Harmful alga trades off growth and toxicity in response to cues from dead phytoplankton

Organisms are under selection pressure to recognize predators and assess predation risk to avoid becoming prey. In some cases, the presence of injured competitors alerts individuals to the likelihood that predators are nearby. Previous studies have shown that the marine dinoflagellate Alexandrium minutum responds to chemical cues from copepods by dramatically upregulating sodium channel-blocking toxins that appear to function as defenses against copepod grazing. However, it is unknown whether A. minutum uses other cues, such as damaged phytoplankton, some of which are its competitors, to assess predation risk and subsequently increase its resistance to predators. To investigate the role of dead phytoplankton cues in chemical defense plasticity, A. minutum was exposed for 3 days to chemical cues from six different phytoplankton. Chemical cues from dead, unrelated, historically co-occurring phytoplankton species induced toxin production in A. minutum coincident with a decrease in growth. In contrast, exposure to chemical cues from more closely related dead phytoplankton, either conspecific or congeneric, suppressed toxin production in A. minutum relative to their absence. This was coupled with a modest, yet significant, increase in growth. The consistent inverse relationship between toxin production and growth suggests that A. minutum experiences a trade-off. Together, these results reveal that relatedness of dead phytoplankton is important in how A. minutum utilizes resources for growth and defense.