Trace metal uptake by Pichia spartinae, an endosymbiotic yeast in the salt marsh cord grass Spartina alterniflora
The ascosporogenous marine yeast Pichia spartinae is a dominant endosymbiont of the marsh grass Spartina alterniflora. Results of previous studies suggested that P. spartinae is involved in iron transport processes in the grass. Of particular interest has been the mechanisms of metal uptake and metabolism by the yeast, and the ecological and plant biochemical significance of these processes. This investigation examined the uptake of iron and other metals (Zn, Cu, Cd, Ni, Pb, Cr) by P. spartinae, and provides data on possible mechanisms of this activity. The results suggest a) the yeast can assimilate divalent and trivalent forms of inorganic iron, as well as large organic-Fe(III) complexes, b) the uptake of inorganic trivalent iron under soluble iron-deficient conditions proceeded by a different mechanism than that of soluble Fe(II) , with intracellular loadings of iron much increased under the former conditions; c) trivalent iron uptake is not mediated by hydroxamate siderophores at levels detectable by sensitive screening assays; d) the assimilation of some trace metals (Cu, Zn, Cd, Ni) is likely to be mediated by low molecular weight cysteine rich proteins, possibly metallothionein, and; e) siderophores from other fungi can provide iron for P. spartinae. The iron assimilation data suggested that multiple mechanisms are involved, and are influenced by the concentration and speciation of iron in the system. In general, iron assimilation mechanisms are comparable to those described for closely related yeasts, such as Sacckaromyces cerevisiae. Among other things, these results indicated that future studies of trace metal mobilization and plant assimilation in salt marsh ecosystems must account for the activities of microbial symbionts associated with the plants.
Catallo, WJ; Henk, W; Younger, L; Mills, O; Thiele, DJ; Meyers, SP
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