rate of ingestion of P. lutheri. P. vestita was, therefore, able to modify its rate of ingestion on the basis of prey type and prey C:N ratio and to discriminate between alternative prey of similar size in mixed-prey assemblages.