Effect of light on N-2 fixation and net nitrogen release of Trichodesmium in a field study

Dinitrogen fixation (NF) by marine cyanobacteria is an important pathway to replenish the oceanic bioavailable nitrogen inventory. Light is the key to modulating NF; however, field studies investigating the light response curve (NF-I curve) of NF rate and the effect of light on diazotrophderived nitrogen (DDN) net release are relatively sparse in the literature, hampering prediction using models. A dissolution method was applied using uncontaminated N-15(2) gas to examine how the light changes may influence the NF intensity and DDN net release in the oligotrophic ocean. Experiments were conducted at stations with diazotrophs dominated by filamentous cyanobacterium Trichodesmium spp. in the western Pacific and the South China Sea. The effect of light on carbon fixation (CF) was measured in parallel using the C-13 tracer method specifically for a station characterized by Trichodesmium bloom. Both NF-I and CF-I curves showed a I-k (light saturation coefficient) range of 193 to 315 mu Em(-2) s(-1), with light saturation at around 400 mu Em(-2) s(-1). The proportion of DDN net release ranged from similar to 6 to similar to 50 %, suggesting an increasing trend as the light intensity decreased. At the Trichodesmium bloom station, we found that the CF/NF ratio was light-dependent and the ratio started to increase as light was lower than the carbon compensation point of 200 mu Em(-2) s(-1). Under low-light stress, Trichodesmium physiologically preferred to allocate more energy for CF to alleviate the intensive carbon consumption by respiration; thus, there is a metabolism tradeoff between CF and NF pathways. Results showed that shortterm (< 24 h) light change modulates the physiological state, which subsequently determined the C/N metabolism and DDN net release by Trichodesmium. Reallocation of energy associated with the variation in light intensity would be helpful for prediction of the global biogeochemical cycle of N by models involving Trichodesmium blooms.