Trophic interactions mediated by the freshwater cyanobacterium Lyngbya wollei.

摘要

The freshwater cyanobacterium Lyngbya wollei forms dense mats in lentic systems throughout the southeastern United States and synthesizes saxitoxins and an extracellular polysaccharide sheath (EPS). Because this cyanobacterium persists in habitats with high densities of the herbivorous amphipod Hyalella azteca, the role of saxitoxins and the EPS in deterring amphipod herbivory was investigated. While amphipods avoided consuming live and ground L. wollei, amphipods consumed significantly greater amounts of artificial food containing saxitoxins. However, when sheath material was added to artificial foods, even in combination with crude extracts, amphipods preferred to eat paired control food. Thus, structural, rather than chemical, traits may play a more important role in helping to maintain the abundance of L. wollei in systems under intense herbivory.; To investigate the potential for cyanobacteria to reduce bluegill sunfish predation on H. azteca, amphipods were placed in tanks containing L. wollei and Rhizoclonium hieroglyphicum in the presence and absence of fish and chemical cues of fish predation. Choice and no-choice assays revealed that amphipods experienced lower mortality on L. wollei mats when fish were present but migrated to the more palatable R. hieroglyphicum in their absence. Furthermore, waterborne cues from sunfish decreased H. azteca migration rates between mats. This may further reduce the impact of herbivory and promote coexistence of algae and cyanobacteria.; Phylogenetic analysis of DNA sequences coding for the 16S ribosomal subunit supported the monophyly of L. wollei samples from Alabama and Canada, with 99.3% sequence similarity over 1,326 base pairs; however, L. wollei failed to form a unique clade with any other cyanobacteria. While the Oscillatoriales appeared to be a paraphyletic group in need of revision, the heterocystous cyanobacteria formed a well-defined clade with strong bootstrap support.; The results of these studies indicate that L. wollei can resist amphipod herbivory while providing shelter from predation. Amphipods also increase their survivorship by decreasing their between-patch migration rates in response to waterborne predator cues. Lastly, 16S rDNA revealed that L. wollei from Alabama shares strong affinity to a recently discovered population in Betula Lake, Canada. Phylogenetic analysis suggests that its placement in the Oscillatoriales needs to be re-evaluated.