PARENT SESSION
Poster Session # 15: Plant Ecology.

Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Herbivore-mediated competition between cyanobacteria and green algae: Impacts on cyanobacterial toxin production.

Bevis, Kevin^*,1, Thacker, Robert¹, ¹ University of Alabama at Birmingham, Birmingham, AL

ABSTRACT- Although the filamentous, mat-forming cyanobacterium Lyngbya wollei is known to produce saxitoxin, a paralytic shellfish poison, experimental analyses of the potential role of saxitoxin in freshwater ecosystems are limited. Lake Guntersville, AL contains persistent mats of L. wollei that occupy many shoreline areas, and in most cases are found with sympatric green algae and high densities of herbivorous snails (Pleurocera annuliferum). Competition between L. wollei and a common sympatric green alga, Rhizoclonium hieroglyphicum, was studied in a response surface design in the presence and absence of P. annuliferum. Both L. wollei and R. hieroglyphicum show strong intraspecific competition at high densities. Although L. wollei growth was not significantly affected by R. hieroglyphicum density, L. wollei exhibited a density-dependent facilitative effect on R. hieroglyphicum growth in the absence of herbivory. The presence of snails had no effect on R. hieroglyphicum growth, but increased L. wollei growth. Saxitoxin concentrations were highest in L. wollei grown at high density. In the presence of herbivores, saxitoxin concentrations decreased with increasing R. hieroglyphicum density. These results suggest that saxitoxin plays a limited role in interspecific competition and may be produced primarily in response to intraspecific competition. Since the presence of palatable green algae reduces herbivory on this cyanobacterium, L. wollei may decrease saxitoxin production and gain an associational defense when both herbivores and alternative foods are present.

Key words: competition, saxitoxin, chemical ecology, cyanobacteria