The exopolymer secretions (EPS) layer surrounding Aureoumbra lagunensis cells affects growth, grazing, and behavior of protozoa

Texas brown tide, a nuisance algal bloom formed by the marine pelagophyte Aureoumbra lagunensis, persisted without interruption from 1990-1997 in Laguna Madre, Texas. We hypothesize that, in addition to other factors, the presence of a mucus layer of exopolymer secretions (EPS) outside its cell wall may have played a role in reducing microzooplankton grazing on A. lagunensis, and therefore contributing to the persistence of the brown tide bloom. Three protozoan species common to the bloom waters fed A. lagunensis with low or high EPS showed significantly reduced growth rates with a high EPS diet. An additional experiment with the ciliate Aspidisca sp. revealed that the lower growth rate was caused by a reduced grazing rate on A. lagunensis with high EPS. A. lagunensis cells with high EPS might be unpalatable to Aspidisca, hence forced it to feed on bacteria as an alternate food source. Also, the EPS mucus might have affected the feeding mechanics of ciliates by adhering to their cilia and clogging their feeding apparatus. Motion analysis of swimming behavior of Aspidisca fed high and low EPS A. lagunensis revealed altered behavior in the presence of high EPS that could affect their grazing efficiency.