(Georgia Institute of Technology, 2006-03-29)
Krauk, Jamie M.; Villareal, Tracy A.; Sohm, Jill A.; Montoya, Joseph P.; Capone, Douglas G.
We followed changes in N:P ratios in batch cultures of the planktonic marine cyanobacterium Trichodesmium (IMS 101) grown in 2 different media and in field populations from 4 different oceanic regions. Cultures grown on low P media showed a rapid rise in N:P ratio upon depletion of phosphate. Ratios exceeding 125 were reached in 1 experiment before attaining stationary phase. A transect across the North Atlantic Ocean along 32°N showed a monotonic decrease in the N:P ratio of field collected colonies, dropping from about 60:1 on the western side of the basin to about 30:1 on the eastern side. A second cruise sampled colonies and surface slicks in waters along the north coast of Australia, where ratios of N:P were generally lower than in the North Atlantic, ranging from 11:1 to 47:1 with an average of 22:1. A comparison of rising and sinking colonies collected at 8 stations in the Gulf of Mexico shows a higher mean N:P ratio among sinking colonies than floating colonies. Overall, the average N:P in the Gulf of Mexico was about 68:1. N:P ratios of Trichodesmium around the Hawaiian Islands were very consistent between 2 consecutive years of sampling, with an average colony N:P for both years of about 38:1. Our research demonstrates high variability in the cellular N:P in Trichodesmium both in the laboratory and in the field. Trichodesmium N:P ratio may provide an index to the relative severity of P limitation in these diazotrophs. Geochemical and ecological modeling efforts which rely on using the N:P ratio of diazotrophs in deriving nitrogen fixation rates should account for the variability of these ratios in situ.
(Georgia Institute of Technology, 2005)
Capone, Douglas G.; Burns, James A.; Montoya, Joseph P.; Subramaniam, Ajit; Mahaffey,; Gunderson, Troy; Michaels, Anthony F.; Carpenter, Edward J.
The broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth-integrated N₂ fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO₃ into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N₂ fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N₂ fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 X 10 ¹² mol N by Trichodesmium N₂ fixation alone. This input can account for a substantial fraction of the N₂ fixation in the North Atlantic inferred by several of the geochemical approaches.