Marine aggregates play an important role in the cycling of carbon, nutrients and trace metals. Within aggregates the oxygen depleted by aerobic microbial respiration may not be replaced rapidly, generating anoxic or suboxic microzones. Reduced compounds that are unstable in the oxygenated water column have been previously found associated with marine snow. Therefore, in the experiments described in this thesis, artificial aggregates were made in the laboratory from senescent phytoplankton material and incubated to investigate the role of the associated microbial populations to the biogeochemical redox cycling of iron and manganese and to the degradation of organic matter. The release of dissolved iron from artificial aggregates which did not contain any measurable (~10 μm) anoxic microzones, 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). Moreover highly significant releases in reduced Mn were detected from aggregates incubated under a constant velocity shear, although the same aggregates did not affect the dissolution of iron. A possible reason is likely associated with the higher stability of Mn(II), compared to Fe(II) in aerobic environments. Molecular (16S rRNA gene) analyses showed the bacterial community associated with artificial aggregates to be similar to that found in natural aggregates and dominated by (predominantly uncultured) �- and �-Proteobacteria, Bacteroidetes, Planctomycetes and Cyanobacteria. It was possible to culture NO3 --, Fe(III)- and Mn(IV)-reducing bacteria from the artificial aggregates, and marine particles incubated with Fe(III) under anaerobic conditions contained a range of �- and �-Proteobacteria known to respire Fe(III) and in most cases Mn(IV). Moreover several microorganisms belonging to �-Proteobacteria were isolated from marine aggregates and strains affiliated to the genera Amphritea, Marinobacterium and Marinobacter, were demonstrated to grow through the reduction of Fe(III), with Marinobacter also capable of respiring Mn(IV). Whilst the precise mechanism of reduction is not clear, it is evident that marine aggregates can be a source of Fe(II) and dissolved Mn, in coastal waters and most probably other natural water systems. Fatty acid analyses revealed the prevalence of saturated over unsaturated fatty acids indicating that aggregates were already partially degraded when incubation started. Nonetheless, the lipids in the artificial aggregates were rapidly degraded further as indicated by a depletion in short chain (<20) saturated and monounsaturated fatty acids. In contrast, the concentrations of linear and branched, saturated long chain (>20) fatty acids fluctuated, suggesting that some of these lipids could have been produced in situ by marine microorganisms rather than deriving from II higher plant debris. In addition, a bacterial branched monounsaturated fatty acid (11- methyl-octadecenoic acid), which has not previously been found in marine particles was present in artificial aggregates. Roseobacter litoralis found among the aggregateattached bacteria contains 11-methyl-octadecenoic acid, and other bacteria present in artificial aggregates have the potential to produce long-chain saturated and polyunsaturated fatty acids. Thus, the fatty acid assemblage appears to reflect both organic matter degradation, including selective preservation, but also changes in the microbial assemblage. A range of future studies are suggested to elucidate the mechanisms for Fe(III) and Mn(IV) reduction in aggregates. These include microscale analyses of dissolved species and evaluation of the presence of metal binding ligands associated with aggregates. Moreover it is important to assess the activity of the Fe(III)- and Mn(IV)- reducing bacteria present in aggregates in situ and the production of long chain fatty acids in degrading aggregates.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:525722 |
| Date | January 2009 |
| Creators | Balzano, Sergio |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/168945/ |
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