ABSTRACT: The potential role of the diazotrophic cyanobacterium Trichodesmium spp. in nitrogen and phosphorus dynamics of the euphotic zone of the North Pacific subtropical gyre was investigated as one component of the Hawaii Ocean Time-series (HOT) program. Experiments were conducted with natural samples collected at Stn ALOHA (22°45'N, 158°W) and with isolated cultures in laboratory. In both sets of experiments, we documented aerobic nitrogenase activity in Trichodesmium by acetylene (C₂H₂) reduction to ethylene (C₂H₄). Although average C₂H₄ evolution per unit chlorophyll a (chl a) was lower in naturally occurring single trichomes relative to colonies [3.9 vs 12.5 nmol C₂H₄ (µg chl a)^-1 h^-1, respectively], the generally greater biomass of single trichomes in the North Pacific Ocean suggests that trichomes may be important in the oceanic N cycle. Disrupted colonies display the lowest nitrogenase activities, but these activities increase with time in cultures. These observations and the relatively high dark oxygen consumption rates observed for Trichodesmium [0.18 µmol O₂ (µg chl a)^-1 h^-1] suggest that, in nature, this cyanobacterium may be able to protect nitrogenase from oxygen inactivation, and that colony formation enhances, but is not prerequisite for, nitrogenase activity. Trichodesmium spp. collected from different depth strata at Stn ALOHA were also used to study variations in the C:N:P elemental composition of rising and sinking colonies. Although changes in elemental ratios were small, the relative C:N increase in all sinking colonies and the N:P decrease in rising colonies, sampled at approximately 100 m depth, is consistent with the model of Trichodesmium storage of carbohydrate in shallow waters (<20 m) and uptake of P at depth. The active uptake of inorganic phosphorus measured in sinking colonies incubated in the dark combined with a change toward positive buoyancy in colonies during the incubation supports the hypothesis that vertical migrations of Trichodesmium may represent an upward transport of P into the euphotic zone and a potential decoupling of N and P nutrient cycles. However, these results do not explain the large concentration of non-migratory single trichomes observed in the upper water column of Stn ALOHA, unless colony versus free trichome morphology is a transient condition that is under cellular control.

KEY WORDS: Nitrogen fixation · Trichodesmium · Nutrients · North Pacific

Published in AME Vol. 15, No. 3 (1998) on August 31
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1998