Differential Effects of Varying Concentrations of Phosphorus, Iron, and Nitrogen in N2-Fixing Cyanobacteria

Diazotrophs or N-2-fixers are one of the most ecologically significant groups in marine ecosystems (pelagic and benthic). Inorganic phosphorus (PO43-) and iron (Fe) can limit the growth and N-2-fixing capacities of cyanobacteria. However, studies investigating co-limitation of these factors are lacking. Here, we added different concentrations of PO(4)(3-)and Fe in two cyanobacterial species whose relatives can be found in seagrass habitats: the unicellularHalothecesp. (PCC 7418) and the filamentousFischerella muscicola(PCC 73103), grown under different nitrate (NO3-) concentrations and under N(2)as sole N source, respectively. Their growth, pigment content, N-2-fixation rates, oxidative stress responses, and morphological and cellular changes were investigated. Our results show a serial limitation of NO(3)(-)and PO43-(with NO(3)(-)as the primary limiting nutrient) forHalothecesp. Simultaneous co-limitation of PO(4)(3-)and Fe was found for both species tested, and high levels of Fe (especially when added with high PO(4)(3-)levels) inhibited the growth ofHalothecesp. Nutrient limitation (PO43-, Fe, and/or NO3-) enhanced oxidative stress responses, morphological changes, and apoptosis. Furthermore, an extensive bio-informatic analysis describing the predicted Pho, Fur, and NtcA regulons (involved in the survival of cells to P, Fe, and N limitation) was made using the complete genome ofHalothecesp. as a model, showing the potential of this strain to adapt to different nutrient regimes (P, Fe, or N).