AME 59:273-282 (2010)  -  DOI: https://doi.org/10.3354/ame01397

ABSTRACT: We investigated the distribution of nano-sized unicellular Cyanobacteria (nanocyanobacteria) in the Pacific Ocean. Nanocyanobacteria were distributed in the mixed layer throughout the western and central regions of the North and South Pacific subtropical gyres, where nitrate depletion and high irradiance are maintained throughout the year. Their maximum occurrence of >5 × 10⁹ cells m^–2 was found between 12 and 27°N and 10 and 21°S. These areas with high nanocyanobacterial abundance largely correspond with those with high nitrogen fixation activity. In particular, the biomass of nanocyanobacteria was comparable with or larger than Synechococcus between 10 and 20°N, while in the other regions, Synechococcus was overwhelmingly dominant. Where phosphorus was exhausted at the surface, they were comparatively sparse, even within the western subtropical gyres. In the subtropical and tropical provinces other than the western subtropical gyres, nanocyanobacteria were very sparse, even under similar nutrient conditions where nitrate was depleted and phosphorus was detectable. They may be competing with pico-sized Synechococcus and large filamentous diazotrophic Cyanobacteria Trichodesmium for various resources. Their patchy distribution suggests that the mechanism of nitrogen cycling also varies patchily in the oligotrophic region of the Pacific Ocean.

KEY WORDS: Picophytoplankton · Nanophytoplankton · Cyanobacteria · Synechococcus · Pacific subtropical gyres · Cross-basin distribution · Nitrogen fixation