Bacterial Community Dynamics on Suspended Particle Microscopic Islands and Implications for the Theory of Island Biogeography

Abstract

Suspended particles and aggregates are ubiquitous in most of aquatic environments. Aggregates have enriched nutrient and organic matter concentration compared to equal volumes of surrounding water, which can provide a favorable microscopic habitat for microorganism in aquatic ecosystems. So far, very few studies have been done regarding the composition of bacterial communities on aggregates in freshwater. Spatial distributions of particle-associated (PA) and free-living (FL) bacterial communities were characterized and compared at five sites along a transect from the St. Louis River estuary to nearshore Lake Superior in July and October 2013. The bacterial richness, Shannon-Wiener diversity and evenness indices of PA bacterial communities were all higher than for FL communities. The PA bacterial community in nearshore Lake Superior was significantly different from PA communities in the estuary (harbor and river). The PA bacterial community varied monthly, but no spatial or temporal differences were observed in FL bacterial communities along the transect. Seasonal dynamics of bacterial communities associated with sinking particles (SP), artificial aggregates (AA), and aggregates-free water (AFW) were investigated and compared from October 2012 to December 2013 at two sites in nearshore Lake Superior and the Duluth-Superior Harbor. Clear seasonal changes were observed on SP, AA and in AFW bacterial communities, but the seasonal dynamic in AFW bacterial communities was less pronounced than SP and AA bacterial communities at the lake and harbor sites. Bacterial communities on SP, AA and in AFW were different between the lake and harbor sites. In addition, bacterial communities on SP and AA, as well as communities on AA and in AFW were significantly different from each other at both sites. Flocculent organic aggregates were used to test whether the Theory of Island Biogeography applies at a microbial scale, and to examine how bacterial community develop and evolve. Bacterial richness increased with closer colonizer distances on lake aggregates in harbor water, supporting Theory of Island Biogeography predictions. However, this trend was not observed on harbor aggregates in lake water. It was also observed that bacterial communities on different aggregate types had their own distinctive pattern of succession during the re-colonization process.