The effect of growth rate, phosphorus concentration, and temperature on N2 fixation, carbon fixation, and nitrogen release in continuous cultures of Trichodesmium IMS101

With the use of continuous culture systems, rates of dinitrogen (N-2) and carbon (C) fixation and nitrogen (N)- and C-based doubling times were assessed in Trichodesmium IMS101 growing exponentially at steady state dilution rates of 0.10, 0.20, and 0.33 d(-1) (doubling times of 10, 5, and 3 d - within the range reported for natural populations). Rates of C fixation, N-2 fixation, and N release were examined in replicate culture systems with several techniques. Biomass-specific C uptake varied little with population doubling time, but N-2 fixation and N release varied markedly among treatments. Total daily gross N-2 fixation rates and estimated N release rates were higher in cultures with higher dilution rates. Cultures grown at lower dilution rates had higher daily C:N, fixation ratios and lower N release rates. Consistent with other studies, it was estimated that Trichodesmium released about 80-90% of their recently fixed N-2 during growth. Turnover of cellular C estimated from carbon fixation was a good estimator of population growth rates in steady state cultures, whereas turnover of cellular N estimated from gross or net N, fixation was not. Small changes in temperature (24 degrees C vs. 28 degrees C) did not appear to affect gross N2 fixation, whereas inorganic phosphorus (1 vs. 5 mu mol L-1) supply had a large effect on N-2 fixation. These results suggest that continuous culture systems are excellent for elucidating physiological responses of Trichodesmium under ecologically relevant growth conditions and provide a framework for assessing highly variable field estimates of N-2 and C fixation.