HABS-BLOCKS©, a Floating, Slow-Release Glucose Source, Promoted the Growth of Heterotrophic Bacteria Relative to Toxic Cyanobacteria in Lake Water Mesocosms.

Previously, we demonstrated that the addition of glucose to lake water could alter the composition of the microbial community so that heterotrophic bacteria came to dominate the cyanobacteria. To target the glucose additions to the euphotic zone, a floating, slow-release glucose source, designated HABS-BLOCKS©, was created. HABS-BLOCKS© consist of blocks of pumice stone, vacuum infiltrated with glucose, and covered in layers of soy wax. In this study, the HABS-BLOCKS© were tested in 7-liter mesocosm vessels (n = 4) that received an initial 750 ml of lake water, followed by weekly additions of 500 ml of freshly collected lake water. Three HABS-BLOCKS© were added to each of two replicate mesocosms. For controls, one mesocosm was left untreated and one mesocosm received three "Dummy" HABS-BLOCKS© (contain no glucose). During a ten-week experiment, 25 ml samples were obtained from each mesocosm weekly, which were then filtered, frozen and latter processed for 16S rRNA sequencing. Planktothrix and Cyanobium were the most abundant cyanobacteria in the lake water. Within three weeks of the start of the experiment, the bacterial community in the HABS-BLOCKS© treated mesocosms became dominated by heterotrophic bacteria, e.g., Asticcacaulis, relative to the control mesocosms. Heterotrophic domination in the HABS-BLOCKS© treated mesocosms continued for the rest of the experiment. HABS-BLOCKS© appears to provide a competitive edge for the heterotrophic bacteria that allows them to dominate the toxin-producing cyanobacteria.