Trichodesmium spp. physiology and nutrient fluxes in the North Pacific subtropical gyre

Abstract

The potential role of the diazotrophic cyanobactenum Trichodesn~ium 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 13.9 vs 12.5 nmol C2H, (pg chl a)-' h-', respectively], the generally greater blomass 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 pm01 O2 (pg chl a)-' h-'] 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 wlth the model of Tr~chodesmium 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.

Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

“…comm.). It is generally accepted that diatoms are 425 characterized by a low PON:POP ratio , which could contribute to reducing the observed PON excess in comparison with the ratios measured by Karl et al (1992) and Letelier and Karl (1998).…”

“…This difference could be explained because PON:POP ratio of 125 observed by Karl et al (1992) is an exceptionally high value related to an intense bloom that probably does not reflect the general relationships associated with diazotrophs growth. Letelier and Karl (1998) observed mean ratios associated with Trichodesmium sp.…”

Modeled contributions of three types of diazotrophs to nitrogen fixation at Station ALOHA

“…More generally, temporal and spatial sampling resolution as well as other model assumptions and parameter choices also affected the accuracy of the modeled results. The relatively wellcharacterized Trichodesmium is well established as an important contributor to N 2 fixation in subtropical and tropical ocean waters (Capone et al, , 2005Letelier and Karl, 1998;Capone and Carpenter, 1999). This study attempts to quantify the impact of relatively uncharacterized, smaller diazotrophs to this process.…”

“…The filamentous, nonheterocyst-forming cyanobacterium Trichodesmium is thought to be responsible for the majority of N 2 fixation, and therefore a major source of nitrogen, in the photic zone of oligotrophic marine ecosystems (Capone et al, , 2005Letelier and Karl, 1998;Capone and Carpenter, 1999). These relatively large microorganisms can form colonies that are visible to the unaided eye.…”

The effect of growth rate, phosphorus concentration, and temperature on N2 fixation, carbon fixation, and nitrogen release in continuous cultures of Trichodesmium IMS101

“…Trichodesmium occur throughout the upper water column and frequently amass as large surface aggregations. High rates of N 2 fixation have been observed in surface populations (Capone et al 1997), whereas lower N 2 fixation rates are characteristic of cells collected from depth and of cells incubated at low light levels (Letelier and Karl 1998). Because photosynthesis supplies energy and C skeletons for N uptake and assimilation, the availability of light and low photosynthetic rates might limit N 2 fixation at depth.…”