Iron requirements for dinitrogen‐ and ammonium‐supported growth in cultures of Trichodesmium (IMS 101): Comparison with nitrogen fixation rates and iron: carbon ratios of field populations

Abstract

We quantified the relationships among the specific growth rate, intracellular iron content, and steady state iron uptake rate for cultures of the marine diazotrophic cyanobacterium Trichodesmium (IMS 101) grown under differing conditions of Fe and N availability. The Fe quotas necessary to support a moderately Fe‐limited growth rate (70% mmax) of 0.1 d−1 under diazotrophy and ammonium were 38 and 8 µmol mol−1, indicating a fivefold increased Fe cost for diazotrophy. This increased demand reflects the influences of both the ninefold lower marginal use efficiency and the greater maintenance Fe :C requirement at zero growth rate under diazotrophy (13.5 versus 5.2 µmol mol−1). For diazotrophic growth at µ = 0.1 d−1, we estimate that 19%–53% of the cellular Fe is bound in nitrogenase. Trichodesmium is capable of luxury uptake of at least 13‐fold greater amounts of Fe than needed for moderately Fe‐limited growth. At least half of the populations sampled from the continental shelf of northern Australia had Fe :C ratios within the range of iron limitation of laboratory cultures. Bottle incubations showed that, after 3–4 d with added iron, a low Fe :C population (21 µmol mol−1) had higher N2 fixation rates than controls, whereas a high Fe :C population (78 µmol mol−1) showed no response. The cellular N: P quotas and N2 fixation rates were compared to those previously reported for the Atlantic Ocean. Empirically, these data suggest a critical N: P ratio ~40–50 for the onset of P limitation of growth. Only 10% of the Trichodesmium populations from coastal Australia had N: P ratios within the putative P‐limitation range (>40), whereas 40% the Atlantic populations fell within this range. These comparisons, along with those for N fixation versus colony Fe :C ratio, suggest that Fe limitation is more prevalent in the Australia Trichodesmium populations, while P limitation predominates in the Atlantic populations.