Role of microbial populations in the release of reduced iron to the water column from marine aggregates

The release of dissolved iron from artificial aggregates formed from oxic natural coastal water and senescent phytoplankton material was demonstrated under dark conditions. The rate of release was controlled by the amount of reducible Fe(III) available, and appears to be limited by the competing oxidation of Fe(II). Molecular (16S rRNA gene) analyses showed the bacterial community associated with the aggregates, originating from estuarine water, to be dominated by oxic heterorophs. However, it was possible to culture NO3- and Fe (III)-reducing bacteria from the artificial aggregates, and marine particles incubated with Fe(III) under anaerobic conditions contained microorganisms belonging to the genera Desulfovibrio and Marinobacter, bacteria known to reduce Fe(III). Whilst the precise mechanism of reduction is not clear, it is evident that marine aggregates can be a source of Fe(II), and thus ultimately iron in other forms, in coastal waters and most probably other natural water systems.