Photosynthetic characteristics ofTrichodesmiumin the southwest Pacific Ocean: importance and significance

The photosynthetic capacities ofTrichodesmiumwere investigated in a multidisciplinary 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 photosynthesis in natural communities ofTrichodesmiumusing an O₂ electrode with the addition of a14C-tracer gave a mean photosynthetic quotient of 1.19, quite close to the theoretical value. Seven photosynthesis vs. irradiance curves exhibited typically high light-saturated and compensation photosynthetic parametersI kandI c(327 and 77 μE m–2s–1, respectively), implying thatTrichodesmiumrequires a stronger irradiance for growth than other phytoplankton typical of oligotrophic systems. The vertical profiles ofin situproductivity ofTrichodesmiumgenerally 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 productivity data and the mean measured C:chlaratio of 188 g C g chla –1, the maximumTrichodesmiumgrowth 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 ofTrichodesmiumcolonies 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 ofTrichodesmiumpopulations, which tends to be minimum in winter and spring and maximum in summer.