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

• Published in: Marine ecology. Progress series (Halstenbek) 2011-01, Vol.421, p.25-38
• Main Authors: Chen, Yuh-ling Lee, Tuo, Sing-how, Chen, Houng-Yung
• Format: Article
• Language: English
• Subjects: Diatoms; Nitrogen; Ocean currents; Oceans; Phytoplankton; Plankton; Sea water; Seas; Seasons; Surface water
• ISSN: 0171-8630; 1616-1599

Actively growingTrichodesmiumspp. 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 ofTrichodesmiumspp. and other co-occurring cells, i.e. diatoms, have never been well documented. We conducted laboratory15N tracer experiments to investigate the trophic interactions ofT. erythraeumand diatoms in culture. Field studies were carried out to examine spatial-temporal distributions ofTrichodesmiumspp. and diatoms in the low-latitude Kuroshio Current and the neighboring northern South China Sea (SCS). Enrichment of15N occurred in 3 species of cultured diatoms when they were incubated withT. erythraeumIMS101 in culture media enriched with gaseous15N₂, which confirmed the transfer of newly fixed N fromT. erythraeumto diatoms. Field results showed that in the Kuroshio, whereTrichodesmiumspp. flourished in the warm seasons, the abundances ofTrichodesmiumspp. 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 ofTrichodesmiumspp. and a second abundance maximum near the nitracline. The diatom surface-abundance maximum disappeared in the cold seasons, when theTrichodesmiumspp. population languished. The bimodal distribution of diatoms and the co-occurrence of diatom andTrichodesmiumspp. abundance maxima recurred asTrichodesmiumspp. abundance increased when the warm seasons returned. In contrast to the Kuroshio, the SCS, inhabited sparsely byTrichodesmiumspp., showed no surface abundance maxima in the vertical distributions ofTrichodesmiumspp. or diatoms. Both field and laboratory data suggest that the co-occurrence ofTrichodesmiumspp. and diatoms might be driven by the transfer of N fixed byTrichodesmiumspp. to diatoms in the N-limited oligotrophic ocean. Because diatoms are more effective thanTrichodesmiumspp. in exporting N and carbon to depth, the N exudation byTrichodesmiumspp. that supports diatom growth could represent an important pathway of N and carbon export to depth in tropical and subtropical oceans.