Photosynthetic characteristics of Trichodesmium in the southwest Pacific Ocean: importance and significance

The photosynthetic capacities of Trichodesmium were investigated in a multidiscipli- nary study comprising 9 cruises in a region of the Coral Sea, southwest Pacific Ocean, where these diazotrophic cyanobacteria are particularly abundant. Thirty specific measurements of photosynthe- sis in natural communities of Trichodesmium using an O2 electrode with the addition of a 14 C-tracer gave a mean photosynthetic quotient of 1.19, quite close to the theoretical value. Seven photosynthe- sis vs. irradiance curves exhibited typically high light-saturated and compensation photosynthetic parameters Ik and Ic (327 and 77 µE m -2 s -1 , respectively), implying that Trichodesmium requires a stronger irradiance for growth than other phytoplankton typical of oligotrophic systems. The vertical profiles of in situ productivity of Trichodesmium generally showed a maximum at 10 or 20 m depth and a lower value at the surface, the latter probably being due to photoinhibition. Based on produc- tivity data and the mean measured C:chl a ratio of 188 g C g chl a -1 , the maximum Trichodesmium growth rate ranged between 0.18 and 0.32 d -1 . The high level of energy required by these organisms to grow could explain why the vertical distribution of Trichodesmium colonies is generally restricted to well-lit surface waters. Furthermore, our observations suggest that the presence of a shallow mixed-layer is a prerequisite for an optimal light regime and a maximum growth rate for this genus. Hence, the seasonal changes in both incident radiation and water column stratification would strongly control the variations in the abundance of Trichodesmium populations, which tends to be minimum in winter and spring and maximum in summer.