Comparison of limnic and marine particle-associated microbial communities

Marine and limnic particles are hotspots of organic matter mineralization significantly affecting biogeochemical element cycling. Fluorescence in-situ hybridization with rRNA-targeted probes and tag pyrosequencing of 16S rRNA genes were combined to investigate bacterial diversity and community composition on limnic and coastal marine particles, defined operationally by a diameter of >5 and >10 µm, respectively. Samples were obtained from Lake Stechlin and Grosse Fuchskuhle, the Adriatic Sea and the North Sea. On average, particle abundance was higher in limnic (1×107 L-1) than in marine systems. Whereas limnic particles were smaller in size (average areas of 471 vs. 2050 µm2), they were more densely colonized compared to marine ones (average bacterial densities of 7.3 vs. 3.6 cells per 100 µm2). Therefore, particle-associated (PA) bacteria accounted on average for a larger fraction of total bacteria in limnic (15%) than in marine environments (4%). Unlike previously suggested limnic PA bacteria harbored beyond Alphaproteobacteria and Betaproteobacteria sizeable populations of Gammaproteobacteria, Actinobacteria and Bacteroidetes. Marine particles were colonized by Planctomycetes and Betaproteobacteria additionally to Alphaproteobacteria, Bacteroidetes and Gammaproteobacteria. PA bacteria were in 2/3 of the cases more diverse than free-living (FL) bacteria. Accordingly, we propose a new index to characterize particle colonization based on the PA/FL Chao1 diversity index ratio. Large heterogeneities in individual particle colonization could be shown. However, high-throughput sequencing revealed significant overlap between PA and FL communities and thus highlights an underestimated connectivity between these fractions. Overall, despite seemingly similar ecological niches our study found large differences between limnic and marine PA communities.