Graduate Thesis Or Dissertation
 

Rapid equilibrium response of a marine diatom to external and internal nutrient concentrations

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  • Batch cultures of Skeletonema costatum were grown under continual light saturation, with nitrogen nutrients in limiting concentrations. Population parameters measured included ¹⁴C uptake, chlorophyll a, particulate carbon and nitrogen, and ammonia, nitrate, and nitrite, concentrations. A method was developed for measuring cellular nutrient reservoirs in laboratory cultures of marine phytoplankton. The method seems compatible to field measurements of cellular reservoirs in natural plankton communities. Significant cellular nutrient reservoirs of nitrate and ammonia were measured in all three experiments, and the ratio of mg/1 of cellular nutrient reservoir to mg/1 of particulate nitrogen ranged from zero to 1.0. Cellular nitrite reservoirs ranging from 0-1μM were observed during nitrite assimilation, A practical method of measuring nitrate reservoirs was developed. From the raw data, parameters calculated were nutrient specific uptake rates, particulate nitrogen specific growth rates, relative cellular reservoir size, percent soluble organic nitrogen, photosynthetic assimilation ratios, chlorophyll a / carbon ratios, and nitrogen to carbon ratios. Specific uptake rates were found to be highly variable, and at times were ca. 20 times the highest measured population growth rate. It was concluded that specific uptake rates showed poor correlation to specific growth rates. Calculated relative cellular nutrient reservoir sizes do not correspond directly to trends in specific growth rates, although it was noted that large relative cellular nutrient reservoir sizes preceded the initial large increases in particulate nitrogen specific growth rates. Photosynthetic assimilation ratios and percent soluble organic nitrogen correlate with trends in population growth rate very well. Changes in N/C ratios, and chlorophyll a / carbon ratios were also found to be related to changes in population growth rates. The N/C values ranged from 0.04 to 0.16, and the corresponding C/N ratios from 15 to 6. 2. Induction of the chlorophyll a synthesis pathway by internal ammonia concentrations is discussed. Four basic models of phytoplankton growth dynamics are discussed, and the inadequacy of each model, in light of past and present experimental evidence is pointed out. A new conceptual model is presented and discussed, which appears to be biologically sound and compatible with experimental evidence. The most important aspect of the model is the induction of assimilatory enzyme systems by internal nutrient concentrations.
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