MEPS 421:25-38 (2011)  -  DOI:

Co-occurrence and transfer of fixed nitrogen from Trichodesmium spp. to diatoms in the low-latitude Kuroshio Current in the NW Pacific

Yuh-ling Lee Chen1,2,*, Sing-how Tuo1, Houng-Yung Chen2,3

1Department of Marine Biotechnology and Resources, 2Asia-Pacific Ocean Research Center, and 3Institute of Marine Biology, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, ROC

ABSTRACT: Actively growing Trichodesmium spp. are known to release their newly fixed nitrogen (N) into the surrounding N-limited water. Although newly released fixed N could potentially enhance the growth of nearby non-diazotrophic phytoplankton, the interactions of Trichodesmium spp. and other co-occurring cells, i.e. diatoms, have never been well documented. We conducted laboratory 15N tracer experiments to investigate the trophic interactions of T. erythraeum and diatoms in culture. Field studies were carried out to examine spatial-temporal distributions of Trichodesmium spp. and diatoms in the low-latitude Kuroshio Current and the neighboring northern South China Sea (SCS). Enrichment of 15N occurred in 3 species of cultured diatoms when they were incubated with T. erythraeum IMS101 in culture media enriched with gaseous 15N2, which confirmed the transfer of newly fixed N from T. erythraeum to diatoms. Field results showed that in the Kuroshio, where Trichodesmium spp. flourished in the warm seasons, the abundances of Trichodesmium spp. and diatom cells with >10 µm diameter were positively correlated (p < 0.05). During the warm seasons, diatom vertical distributions showed a surface-abundance maximum that coincided with the depth of highest cell densities of Trichodesmium spp. and a second abundance maximum near the nitracline. The diatom surface-abundance maximum disappeared in the cold seasons, when the Trichodesmium spp. population languished. The bimodal distribution of diatoms and the co-occurrence of diatom and Trichodesmium spp. abundance maxima recurred as Trichodesmium spp. abundance increased when the warm seasons returned. In contrast to the Kuroshio, the SCS, inhabited sparsely by Trichodesmium spp., showed no surface abundance maxima in the vertical distributions of Trichodesmium spp. or diatoms. Both field and laboratory data suggest that the co-occurrence of Trichodesmium spp. and diatoms might be driven by the transfer of N fixed by Trichodesmium spp. to diatoms in the N-limited oligotrophic ocean. Because diatoms are more effective than Trichodesmium spp. in exporting N and carbon to depth, the N exudation by Trichodesmium spp. that supports diatom growth could represent an important pathway of N and carbon export to depth in tropical and subtropical oceans.

KEY WORDS: Trichodesmium spp. · Diatoms · Kuroshio · South China Sea · N2 fixation

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Cite this article as: Lee Chen Yl, Tuo Sh, Chen HY (2011) Co-occurrence and transfer of fixed nitrogen from Trichodesmium spp. to diatoms in the low-latitude Kuroshio Current in the NW Pacific. Mar Ecol Prog Ser 421:25-38.

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