REEVALUATION OF THE NITROGEN FIXATION BEHAVIOR IN THE MARINE NON‐HETEROCYSTOUS CYANOBACTERIUM LYNGBYA MAJUSCULA1

Lyngbya majuscula Harvey ex Gomont is a common marine cyanobacterium in tropical and subtropical near‐shore waters. A few reports have indicated that L. majuscula fixes nitrogen only in the light. Because this feature is uncommon among non‐heterocystous cyanobacteria, we attempted a reevaluation. Nitrogenase activity, regulation, and localization were examined over diel cycles on natural populations of L. majuscula growing in subtidal zones off Zanzibar in the western Indian Ocean. The data show that L. majuscula fixed nitrogen and synthesized nitrogenase in all cells during the dark phase of a diel cycle. During the light phase, nitrogenase was degraded to undetectable levels.     

GROWTH AND NITROGEN FIXATION OF THE DIAZOTROPHIC FILAMENTOUS NONHETEROCYSTOUS CYANOBACTERIUM TRICHODESMIUM SP. IMS 101 IN DEFINED MEDIA: EVIDENCE FOR A CIRCADIAN RHYTHM1

Trichodesmium sp. IMS 101, originally isolated from coastal western Atlantic waters by Prufert-Bebout and colleagues and maintained in seawater-based media, was successfully cultivated in two artificial media. Its characteristics of growth, nitrogen fixation, and regulation of nitrogen fixation were compared to those of natural populations and Trichodesmium sp. NIBB 1067. Results indicate that the culture grown in artificial media had nitrogen fixation characteristics similar to those when the culture is grown in seawater-based medium and to those of Trichodesmium sp. in the natural habitat. The study provides practical artificial media to facilitate the physiological studies of these important diazotrophic cyanobacteria, as well as the cultivation of other Trichodesmium species in future studies. Manipulations of the light/dark cycle were performed to determine whether or not the daily cycle of nitrogen fixation is a circadian rhythm. Cultures grown under continuous light maintained the cycle for up to 6 days. We demonstrated that the daily cycle of nitrogen fixation in Trichodesmium sp. IMS 101 was at least partially under the control of a circardian rhythm.     

蓝藻Anabaena 7120的固氮放氢和同化力之间的关系

分子氮和二氧化碳对蓝藻Anabaena 7120固氮的抑制作用可因反应系统中pH值的提高以及对蓝藻进行预照光处理而削弱或消除。分子氢对经预照光处理的蓝藻固氮活性不但不支持,且有削弱。预暗处理的效应恰好相反。蓝藻经低温(4℃)预处理后,分子氢对其固氮活性支持减弱,甚至抑制。蓝藻放氢对分子氢和同化力水平的反应规律在趋势上与固氮基本相同。     

NITROGEN UTILIZATION AND METABOLISM RELATIVE TO PATTERNS OF N2 FIXATION IN CULTURES OF TRICHODESMIUM NIBB1067

We measured uptake kinetics for four combined N sources, ambient rates of N uptake and N₂ fixation, glutamine synthetase activity (transferase and biosynthetic), and concentrations of intracellular pools of glutamate (glu) and glutamine (gln) in cultures of Trichodesmium NIBB1067. N dynamics and metabolism were examined to assess the relative importance of N₂ fixation and N uptake to Trichodesmium nutrition. Comparisons were made between cultures grown on medium without added N, with excess NO, or with excess urea. Of the combined N sources tested, Trichodesmium NIBB1067 had the highest affinity for NH; high uptake capacities for NH, urea, and glu; and little capacity for NO uptake. In cultures grown on medium without added N, NH accumulated in the medium during growth, resulting in high NH uptake rates relative to rates of N₂ fixation. Glu uptake rates were low but consistent throughout the diel period. In cultures grown on excess NO or urea, uptake of these compounds supplied the majority of the daily N demand, although some N₂ fixation occurred during the light period. NO uptake rates were reduced when N₂-fixation rates were high. In all of the cultures, the highest gln/glu ratios and the lowest glutamine synthetase transferase/biosynthetic ratios were observed during the period when rates of total N uptake were highest. In cultures growing exponentially on medium without added N, N₂ fixation accounted for 14%– 16% of the total daily N uptake. Uptake of NH and glu, presumably regenerated within the culture vessels, represented 84%–86% of the daily N uptake. Because these systems were closed, net growth was constrained by the rate at which N₂ could be fixed into the system. However, total daily N turnover was greater than that necessary to accommodate the observed increase in culture biomass. The rapid N turnover rates observed in these cultures may support gross productivity and balance the high rates of C fixation observed in natural populations of Trichodesmium.     

DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2‐FIXATION RATE,BIOMASS, AND C:N:P STOICHIOMETRY1

Trichodesmium N₂ fixation has been studied for decades in situ and, recently, in controlled laboratory conditions; yet N₂‐fixation rate estimates still vary widely. This variance has made it difficult to accurately estimate the input of new nitrogen (N) by Trichodesmium to the oligotrophic gyres of the world ocean. Field and culture studies demonstrate that trace metal limitation, phosphate availability, the preferential uptake of combined N, light intensity, and temperature may all affect N₂ fixation, but the interactions between growth rate and N₂ fixation have not been well characterized in this marine diazotroph. To determine the effects of growth rate on N₂ fixation, we established phosphorus (P)–limited continuous cultures of Trichodesmium, which we maintained at nine steady‐state growth rates ranging from 0.27 to 0.67 d^?1. As growth rate increased, biomass (measured as particulate N) decreased, and N₂‐fixation rate increased linearly. The carbon to nitrogen ratio (C:N) varied from 5.5 to 6.2, with a mean of 5.8 ± 0.2 (mean ± SD, N = 9), and decreased significantly with growth rate. The N:P ratio varied from 23.4 to 45.9, with a mean of 30.5 ± 6.6 (mean ± SD, N = 9), and remained relatively constant over the range of growth rates studied. Relative constancy of C:N:P ratios suggests a tight coupling between the uptake of these three macronutrients and steady‐state growth across the range of growth rates. Our work demonstrates that growth rate must be considered when planning studies of the effects of environmental factors on N₂ fixation and when modeling the impact of Trichodesmium as a source of new N to oligotrophic regions of the ocean.