Cell-growth gene expression reveals a direct fitness cost of grazer-induced toxin production in red tide dinoflagellate prey

Predation in a Microbial World: Mechanisms and Trade-Offs of Flagellate Foraging

Heterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean and thus play a key role in ocean biogeochemistry. They are found in all major branches of the eukaryotic tree of life but are united by all being equipped with one or a few flagella that they use to generate a feeding current. These microbial predators are faced with the challenges that viscosity at this small scale impedes predator-prey contact and that their foraging activity disturbs the ambient water and thus attracts their own flow-sensing predators. Here, I describe some of the diverse adaptations of the flagellum to produce sufficient force to overcome viscosity and of the flagellar arrangement to minimize fluid disturbances, and thus of the various solutions to optimize the foraging-predation risk trade-off. I demonstrate how insights into this trade-off can be used to develop robust trait-based models of microbial food webs.

Nitrogen liberated via allelopathy can promote harmful algal blooms

Allelopathy is a biological mechanism that can promote harmful algal blooms (HAB) via the inhibition of sympatric phytoplankton. While nutrient loading can also promote HABs, the ability of allelopathy to stimulate HABs via the regeneration of nutrients has yet to be explored. To examine the impacts of allelopathically liberated N on HAB species, a series of experiments were performed using multiple allelopathic HAB species including the dinoflagellates Alexandrium catenella and Margalefidinium polykrikoides, and the pelagophyte, Aureoumbra lagunensis. These HAB species were paired with the cosmopolitan dinoflagellate, Akashiwo sanguinea, that was labeled with 15NO3- or 15NH4+, allowing the release and transfer of N to be traced as a time course during allelopathic interactions. During all experiments, the allelopathic inhibition of Akashiwo was accompanied by increases in cell densities, growth rates, and the δ15N content of the HAB species, evidencing the transfer of N liberated from Akashiwo. The cellular transfer of 15N and release of dissolved N was higher when Akashiwo was grown with 15NO3- compared to 15NH4+ suggesting a differential subcellular-compartmentalization of N sources. Regardless of the type of N, HAB species obtained 60 - 100% of their cellular N from lysed Akashiwo cells and there was an enrichment of the δ15N content of the dissolved NH4+ pool post-lysis of Akashiwo. Collectively, the results demonstrate that beyond facilitating species succession, allelopathy can supply HABs with N and, therefore, is likely important for promoting and sustaining HABs. Given that allelopathy is known to be a dose-dependent process, allelopathy may induce a positive feedback loop, whereby competitors are lysed, N is liberated, HABs are intensified and, in turn, become more strongly allelopathic.

Evolution of toxins as a public good in phytoplankton

Toxic phytoplankton blooms have increased in many waterbodies worldwide with well-known negative impacts on human health, fisheries and ecosystems. However, why and how phytoplankton evolved toxin production is still a puzzling question, given that the producer that pays the costs often shares the benefit with other competing algae and thus provides toxins as a 'public good' (e.g. damaging a common competitor or predator). Furthermore, blooming phytoplankton species often show a high intraspecific variation in toxicity and we lack an understanding of what drives the dynamics of coexisting toxic and non-toxic genotypes. Here, by using an individual-based two-dimensional model, we show that small-scale patchiness of phytoplankton strains caused by demography can explain toxin evolution in phytoplankton with low motility and the maintenance of genetic diversity within their blooms. This patchiness vanishes for phytoplankton with high diffusive motility, suggesting different evolutionary pathways for different phytoplankton groups. In conclusion, our study reveals that small-scale spatial heterogeneity, generated by cell division and counteracted by diffusive cell motility and turbulence, can crucially affect toxin evolution and eco-evolutionary dynamics in toxic phytoplankton species. This contributes to a better understanding of conditions favouring toxin production and the evolution of public goods in asexually reproducing organisms in general.

Cell-growth gene expression reveals a direct fitness cost of grazer-induced toxin production in red tide dinoflagellate prey

Induced prey defences against consumers are conspicuous in microbes, plants and animals. In toxigenic prey, a defence fitness cost should result in a trade-off between defence expression and individual growth. Yet, previous experimental work has failed to detect such induced defence cost in toxigenic phytoplankton. We measured a potential direct fitness cost of grazer-induced toxin production in a red tide dinoflagellate prey using relative gene expression (RGE) of a mitotic cyclin gene (cyc), a marker that correlates to cell growth. This approach disentangles the reduction in cell growth from the defence cost from the mortality by consumers. Treatments where the dinoflagellate Alexandrium catenella were exposed to copepod grazers significantly increased toxin production while decreasing RGE of cyc, indicating a defence-growth trade-off. The defence fitness cost represents a mean decrease of the cell growth rate of 32%. Simultaneously, we estimate that the traditional method to measure mortality loss by consumers is overestimated by 29%. The defence appears adaptive as the prey population persists in quasi steady state after the defence is induced. Our approach provides a novel framework to incorporate the fitness cost of defence in toxigenic prey-consumer interaction models.