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 CO2 fixation, and NH4+ uptake were measured in cultures of Trichodesmium IMS101 growing on an artificial culture medium wi thout added N substrates. Cultures exhibited a doubling time of about 4 d b ased on the accumulation of PON, POC, chlorophyll a, and cell number. Cell- specific rates of N-2 and CO2 fixation were highest during midday and durin g the initial stages of the growth cycle. Up to midlog phase, integrated estimates of CO2 fixation (based on short-te rm (CO2)-C-13 uptake) closely tracked increases in POC. However, after day 8, estimates of C accumulation based on CO2 fixation exceeded observed POC increases. Within the error of the measurements, N-2 fixation estimates (ba sed on C2H2 reduction) also closely tracked, and could largely account for, PON increases over most of the experiment. N demand based on CO2 fixation also followed PON increases during early to midlog phase but diverged durin g late exponential phase and thereafter by greater than 100%. Estimates of short-term C fixation rates during and after late exponential phase overest imate net growth of Trichodesmium IMS101, whereas estimates of N-2 fixation rates approximated net growth. These observations may partially explain th e disparity of estimates of growth in held populations based on (CO2)-C-14 fixation compared to N-2 fixation. Ammonium accumulated in the culture medium during the experiment, and rates of NH4+ uptake were high throughout the day and over the whole growth cycl e. Rates of NH4+ uptake generally exceeded N-2 fixation. The turnover of N was therefore much greater than the net accumulation of N in biomass and im plies a concurrent high rate of NH4+ release.