Costs and benefits of predator-induced defence in a toxic diatom

Abstract

Phytoplankton employ a variety of defence mechanisms against predation, including production of toxins. Domoic acid (DA) production by the diatom Pseudo-nitzschia spp. is induced by the presence of predators and is considered to provide defence benefits, but the evidence is circumstantial. We exposed eight different strains of P. seriata to chemical cues from copepods and examined the costs and the benefits of toxin production. The magnitude of the induced toxin response was highly variable among strains, while the costs in terms of growth reduction per DA cell quota were similar and the trade-off thus consistent. We found two components of the defence in induced cells: (i) a ‘private good’ in terms of elevated rejection of captured cells and (ii) a ‘public good’ facilitated by a reduction in copepod feeding activity. Induced cells were more frequently rejected by copepods and rejections were directly correlated with DA cell quota and independent of access to other food items. By contrast, the public-good effect was diminished by the presence of alternative prey suggesting that it does not play a major role in bloom formation and that its evolution is closely associated with the grazing-deterrent private good.

Short handling times allow for active prey selection in suspension feeding copepods

“…6). In fact, copepods are highly selective feeders and may select prey based on its nutritional value (Cowles et al 1988; Meunier et al 2016), its contents of toxins (Teegarden 1999; Ryderheim et al 2021; Olesen et al 2022), or other properties of the cell (Stemberger 1985; Atkinson 1995; Ryderheim et al 2022 a ), and may at times reject > 90% of the prey captured (Xu and Kiørboe 2018). Foraging on small prey leaves less room for prey selection (Figs.…”

Grazer‐induced aggregation in diatoms

“…Many diatoms may further harness this defense by thickening the shell in response to copepod cues (Pondaven et al 2007;Grønning and Kiørboe 2020). Other inducible defense mechanisms include the modification of colony size in some chainforming diatoms-shorter chains reduce the risk of being consumed by a copepod (Rigby and Selander 2021;Ryderheim et al 2022b)-and the production of toxins in Pseudo-nitzschia species that make copepods deselect toxin-producing cells (Zhang et al 2021;Olesen et al 2022). It has similarly been described that viral infection induces formation of resting spores that may subsequently germinate as healthy cells (Pelusi et al 2021), and viral infections can also cause aggregation and mass sinking (Yamada et al 2018).…”

Microzooplankton grazers induce chain length plasticity in colonial diatoms

“…Incubations lasted for 72 h ( S. marinoi – T. longicornis ) or 96 h (all other combinations) h at 16°C. These exposure times have been sufficient to trigger defensive responses in other diatoms (Grønning and Kiørboe 2020; Olesen et al 2022). With C. affinis we also performed two experiments with longer incubation times (120 h and 144 h), as this species have a lower growth rate compared with the other two.…”