Potential Impacts of N2-fixing Trichodesmium on Heterotrophic Bacterioplankton Turnover Rates and Organic Carbon Transfer Efficiency in the Subtropical Oligotrophic Ocean System


Bacterial production (BP), primary production (PP) and N2-fixing cyanobacteria, Trichodesmium spp. abundance (i.e., [Tricho#]) were measured in the N. Pacific west-boundary Kuroshio Current in summer and autumn, 2001. The warm (28 - 29°C) upper water column (0 - 30 m) in summer was low in nitrate (NO3; 0.08 ± 0.08 µM) with Trichodesmium spp. (192 - 2,115 trichomes L−1 ) observed at all sampling stations. Averaged NO3 in the upper 30 m were higher (0.39 ± 0.27 µM) in autumn but with no Trichodesmium observed except 1 station with an abundance of 962 trichomes L−1 in surface waters. PP (21 ± 7 µgC L-1 d-1 ) and phytoplankton turnover rate (Pµ ; 0.48 ± 0.19 d-1 ) in summer were at least 1.5-fold higher than those recorded in autumn. BP (summer, 2.5 ± 0.6 µg C L-1 d-1 ; autumn, 2.4 ± 0.3 µg C L-1 d-1 ) showed no seasonal difference, but bacterial turnover rate (Bµ ; 0.11 ± 0.03 d-1 ) was 57% higher iegative trend of [Tricho#] vs. NO3 and higher transmittance in summer indicate that NO3 depletion and better light availability might trigger the development of N2-fixing cyanobacteria. Bµ  changed positively with [Tricho#] suggesting that bacterial growth might be enhanced by the supply of "new" inorganic (i.e., NH4) and/or dissolved organic nitrogen from N2-fixing cyanobacterium. From the negative relationship of BP : PP ratio (7 - 21%) vs. [Tricho#], we deduce that the occurrence of high Trichodesmium abundance might potentially reduce the magnitude of NH4 competition between algae and bacteria. In the open ocean, organic carbon transfer efficiency, either to higher trophic levels or to system exportation, can be

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