The Effects of Ecological Factors on Freshwater Biofilm Colonization by Planctomycetes

Abstract

Understanding the pivotal roles played by different bacterial types within biofilms can lead to effective management and/or utilization of biofilms. Freshwater and marine bacteria from the phylum Planctomycetes populate biofilm-specific clusters in a way that has led researchers to conclude that an important relationship exists between planctomycetes and algae and cyanobacteria. This study seeks to investigate this relationship. Replicate stream mesocosms were established using flow channels with half exposed to normal springtime daylight/night-time conditions and half kept in complete darkness to investigate how the different lighting conditions affected community assembly of planctomycetes and photosynthetic microbiota. Biofilms collected from rocks in a natural stream were used as an initial colonisation inoculum for slides placed within the flow channels and these operated for six weeks. Biofilm was collected from one half of a slide per channel on a weekly basis and stored at -20°C. The other half of the slide was scraped clean for Chlorophyll concentration analysis. After the six-week sampling period, frozen biofilm samples were removed from collection sponges and processed to extract DNA, amplify selected bacterial communities using PCR, and then sequenced. Sequences were sorted and assigned operational taxonomic units followed by reducing the data set to the most frequently occurring planctomycetes and potential phototrophic microorganism OTUs. Data-mining, bioinformatics, and statistical methods were employed to look for patterns of occurrence and correlations between planctomycetes OTUs and the other selected OTUs based upon lighting conditions, temperature, community changes over time, and as related to the presence of chlorophyll a, chlorophyll b, and bacteriochlorophyll a. The hypothesis of relationship between planctomycetes and photosynthetic microorganisms was not established, however results of this experiment suggest differing patterns of occurrence for each of the 24 most frequently occurring planctomycetes OTUs. Additionally, several correlations exist between planctomycetes and proteobacterial OTUs, certain cyanobacteria and with members of Verrucomicrobia, although none confirm a dependent relationship. The relative abundance of planctomycetes was not significantly different between daylight and dark conditions. This study provides a better understanding of community assembly within freshwater biofilms.