Resolving the fate of trace metals during microbial remineralization of phytoplankton biomass in precursor banded iron formation sediments

Banded Iron Formations (BIFs) have long been considered a sedimentary record of seawater trace metal composition during the Precambrian. However, recent work has suggested that the trace metal composition of BIFs was derived from phytoplankton biomass, not seawater. In this model, phytoplankton biomass settles from the photic zone to the seafloor sediments, where it is then oxidized by heterotrophic microbes, such as dissimilatory Fe(III) reducing (DIR) bacteria, for energy generation. Remineralization of this biomass released the trace metals associated with organic molecules from phytoplankton (i.e., in metalloproteins), allowing these metals to be captured by Fe (oxyhydr)oxides and preserved in BIFs. While there is compelling evidence that the phytoplankton biomass served as a trace metal shuttle to precursor BIF sediments, it is unclear whether the degradation of biomass by DIR bacteria would liberate the biogenic trace metals as the model proposes. This work tests this hypothesis by using anoxic incubations of a model DIR bacterium (Shewanella oneidensis MR-1) with phytoplankton biomass as energy and carbon sources and ferrihydrite, a poorly crystalline Fe(III) oxyhydroxide (Fe(OH)3), as electron acceptor. Our results show that while S. oneidensis MR-1 can consume some of the carbon substrates found in phytoplankton biomass, there is no evidence that S. oneidensis MR-1 degraded metalloproteins which would have liberated trace metals. In the context of the Precambrian, these data imply that other heterotrophic bacteria, such as fermenters, may have had a larger role in the liberation of trace metals from dead biomass during early BIF development.