Physiological and elemental changes of Trichodesmium in response to growth limitation by phosphorus, iron and zinc

Trichodesmium spp. is a colonial diazotrophic cyanobacterium found in the oligotrophic (sub)tropical oceans, where dissolved inorganic phosphorus (DIP) can be depleted. To cope with low P concentrations, P can be scavenged from the dissolved organic P (DOP) pool. This requires deployment of multiple enzymes that are activated by trace metals such as iron (Fe) and zinc (Zn), as the concentrations of these metals in the open ocean can often be extremely low and their physicochemical speciation further constrains their bioavailability, the potentially enhanced trace element requirements for P acquisition enzymes under stronger P limitation has the potential to exert control on the growth of Trichodesmium and thereby impact global nitrogen and carbon cycles. In this thesis, I applied a continuous culture technique under trace-metal-controlled conditions to vary the supply of DIP, DOP, Fe and Zn to test the impact of the relative availability of P sources, Fe, and Zn availability on the growth and physiological changes in Trichodesmium. Firstly, this study revealed 3.5-fold elevated Zn requirements of Trichodesmium when MPA was used as the sole source of P in comparison to growth on DIP. Secondly, this study provided for the first time the elemental stoichiometry of Trichodesmium grown under Fe limitation and Fe-P co-limitation conditions and revealed the response of Trichodesmium to nutrient limitation from the perspective of metabolite changes. Thirdly, this study revealed ~10-fold elevated nickel (Ni) requirement of Trichodesmium under Zn limitation. Together, the research presented in this thesis demonstrates that Trichodesmium can significantly change its elemental stoichiometry in response to different elemental limitation conditions. Ultimately, these data will provide a useful resource for ocean biogeochemical models that may in the future resolve P, Fe and Zn.