title: DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2 -FIXATION RATE, BIOMASS, AND C:N:P STOICHIOMETRY(1).
abstract: 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.
authors.affiliation: School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
authors.forename: Carolyn M
authors.initials: CM
authors.lastname: Holl
authors.affiliation: School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Montoya
keywords: Trichodesmium; continuous culture; diazotroph; growth rate; nitrogen fixation