UC San Diego
The role of siderophores in algal-bacterial interactions in the marine environment
- Author(s): Amin, Shady Ahmed
- et al.
Iron is arguably the most important micronutrient in the marine environment due to its involvement in many vital physiological processes, such as photosynthesis, oxygen transport, electron transfer and nitrogen fixation. However, the availability of iron is hampered by its poor solubility and tendency to form biologically inaccessible colloidal and oxo-polymeric species. This lack of availability is more pronounced in the oceanic environment where the solubility of iron(III) is even lower than in most terrestrial environments and where the majority of soluble iron is complexed by organic ligands that dominate its speciation. Consequently, iron has been shown to limit microorganisms in large areas of the ocean. In response to iron limitation, many marine bacteria have been shown to produce siderophores, LMW complexing agents that possess high affinity for Fe(III). Recently, the traditional view of siderophores as sole iron chelators has been challenged by the discovery of their role in the transport of other metals and their interactions with cell-to-cell communication systems in bacteria. Chapters 2 and 3 of this dissertation discuss another unique trait of siderophores: the ability to efficiently bind borate. The high concentration of boron in the ocean (0.4 mM) and the affinity of two major siderophore families, the catecholates and the citrates, to this element suggest a possible unknown biological function. Further proteomic studies are underway to examine the importance of boron to marine bacteria. Chapter 4 investigates siderophore production of a group of marine bacteria belonging to the Marinobacter genus, some of which were isolated from lab cultures of dinoflagellates and coccolithophores, in response to iron limitation. Two algal-associated subclades of Marinobacter spp. were found to produce the photoactive siderophore vibrioferrin (VF) while non-algal associated species did not. This correlation between algal association and VF production leads to what appears to be a mutualistic relationship between both organisms where the bacteria contribute iron via the photochemistry of VF and the phytoplankton release dissolved organic matter (DOM) that support the growth of the bacteria and ultimately fuel the biosynthesis of VF. The photochemistry of VF and its influence on iron speciation is further elucidated in chapter 5, where it is shown that ferric- vibrioferrin rapidly degrades in sunlight leaving behind a highly bioavailable form of iron, Fe'. Because light is central to the photochemistry of VF and consequently to any interactions between VF-producers and phytoplankton, I sought to investigate the influence of light on gene expression of iron uptake genes in M. algicola DG893. Chapter 6 reveals a selective response of iron uptake genes related to only the metabolism of VF. In addition, environmental samples collected during a research cruise in the North Atlantic in 2009 demonstrate that VF- producers are concentrated near the surface during daytime. This pattern of vertical distribution demonstrates that although these organisms are not abundant in the ocean, they are relevant to iron biogeochemistry and further work is needed to elucidate the extent of their contribution to iron speciation and to algal iron uptake