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Macrofaunal impact on microbial diversity and organic contaminant degradation in marine sediments

Final Activity Report Summary - MACROMICRO (Macrofaunal impact on microbial diversity and organic contaminant degradation in marine sediments)

The overall objective of MACROMICRO was to investigate if and how burrowing macrofauna affect the structure and function of bacterial communities in marine sediments. Molecular techniques were used for analysing bacterial genetic diversity in sediments collected from different compartments (surface, burrows and bulk sediments). DNA was extracted from the complex sediment matrix and the rpoB gene, which codes for a subunit of RNA polymerase, was amplified using the polymerase chain reaction (PCR). This gene differs in sequence among bacterial "species" and can therefore be used to "measure" bacterial diversity. Denaturing gradient gel-electrophoresis (DGGE) was used to create fingerprints of bacterial communities from different sediments. The function studied was the ability and efficiency by which bacterial communities degrade oil-derived pollutants (polycyclic aromatic hydrocarbons-PAHs) present in marine sediments. Particular focus was placed on exploring temporal and spatial variation in natural bacterial community structure and function, as affected by contaminant exposure, especially in and around burrows made by sediment dwelling macrofauna.

The original project design involved research both in the laboratory and the field. Field investigations were planned to be conducted along pollution gradients across a larger geographical area including Kattegat and Öresund. An opportunity to carry out research in Greenland arose in conjunction with ongoing projects within the host institute research group. This aspect was not included in the initial proposal plan but added a fortunate general aspect to the project, where temperate and Arctic marine sediment systems could be compared.

Three field campaigns were performed involving four different investigations. Three of these were designed identically comparing pristine and polluted sites, while the fourth involved sampling of several pristine sites at an extended geographical range to investigate natural variation in bacterial community structure. Preliminary results show that bacterial degradation of PAHs differ between sediment compartments, with higher rates observed in surface and burrow sediments compared to bulk sediments. Degradation rates also appear dependent on historical contamination, indicating structural and/or functional microbial community change.

Degradation rates were generally found to be up to two orders of magnitude lower in Arctic compared to temperate sediments for both contaminated and pristine sites. DGGE banding patterns showed that bacterial community composition differs both between sites and among sediment compartments. Banding patterns also show many similarities which may be attributed to presence of old DNA from resting or dead bacteria in the sediments.

Spin-off projects include development of new methods for separating the active bacterial community structure signal from the old background DNA diversity structure signal in marine sediments. This will be done by performing simultaneous extraction of mRNA and DNA from the complex sediment matrix.