Oxylipins from freshwater diatoms act as attractants for a benthic herbivore

Lipoxygenase products (often called oxylipins) are degradation products of fatty acids. They are known to regulate many defensive and developmental pathways in plants. Among these lipoxygenase products are volatile aldehydes, that are released from wound activated diatom cells and that are potent inhibitors of mitotic proliferation in sea urchin embryos. They have been shown to dramatically lower the hatching success of herbivorous copepods and therefore are considered to constitute an activated defence strategy. Alternatively, lipoxygenase products might also serve as infochemicals, especially in biofilms of benthic algae and cyanobacteria. Here, we demonstrate that the bouquet of volatile lipoxygenase products released from a benthic diatom (Achnanthes biasolettiana) is attractive to an herbivorous gastropod grazer (Radix ovata). The volatiles are released from the algae upon cell damage and can be utilized as food-finding cues by the herbivorous snails. In contrast to this observation, the odour bouquet from another benthic diatom species (Gomphonema parvulum) did not show any attractant activity to the snail, which is probably caused by differences in the bouquet of odorous substances released by the two diatom isolates. In contrast to G. parvulum, the odour bouquet of A. biasolettiana consisted of mono- and diunsaturated alcohols and ketones, primarily with a C-8-skeleton. Most of the compounds have never before been described from diatoms. This study shows that diatom lipoxygenase products cannot only be involved in activated defence, but can also be utilised as food-finding cues by herbivores. Thus, the ecological role of these volatile organic compounds can be very complex and will strongly depend on the ecological context of the chemical interaction.