Composition and nutrient dynamics of bacterial communities associated with Microcystis blooms in Lake Erie

Background/Question/Methods

Microcystis aeruginosa causes harmful blooms in highly productive lakes releasing a neuro- and hepatotoxin called microcystin. The western basin of Lake Erie, including Sandusky and Maumee Bays, has experienced repeated Microcystis blooms since the 1990s.  Bacterial communities associated with Microcystis have been hypothesized to be distinct from communities in surrounding water due to the presence of a microhabitat formed by the mucilaginous matrix of the cyanobacteria. This in turn could affect mobilization of resources, like phosphorus and carbon, to higher trophic levels. Phosphorus is also a limiting nutrient for cyanobacteria and so higher rates of phosphorus uptake by Microcystis-attached bacteria might contribute to bloom formation. In this study, we compared community composition and nutrient dynamics of attached and free-living bacteria before, during, and after cyanobacterial blooms at two sites each in Sandusky and Maumee Bays. Differences between attached and free-living (planktonic) bacteria were measured after differential filtration on 1µm and 0.22 µm filters respectively. Bacterial productivity and nutrient uptake were measured using radioactive isotopes and bacterial numbers calculated from DAPI counts. Community composition was examined based on amplification of 16S rRNA genes in DNA extracts and denaturing gradient gel electrophoresis (DGGE).

Results/Conclusions Three of the four sites discussed hereafter showed Microcystis blooms; with a Planktothrix bloom at the other site. Attached bacterial numbers increased two-fold, on average, through bloom development followed by a post-bloom decline. Productivity (g C/cell/hr) of attached bacteria varied little over time and was, on average, two times higher pre-bloom and ten times higher post-bloom relative to planktonic bacteria. Phosphate uptake assays revealed that cyanobacteria were phosphorus limited in 60% of the cases through bloom formation. While phosphate uptake rates of both bacterial fractions were comparable pre- bloom, those of attached bacteria were 3-fold higher than planktonic bacteria during the bloom and ranged from 4.16X10^-7µmolP/cell/µgm chlorophyll pre- bloom to 1.2X10^-4 post-bloom. As expected, uptake of nascent photosynthate released by the primary producers, was higher in the attached fraction than the planktonic. DGGE revealed that certain bands were absent in the attached fraction during the bloom while bands were similar between the two fractions prior to the bloom. Overall, attached bacteria exhibited increased phosphate uptake over time, with productivity and nascent photosynthate uptake rates always higher than that of planktonic bacteria. DGGE results suggest that not all taxa present in the water can successfully exploit the Microcystis microhabitat during the bloom.