Context-dependent allelopathy in algal interactions: Insights from laboratory and natural phytoplankton communities

A key aspect of algal biology is the production of chemicals used for communication and regulation of interactions. Allelopathic chemicals can facilitate competition between algae, influence community composition and drive succession in the phytoplankton. Much is still unknown about the identities of these allelochemicals, and even the ecological relevance of laboratory derived results is often unclear. We sought to evaluate the variability of allelopathy employing a systematic approach. We used Prymnesium parvum as a model to assess species-specificity of allelopathic interactions in laboratory co-cultivations. We assessed the influence of media, ratio of cell densities of the interaction partners and evaluation methods (monitoring of chlorophyll a versus cell density) on the outcome of co-cultivation experiments. Five tested ecologically relevant phytoplankton species, Dunaliella tertiolecta, Emiliania (presently Gephyrocapsa) huxleyi, Florenciella parvula, Ochromonas triangulata, and Pavlova gyrans, exhibited and experienced varying positive and negative allelopathic effects in interactions with P. parvum. Further, we document pronounced effects of variations of laboratory parameters. Among the ten possible bidirectional allelopathic relationships, half of the allelopathic effects were changed by one or more of the variable parameters tested. Allelopathy is thus not only a species-specific trait but additionally depends on the external factors present during the interactions. We support our findings through the identification of interaction patterns from mining the Tara Oceans database for reoccurring trends in natural and laboratory communities. This study strongly supports the notion that allelopathic activity of an alga is not only a species-specific property but also depends on many external parameters that have to be considered and thoroughly documented in future studies of allelopathy.