The role of microbial populations in the cycling of iron and manganese from marine aggregates

Marine aggregates play an important role in the cycling of carbon, nutrients and trace<br/>metals. Within aggregates the oxygen depleted by aerobic microbial respiration may<br/>not be replaced rapidly, generating anoxic or suboxic microzones. Reduced<br/>compounds that are unstable in the oxygenated water column have been previously<br/>found associated with marine snow. Therefore, in the experiments described in this<br/>thesis, artificial aggregates were made in the laboratory from senescent phytoplankton<br/>material and incubated to investigate the role of the associated microbial populations<br/>to the biogeochemical redox cycling of iron and manganese and to the degradation of<br/>organic matter.<br/>The release of dissolved iron from artificial aggregates which did not contain any<br/>measurable (~10 ?m) anoxic microzones, was demonstrated under dark conditions.<br/>The rate of release was controlled by the amount of reducible Fe(III) available, and<br/>appears to be limited by the competing oxidation of Fe(II). Moreover highly significant<br/>releases in reduced Mn were detected from aggregates incubated under a constant<br/>velocity shear, although the same aggregates did not affect the dissolution of iron. A<br/>possible reason is likely associated with the higher stability of Mn(II), compared to<br/>Fe(II) in aerobic environments.<br/>Molecular (16S rRNA gene) analyses showed the bacterial community<br/>associated with artificial aggregates to be similar to that found in natural aggregates<br/>and dominated by (predominantly uncultured) ?- and ?-Proteobacteria, Bacteroidetes,<br/>Planctomycetes and Cyanobacteria. It was possible to culture NO3<br/>--, Fe(III)- and<br/>Mn(IV)-reducing bacteria from the artificial aggregates, and marine particles incubated<br/>with Fe(III) under anaerobic conditions contained a range of ?- and ?-Proteobacteria<br/>known to respire Fe(III) and in most cases Mn(IV). Moreover several microorganisms<br/>belonging to ?-Proteobacteria were isolated from marine aggregates and strains<br/>affiliated to the genera Amphritea, Marinobacterium and Marinobacter, were<br/>demonstrated to grow through the reduction of Fe(III), with Marinobacter also capable<br/>of respiring Mn(IV). Whilst the precise mechanism of reduction is not clear, it is evident<br/>that marine aggregates can be a source of Fe(II) and dissolved Mn, in coastal waters<br/>and most probably other natural water systems.<br/>Fatty acid analyses revealed the prevalence of saturated over unsaturated fatty<br/>acids indicating that aggregates were already partially degraded when incubation<br/>started. Nonetheless, the lipids in the artificial aggregates were rapidly degraded<br/>further as indicated by a depletion in short chain (&lt;20) saturated and<br/>monounsaturated fatty acids. In contrast, the concentrations of linear and branched,<br/>saturated long chain (&gt;20) fatty acids fluctuated, suggesting that some of these lipids<br/>could have been produced in situ by marine microorganisms rather than deriving from<br/>II<br/>higher plant debris. In addition, a bacterial branched monounsaturated fatty acid (11-<br/>methyl-octadecenoic acid), which has not previously been found in marine particles<br/>was present in artificial aggregates. Roseobacter litoralis found among the aggregateattached<br/>bacteria contains 11-methyl-octadecenoic acid, and other bacteria present in<br/>artificial aggregates have the potential to produce long-chain saturated and<br/>polyunsaturated fatty acids. Thus, the fatty acid assemblage appears to reflect both<br/>organic matter degradation, including selective preservation, but also changes in the<br/>microbial assemblage.<br/>A range of future studies are suggested to elucidate the mechanisms for Fe(III)<br/>and Mn(IV) reduction in aggregates. These include microscale analyses of dissolved<br/>species and evaluation of the presence of metal binding ligands associated with<br/>aggregates. Moreover it is important to assess the activity of the Fe(III)- and Mn(IV)-<br/>reducing bacteria present in aggregates in situ and the production of long chain fatty<br/>acids in degrading aggregates.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Balzano, Sergio