Nitrogen regulation of the <i>xyl</i> genes of <i>Pseudomonas putida</i> mt-2 propagates into a significant effect of nitrate on <i>m</i>-xylene mineralization in soil

The nitrogen species available in the growth medium are key factors determining expression of xyl genes for biodegradation of aromatic compounds by Pseudomonas putida. Nitrogen compounds are frequently amended to promote degradation at polluted sites, but it remains unknown how regulation observed in the test tube is propagated into actual catabolism of, e.g. m‐xylene in soil, the natural habitat of this bacterium. To address this issue, we have developed a test‐tube‐to‐soil model system that exposes the end‐effects of remediation practices influencing gene expression of P. putida mt‐2. We found that NO (3) (−) compared with NH (4) (+) had a stimulating effect on xyl gene expression in pure culture as well as in soil, and that this stimulation was translated into increased m‐xylene mineralization in soil. Furthermore, expression analysis of the nitrogen‐regulated genes amtB and gdhA allowed us to monitor nitrogen sensing status in both experimental systems. Hence, for nitrogen sources, regulatory patterns that emerge in soil reflect those observed in liquid cultures. The current study shows how distinct regulatory traits can lead to discrete environmental consequences; and it underpins that attempts to improve bioremediation by nitrogen amendment should integrate knowledge on their effects on growth and on catabolic gene regulation under natural conditions

A three-step test of phosphate sorption efficiency of potential agricultural drainage filter materials

Phosphorus (P) eutrophication of lakes and streams, coming from drained farmlands, is a serious problem in areas with intensive agriculture. Installation of P sorbing filters at drain outlets may be a solution. Efficient sorbents to be used for such filters must possess high P bonding affinity to retain ortho-phosphate (Pi) at low concentrations. In addition high P sorption capacity, fast bonding and low desorption is necessary. In this study five potential filter materials (Filtralite-P®, limestone, calcinated diatomaceous earth, shell-sand and iron-oxide based CFH) in four particle size intervals were investigated under field relevant P concentrations (0-161μM) and retentions times of 0-24min. Of the five materials examined, the results from P sorption and desorption studies clearly demonstrate that the iron based CFH is superior as a filter material compared to calcium based materials when tested against criteria for sorption affinity, capacity and stability. The finest CFH and Filtralite-P® fractions (0.05-0.5mm) were best with P retention of ≥90% of Pi from an initial concentration of 161μM corresponding to 14.5mmol/kg sorbed within 24min. They were further capable to retain ≥90% of Pi from an initially 16μM solution within 11/2 min. However, only the finest CFH fraction was also able to retain ≥90% of Pi sorbed from the 16μM solution against 4 times desorption sequences with 6mM KNO3. Among the materials investigated, the finest CFH fraction is therefore the only suitable filter material, when very fast and strong bonding of high Pi concentrations is needed, e.g. in drains under P rich soils during extreme weather conditions