Ecological implications and identification of a freshwater red alga chemical defense
Nahabedian, John F., III
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Aquatic generalist herbivores feed selectively when offered several species of macrophytes, preferring more nutritious macrophytes and those with minimal chemical or physical defense. This selective feeding plays an important role in structuring plant communities, sometimes completely altering communities of aquatic plants. While it is known that higher plants in freshwater systems produce chemical defenses against herbivores, the chemical deterrents themselves have rarely been identified, and none are known for freshwater red algae. Batrachospermum helminthosum, a freshwater red alga, exhibits evidence of chemical deterrents to feeding by crayfish. In this experiment, we evaluated whether crayfish unwillingness to consume B. helminthosum was due to morphological or structural feeding deterrents by drying, powdering, reconstituting the alga into sodium alginate gel pellets and offering these, versus pellets made with a different, palatable alga to crayfish. Crayfish consumed pellets made from the green alga Cladophora glomerata in preference to pellets made from B. helminthosum, suggesting that chemical rather than structural traits were responsible for crayfish selective feeding. Bioassay-guided fractionation and the application of proton NMR, carbon-13 NMR, liquid chromatography mass spectrometry (LCMS), and high resolution mass spectroscopy were used to attempt to isolate and identify the chemical compounds responsible for B. helminthosum herbivore defenses. We found evidence of several chemical deterrents ranging from polar to non-polar. Candidate molecular weights for one or two compounds in the most purified, active fraction were 339.5 and 268.2.