On the surface: Are secondary metabolites used to prevent fouling in green algae?

Green seaweeds of the order Bryopsidales are large and abundant primary producers in tropical and subtropical soft-bottom communities along with seagrasses.  Non-polar secondary metabolites (bioactive terpenoids) produced by these siphonous algae are known to function as deterrents against herbivory and show anti-microbial/fungal action in laboratory tests.  Because of the seaweeds’ generally epiphyte-free appearance and the high concentrations of secondary metabolites inside their cells, it has been hypothesized that these compounds might also provide protection from fouling.  For such surface-mediated interactions to occur, these compounds must be presented directly to potential colonizers on the relevant surface. Whole live Caulerpa prolifera, Penicillus capitatus, and Udotea conglutinata specimens collected from the seagrass beds of Tarpon Springs, FL were dipped for 30 s in hexane, dichloromethane (DCM), or 4% DCM in hexane.  After the surface dips were completed, the whole tissue extractions of the same plants were conducted.  ¹HNMR spectra showed non-polar secondary metabolites were absent on the surfaces and present in the tissues of the seaweeds. Scanning electron micrographs of the surfaces of these species revealed the absence of pores on the cell wall, suggesting their morphology is not consistent with the excretion of secondary metabolites. Previous work showing the quick degradation of these compounds in seawater and blade abandonment in this order of algae as a mechanism of removing epiphytes align with our results, suggesting non-polar secondary metabolites are not excreted onto the surfaces of these algae as a defense against fouling.