TRICHODESMIUM SPP. PHYSIOLOGY AND NUTRIENT FLUXES IN THE NORTH PACIFIC SUBTROPICAL GYRE

The potential role of the diazotrophic cyanobacterium Trichodesmium sp p. in nitrogen and phosphorus dynamics of the euphotic zone of the Nor th Pacific subtropical gyre was investigated as one component of the H awaii Ocean Time-series (HOT) program. Experiments were conducted with natural samples collected at Stn ALOHA (22 degrees 45' N, 158 degrees W) and with isolated cultures in laboratory. In both sets of experime nts, we documented aerobic nitrogenase activity in Trichodesmium by ac etylene (C2H2) reduction to ethylene (C2H4). Although average C2H4 evo lution per unit chlorophyll a (chl a) was lower in naturally occurring single trichomes relative to colonies 13.9 vs 12.5 nmol C2H4 (mu g ch l a)(-1) h(-1), respectively], the generally greater biomass of single trichomes in the North Pacific Ocean suggests that trichomes may be i mportant in the oceanic N cycle. Disrupted colonies display the lowest nitrogenase activities, but these activities increase with time in cu ltures. These observations and the relatively high dark oxygen consump tion rates observed for Trichodesmium [0.18 mu mol O-2 (mu g chi a)(-1 ) h(-1)] suggest that, in nature, this cyanobacterium may be able to p rotect nitrogenase from oxygen inactivation, and that colony formation enhances, but is not prerequisite for, nitrogenase activity. Trichode smium spp. collected from different depth strata at Stn ALOHA were als o used to study variations in the C:N:P elemental composition of risin g and sinking colonies. Although changes in elemental ratios were smal l, the relative C:N increase in all sinking colonies and the N:P decre ase in rising colonies, sampled at approximately 100 m depth, is consi stent with the model of Trichodesmium storage of carbohydrate in shall ow waters (<20 m) and uptake of P at depth. The active uptake of inorg anic phosphorus measured in sinking colonies incubated in the dark com bined with a change toward positive buoyancy in colonies during the in cubation supports the hypothesis that vertical migrations of Trichodes mium may represent an upward transport of P into the euphotic zone and a potential decoupling of N and P nutrient cycles. However, these res ults do not explain the large concentration of non-migratory single tr ichomes observed in the upper water column of Stn ALOHA, unless colony versus free trichome morphology is a transient condition that is unde r cellular control.