Microbial community structure and function: the response of denitrifiers on an environmental gradient – stream urbanization.

Denitrification is an important step in the global nitrogen cycle, removing fixed bioavailable nitrogen from aquatic systems and returning it to the atmosphere. Understanding the factors that influence the rate of denitrification is an important management goal in streams receiving high inputs of nitrogen: especially those that discharge to sensitive receiving waters. While many studies have examined the role of physical and chemical parameters on rates of denitrification, fewer studies have explored the effects that these factors have on the distribution of the denitrifying community responsible. Variation in the community structure of denitrifying bacteria was explored at sites influenced by the effects of urbanization. Variables were at two scales; those describing the sediment and stream, and those describing the catchment geography. Using multivariate analyses, significant correlations were found between micro habitat and catchment scale variables, suggesting that catchment scale processes modified by urban development play a part in structuring the dentrifying community. ‘Texture’ was an important variable at the micro scale that described the community structure. However, texture was correlated with other factors at multiple scales and these correlations are discussed in the context of the gradient of urban influences. The amount of community variance explained by this set of correlated variables was less than 20% indicating that other factors were also important. The effect of sediment carbon composition was explored using multivariate decomposition of spectra recorded using Fourier Transform Infra Red (FT-IR) and Excitation Emission Matrices (EEM). Principal Components Analysis reduced the number of variables from these spectra to a few factors. Using the scores from the reduced factors demonstrated that carbon composition of the sediments also had a significant structuring effect on the denitrifying community. Links between the denitrifying community structure and function were explored in a multiway ANOVA manipulation of sediments collected from streams heavily impacted and lightly impacted by urbanization. Sediments were dosed with carbon substrates thought to mimic either an urban or non urban source, and changes in the response of CO2, CH4 and N2 monitored for one month. Additionally, sediments were treated with the heavy metal zinc, found in higher concentrations in the urban sediments. The result of these manipulations suggested that community function was, depending on the treatment, significantly altered but that stream community structure was not: function was not limited by lack of diversity – the community was functionally adaptable. This research contributes to a growing body of knowledge on the factors affecting the community structure of microorganisms and suggests several variables that should be consistently included in examinations of ‘wild’ bacterial communities. If futures studies consistently incorporated an agreed set of basic or core variables, the field of microbial ecology would benefit by making cross comparison and meta-analysis of related studies more practical.