Bacterial and chemical iron oxide deposition in a shallow bay on Palaea Kameni, Santorini, Greece: Microscopy, electron probe microanalysis, and photometry of in situ experiments

The present study was aimed at determining whether microorganisms mediate iron oxide deposition in a shallow bay (Iron Bay) on the still volcanically active island of Palaea Kameni, Santorini, Greece, north of Crete in the Aegean Sea. The results of in situ experiments showed that iron oxide deposition in this marine environment occurs at an exceptionally high rate. It was also found that deposition in the central part of the bay proceeded within specific vertical zones in the water column, showing that the bay may be regarded as a stratified system. Using light and scanning electron microscopic methods, including electron probe microanalysis, it was shown that deposition (binding and oxidation) of Fe(II) was mediated by bacteriological processes, which differed in intensity depending on their specific location in the bay. The most intensive iron oxidations and depositions were found in the hot volcanic springs at the toe of the bay, where rates more than 60 times those found in freshwater iron-oxidizing environments were measured. Bacteria that excreted extracellular slime (EPS), which apparently catalysed the process and itself became encrusted with tiny globules of hydrated iron(III) oxide, were responsible for this extremely high rate of iron deposition. The same process of oxidation catalysed by bacterial EPS was also responsible for iron oxide deposition in the main oxidation zone in the central part of the bay. Thus, precipitation of Fe(II) in Iron Bay is not, as was previously assumed, a purely chemical, Eh- and pH-dependent process, but results from the interaction of water chemistry with a high level of bacterial activity, especially surface reactions on bacterial excretion products.