Iron-stimulated N2 fixation and growth in natural and cultured populations of the planktonic marine cyanobacteria Trichodesmium spp.

In light of recent proposals that iron (Fe) availability may play an important role in controlling oceanic primary production and nutrient flux, its regulatory impact on N2 fixation and production dynamics was investigated in the widespread and biogeochemically important diazotrophic, planktonic cyanobacteria Trichodesmium spp. Fe additions, as FeCl3 and EDTA- chelated FeCl3, enhanced N2 fixation (nitrogenase activity), photosynthesis (CO2 fixation), and growth (chlorophyll a production) in both naturally occurring and cultured (on unenriched oligotrophic seawater) Trichodesmium populations. Maximum enhancement of these processes occurred under FeEDTA- amended conditions. On occasions, EDTA alone led to enhancement. No evidence for previously proposed molybdenum or phosphorus limitation was found. Our findings geographically extend support for Fe limitation of N2 fixation and primary production to tropical and subtropical oligotrophic ocean waters often characterized by Trichodesmium blooms