Abstract
Diazotrophs or N(2)-fixers are one of the most ecologically significant groups in marine ecosystems (pelagic and benthic). Inorganic phosphorus (PO(4) (3-)) 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 unicellular Halothece sp. (PCC 7418) and the filamentous Fischerella muscicola (PCC 73103), grown under different nitrate (NO(3) (-)) 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 PO(4) (3-) (with NO(3) (-) as the primary limiting nutrient) for Halothece sp. 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 of Halothece sp. Nutrient limitation (PO(4) (3-), Fe, and/or NO(3) (-)) 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 of Halothece sp. as a model, showing the potential of this strain to adapt to different nutrient regimes (P, Fe, or N).