High N2-fixation rate by unicellular cyanobacteria Crocosphaera watsonii in the north-western tropical Pacific

Abstract

Microbial N2-fixation is an important source of new nitrogen in oligotrophic open oceans and important in controlling biological productivity and carbon flux. Unicellular N2-fixing cyanobacteria, Crocosphaera watsonii, have only recently been discovered and are widely distributed in tropical waters. To elucidate the abundance and contribution to N2-fixation rates, we examined different water masses of the NW Pacific during October 2007, June 2008 and June 2010 research cruises. As a result, Crocosphaera watsonii showed a high abundance in the oligotrophic tropical waters whereas comparatively limited abundance was found in the temperate regions. In the oligotrophic tropical region, up to 3,500 cells/ml abundance was observed in June which were about 1,000 times higher than other types of diazotrophs. N2-fixation rates were 49.1~2,156.6 μmolN m-2d-1 in June and 20.9~171.7μmolN m-2d-1 in October showing much variation among different water masses. Relatively high N2-fixation rates observed in June for tropical waters were mostly due to high concentrations of Crocosphaera watsonii. Based on these results, we could conclude that Crocosphaera are important nitrogen fixers in the tropical waters and may contribute a significant new nitrogen source in this ecosystem.