PhD defence by Elin Djurhuus Samuelsen

Bioaugmentation of flow-through sand filters

A study of cell surface properties, adhesion and catabolic gene expression dynamics during phenoxy acid herbicide degradation.

Global applications of pesticides in agricultural production have led to the detection of trace amounts of pesticides in groundwater resources in levels exceeding the EU threshold limit for drinking water of 0.1 µg L-1. Pesticide-polluted groundwater may be remediated by inoculating waterworks sand filters with specific degrading bacteria. However, degradation efficiency is often hampered by poor adhesion and a lack of sustained catabolic activity of the introduced bacteria.

The overall objective of this thesis was to investigate the significance of selected bacterial surface properties for degradation performances in  flow-through sand columns, with the aim of identifying a suitable inoculant strain for future environmental applications. Another aim was to identify a suitable genetic marker to monitor phenoxy acid degradation in strain Sphingobium sp. PM2.  We were not able to link motility and biofilm formation to the strains´ ability to adhere to sand. Nevertheless, a correlation was found between cell surface hydrophobicity and adhesion and overall degradation performances in flow-through sand columns. We identified S phingobium sp. PM2 as a promising inoculant strain, displaying efficient MCPA degradation for prolonged periods in flow-through sand columns. 

In an expression study of catabolic genes with putative roles in phenoxy acid degradation, we observed a marked upregulation of catabolic genes cadA and tfdC upon exposure to MCPA, 2,4-D, dichlorprop and mecoprop in strain PM2, which coincided with efficient mineralisation/degradation, and proposed the tfdC gene as a suitable marker for monitoring phenoxy acid degradation in strain PM2. Furthermore, when testing strain PM2s degradation performance in flow-through sand columns, we found that strain PM2 was able to sustain  induced expression of tfdC, which coincided with efficient MCPA degradation. 

In conclusion, we succeeded in identifying a promising inoculant strain, Sphingobium sp. PM2, for future environmental applications, as well as identified a suitable genetic marker for monitoring phenoxy acid degradation and degradation capacities in strain PM2.

Assessment Committee: 
Kirsten Jörgensen, Leading Research Scientist, Finnish Env. Ins. SYKE, Finland

Martin Romantschuk, Professor, University of Helsinki, Finland

Peter Stoumann (chairman), Associate Professor, PLEN, University of Copenhagen, DK