The chemical properties and microbial communities of Gulf of Finland sediments differing in their oxygen status

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Title: The chemical properties and microbial communities of Gulf of Finland sediments differing in their oxygen status
Author: Dia, Nay
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Food and Environmental Sciences
Publisher: Helsingfors universitet
Date: 2016
Language: eng
Thesis level: master's thesis
Discipline: Mark- och miljövetenskap
Environmental Soil Science
Maaperä- ja ympäristötiede
Abstract: The interactions between sediment chemistry and bacterial communities are multidirectional and complex. A hypoxia-driven decrease in dissolved oxygen (O2) leads to changes in sediment chemistry, bacterial community and ultimately alters their interactions. The sediment components very sensitive to changes in O2 are iron (Fe) and manganese (Mn) oxides. When O2 content decreases, they will be reduced by bacteria in order to obtain energy. This study was carried out with sediment samples collected from sites of different oxygen status in the Gulf of Finland. The focus was to unravel the interactions between sediment chemistry and bacterial communities by means of chemical extractions distinguishing between Fe and Mn pools of different solubility and, thus, bioavailability. For this purpose, a two-step chemical extraction was carried out in order to selectively quantify the easily reducible and more crystalline fractions of Fe and Mn oxides. The chemistry of phosphorus (P) is intrinsically linked to Fe and aluminum (Al) oxides but not to Mn oxides which are not able to retain P. Unlike Fe and Mn, Al is not a redox-sensitive element but its oxides are of importance in controlling the release of P from sediments by resorption of P. The extracts were analyzed for Fe, Mn, Al and P. Furthermore, a next generation high-input method was used to extract the DNA from the sediments. The results of the chemical extractions and taxonomical classification of the bacteria were statistically analyzed. Subsequently, the interactions between the easily-reducible fractions of Fe and Mn oxides and bacterial communities were established using correlations (correlation coefficient ≥ 0.7). The extractability of Fe and Mn increased in poorly-oxygenated and hypoxic conditions. Iron seemed to originate in the easily-reducible fraction, while Mn was in a less reactive form than Fe. As expected, the extractability of Al did not vary with changing oxygen status. In addition, the rather low extractability of P suggests a strong initial adsorption of P on Al oxides. In environment low in oxygen, P released from the Fe oxides was resorbed by Al oxides. We conclude that the major bacterial processes in the sediments are related to the reduction of sulfate and sulfur and decomposition of organic matter. The bacterial communities varied both vertically and horizontally. The vertical variation was mainly explained by the redox potential, while the horizontal variation was more complex and essentially related to easily-reducible Fe and total carbon and nitrogen in sediment. The correlations between the easily-reducible Fe and Mn and the bacterial communities revealed taxa that reduce Fe and/or Mn, some that oxidize metals, and others that could benefit from organic rich environments created by Fe, Mn and S-reducing bacteria. The correlations indicate causative relationships and indirect associations, which can provide leads for future research.
Subject: anaerobic iron and manganese reduction
bacterial communities
biogeochemical interactions
chemical extraction
easily reducible Fe and Mn oxides

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