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General Session 3: Biological oceanography & global change |
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Light effects on N2 fixation and diazotroph derived nitrogen net release of Trichodesmium in Western Pacific and South China Sea
Monday 9th @ 1445-1505
Multi-function Hall
Yangyang Lu* , State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Presenter Email: luyangyang1826@163.com
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| Nitrogen fixation is a crucial pathway to supply bioavaliable nitrogen in surface ocean and thus supporting primary productivity. Light is a major controlling factor of nitrogen fixation in marine ecosystems but has been overlooked in terms of light response curve (PI curve) of N2 fixation and light effect on diazotroph derived nitrogen (DDN) net release. We use uncontaminated 15N2 gas dissolution method to examine the influence of light intensity on N2 fixation and DDN net release at four stations in Western Pacific and South China Sea where cyanobacterium Trichodesmium spp. was dominant diazotroph. The light effect on carbon fixation was measured parallel using 13C method in the bloom station.The PI cruves showed N2 fixation rates reached half-saturation with light intensities ranged between 330 and 510 μmol E/m2/s. N2 fixation rates were saturated at around 600μmol E/m2/s. The proportion of DDN net release showed an increase trend with the decline of irradiance from 7%±0.1% to 41%±5%. At the Trichodesmium bloom station, light intensity of half saturation was at 405 and 455 μmol E/m2/s for N2 fixation and carbon fixation and the light intensity of carbon compensation point was at 300 μmol E/m2/s. The C N metabolism of Trichodesmium was also controlled by light intensity. We suggested that there are metabolism tradeoff between carbon and nitrogen fixation pathway under light limitiation. N2 fixation pathway might be preferentially cut off under relatively lower light intensity. Overall, our results suggested that light requirements for Trichodesmium N2 fixation was relatively high and DDN net release fraction showed a increase tendency as the light intensity decrease. |
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