Flexible elemental stoichiometry in Trichodesmium spp. and its ecological implications

We conducted laboratory experiments to assess the bioelemental plasticity of cultures of Trichodesmium IMS101 under phosphorus (P)-replete, P-restricted, and light-limited conditions. The results reveal a high degree of stoichiometric flexibility. Specifically, Trichodesmium IMS101 is capable of growth with carbon (C) : nitrogen (N) : P ratios of C585(plus-minus)56 : N90(plus-minus)10 : P1, approximately six times higher than would be predicted by the Redfield reference ratio (C106 : N16 : P1), thus signifying low cellular P quotas relative to C and N. Luxury consumption of P occurs rapidly after periods of prolonged P restriction, under both light and dark conditions, resulting in substantial increases in P quotas and reductions of C : N : P ratios (C96(plus-minus)8 : N16(plus-minus)1 : P1). Comparisons of laboratory culture data to our field observations from the Northwest Atlantic and the North Pacific indicate that, while natural populations of Trichodesmium exhibit persistently low P content relative to C and N (C290(plus-minus)15 : N53(plus-minus)3 : P1), the highest and lowest C : P and N : P ratios recorded in the laboratory are rarely observed in nature. We have also performed laboratory experiments intended to simulate the energetic and nutritional extremes that would occur as naturally migrating populations of Trichodesmium sink out of the euphotic zone into P-rich regions of the upper disphotic zone. The duration of dark survival for this isolate is on the order of 3-6 d, after which time cells are unable to recover from light deprivation. This finding provides a constraint on the temporal scale of vertical migration.