Mineral iron dissolution in Trichodesmium colonies: The role of O₂ and pH microenvironments

Colonies of the N₂-fixing cyanobacterium Trichodesmium can harbor distinct chemical microenvironments that may assist the colonies in acquiring mineral iron from dust. Here, we characterized O₂ and pH gradients in and around Trichodesmium colonies by microsensor measurements on > 170 colonies collected in the Gulf of Eilat over ∼ 2 months. O₂ concentrations and pH values in the center of single colonies decreased in the dark due to respiration, reaching minimum values of 70 μmol L⁻¹ and 7.7, whereas in the light, O₂ and pH increased due to photosynthesis, reaching maximum values of 410 μmol L⁻¹ and 8.6. Addition of dust and bacteria and increasing colony size influenced O₂ and pH levels in the colonies, yet values remained within the range observed in single natural colonies. However, lower values down to 60 μmol L⁻¹ O₂ and pH 7.5 were recorded in the dark in dense surface accumulations of Trichodesmium. Using radiolabelled ferrihydrite, we examined the effect of these conditions on mineral iron dissolution and availability to Trichodesmium. Dark-incubated colonies did not acquire iron from ferrihydrite faster than light-incubated colonies, indicating that the dark-induced decrease in pH and O₂ within single colonies is too small to significantly increase mineral iron bioavailability. Yet, ligand-promoted dissolution of ferrihydrite, a mechanism likely applied by Trichodesmum for acquiring mineral iron, did increase at the lower pH levels observed in surface accumulations. Thus, Trichodesmium surface blooms in their final stage may harbor chemical conditions that enhance the dissolution and bioavailability of mineral iron to the associated microbial community.