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 abundance of N2-fixing cyanobacterium, Trichodesmium contortum（[Tricho#]）were measured in the North Pacific west-boundary Kuroshio Current in summer and autumn, 2001. The warm（28-29 ℃）upper water column（0-30 m）in summer was low in nitrate（NO3da；0.08 ± 0.08 mM）with T. contortum（192-2,115 trichos L-1）observed at all 5 sampling stations. Averaged NO3 in the upper 30m were higher（0.39 ± 0.27 mM）in autumn but with no T. contortum observed except 1 station with a [Tricho#] of 962 trichos/L in surface waters. PP（21 ± 7 mgC L-1 d-1）and algal turnover rate（Pm；0.48 ± 0.19 d-1）in summer were at least 1.5-folds of those recorded in autumn. BP（summer, 2.5 ± 0.6 mgC L-1 d-1；autumn, 2.4 ± 0.3 mgC L-1 d-1）showed no seasonal difference but with bacterial turnover rate（Bm；0.11 ± 0.03 d-1）57% higher in summer. The negative trend of [Tricho#] v.s. NO3da and higher transmittance in summer indicate that NO3 depletion and better light availability might trigger the development of T. contortum. Bm changed positively with [Tricho#] suggesting that bacterial growth might to 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%）v.s. [Tricho#], it is deduced that high [Tricho#] might potentially reduce the magnitude of NH4+ competition between algae and bacteria. In open ocean, organic carbon transfer efficiency, either to higher trophic levels or to system exportation, can be greatly enhanced at low BP：PP ratio driven by the prevalence of N2-fixing cyanobacterium.