Detecting and Understanding Nitric Oxide Formation during Nitrogen Cycling in Microbial Biofilms

Nitric oxide (NO) is a bio-molecule in multicellular organisms and microorganisms which generate NO during catabolic pathways of the N-cycle. This thesis aimed to measure NO concentrations in N-cycling microbial biofilms, determine the responsible pathways and obtain insights into the regulation of NO turnover. Initially, a NO microsensor that is suitable to detect NO within biofilms and sediments was developed. The sensor was applied to a biofilm with nitrifying and denitrifying activity and dental biofilms. In the nitrifying/denitrifying biofilm, NO was formed by ammonia oxidizing bacteria under oxic and denitrifiers under anoxic conditions. Adding NO2- and decreasing O2 lead to immediate production of NO, followed by its consumption. In dental biofilms, NO is formed by aerobic denitrification and chemo-denitrification in a pH-dependent manner. NO formation in dental biofilms might mediate interactions between plaque bacteria and gingival cells, because NO is an important signaling molecule in the gum. In both biofilms, NO occurs in concentrations in the nanomolar range, because of its fast reduction to N2O which accumulates to higher concentrations.