Changes in microbial community structure in the wake of Hurricanes Katrina and Rita

Abstract

Hurricanes have the potential to alter the structures of coastal ecosystems and generate pathogen-laden floodwaters thatthreaten public health. To examine the impact of hurricanes on urban systems, we compared microbial community structures in samples collected after Hurricane Katrina and before and after Hurricane Rita. We extracted environmental DNA and sequenced small-subunit rRNA (SSU rRNA) gene clone libraries to survey microbial communities in floodwater, water, and sediment samples collected from Lake Charles, Lake Pontchartrain, the 17th Street and Industrial Canals in New Orleans, and raw sewage. Correspondence analysis showed that microbial communities associated with sediments formed one cluster while communities associated with lake and Industrial Canal water formed a second. Communities associated with water from the 17th Street Canal and floodwaters collected in New Orleans showed similarity to communities in raw sewage and contained a number of sequences associated with possible pathogenic microbes. This suggests that a distinct microbial community developed in floodwaters following Hurricane Katrina and that microbial community structures as a whole might be sensitive indicators of ecosystem health and serve as "sentinels" of water quality in the environment.

Figure 1

Map overview of the sampling locations at Lake Pontchartrain and Lake Charles in Louisiana. Only the first sample number in a series of samples collected over time at the same location (see groupings below the map) is indicated on the map. Refer to the sampling codes in Table 1 and Table S1 for additional sample information. Pink squares indicate original transects sampled in Sinigalliano et al. (2007). This study focused on Transect 2 (Transect B in Sinigalliano et al. (2007)). Note that Transect 2, Station 3 and Transect 6, Station 4 refer to approximately the same location. Image modified from http://earth.google.com.

Figure 2

A Detrended Correspondence Analysis (DCA) ordination diagram showing 28 samples (filled circles) and bacterial OTUs at the 99% similarity cut-off value (crosses and x-marks) plotted on the first (horizontal) and second (vertical) DCA axes. The scalings on the axes are in multiples of standard deviation. The length of the gradient of our first axis (X-axis) was > 4 S.D. suggesting that some species show a unimodal response to the 1^st DCA axis. This confirms a non-linear response of the species to their environment and thus justifies the use of a DCA approach. We can interpret our ordination diagram using the distance rule wherein a sample that is close to a species point is more likely to contain that species than one farther away. The first eigenvalue was 0.944 and the second was 0.787.

Figure 3

Canonical Correspondence Analysis (CCA) triplot of samples and bacterial and archaeal OTUs at the 99% similarity cut-off value displaying 100% of the variance of the OTUs with respect to the environmental variables. The canonical eigenvalues for axes 1–2 of the sample analysis are 0.874 and 0.781 respectively. Environmental variables are indicated by red arrows while supplemental variables are shown using grey arrows. Samples are represented by green circles and sample numbers refer to sample codes further detailed in Table 1 and Table S1. Bacterial OTUs are represented by blue crosses while archaeal OTUs are shown as pink diamonds.