Stoichiometry of nitrogen and carbon utilization in cultured populations of Trichodesmium IMS101:: Implications for growth

Rates of particulate organic carbon (POC) and particulate organic nitrogen (PON) accumulation, N(2) and CO(2) fixation, and NH(4)(+) uptake were measured in cultures of Trichodesmium IMS101 growing on an artificial culture medium without added N substrates. Cultures exhibited a doubling time of about 4 d based on the accumulation of PON, POC, chlorophyll a, and cell number. Cell-specific rates of N(2) and CO(2) fixation were highest during midday and during the initial stages of the growth cycle. Up to midlog phase, integrated estimates of CO(2) fixation (based on short-term (13)CO(2) uptake) closely tracked increases in POC. However, after day 8, estimates of C accumulation based on CO(2) fixation exceeded observed POC increases. Within the error of the measurements, N(2) fixation estimates (based on C(2)H(2) reduction) also closely tracked, and could largely account for, PON increases over most of the experiment. N demand based on CO(2) fixation also followed PON increases during early to midlog phase but diverged during late exponential phase and thereafter by greater than 100%. Estimates of short-term C fixation rates during and after late exponential phase overestimate net growth of Trichodesmium IMS101, whereas estimates of N(2) fixation rates approximated net growth. These observations may partially explain the disparity of estimates of growth in held populations based on (14)CO(2) fixation compared to N(2) fixation. Ammonium accumulated in the culture medium during the experiment, and rates of NH(4)(+) uptake were high throughout the day and over the whole growth cycle. Rates of NH(4)(+) uptake generally exceeded N(2) fixation. The turnover of N was therefore much greater than the net accumulation of N in biomass and implies a concurrent high rate of NH(4)(+) release.