Predatory cues drive colony size reduction in marine diatoms.

Colony formation is a common feature among nonmotile marine phytoplankton. Several theories exist around the potential benefits of larger colonies.Here, we test the hypothesis that predation is one of the drivers behind colony formation and chain length plasticity. We exposed cultures of Thalassiosira rotula, Chaetoceros curvisetus, and Chaetoceros affinis to copepodamides, a chemical alarm cue released by copepods and perceived as an indicator of predation threat by their prey. This was coupled with a grazing experiment, which compared copepod grazing rates on different chain lengths.Our results show that T. rotula and C. curvisetus decreased their chain lengths by 79% and 49%, respectively, in response to copepodamides. Single cells and short chains were grazed at lower rates compared with long chains, and the copepodamide-driven size shift led to 30% and 12% lower grazing in T. rotula and C. curvisetus, respectively. In contrast, C. affinis showed a slight increased chain length in response to copepodamides although nonsignificant.We found that 2 of 3 studied species reduce their chain length in response to the presence of copepod grazers. Altered size structure has implications for the route of carbon in the marine food webs and carbon export to deeper strata.

Induction of domoic acid production in diatoms-Types of grazers and diatoms are important.

Grazers can induce toxin (domoic acid, DA) production in diatoms. The toxic response has been observed in two species of Pseudo-nitzschia and was induced by Calanus copepods. In this study, interactions between diatoms and copepods were further explored using different species of diatoms and copepods. All herbivorous copepods induced toxin production, whereas exposure to carnivorous copepods did not. In line with this, increasing the number of herbivorous copepods resulted in even higher toxin production. The induced response is thus only elicited by copepods that pose a real threat to the responding cells, which supports that the induced toxin production in diatoms evolved as an inducible defense. The cellular toxin content in Pseudo-nitzschia was positively correlated to the concentration of a group of specific polar lipids called copepodamides that are excreted by the copepods. This suggests that copepodamides are the chemical cues responsible for triggering the toxin production. Carnivorous copepods were found to produce less or no copepodamides. Among the diatoms exposed to grazing herbivorous copepods, only two of six species of Pseudo-nitzschia and none of the Nitzschia or Fragilariopsis strains responded by producing DA, indicating that not all Pseudo-nitzschia species/strains are able to produce DA, and that different diatom species might have different strategies for coping with grazing pressure. Growth rate was negatively correlated to cellular domoic acid content indicating an allocation cost associated with toxin production. Long-term grazing experiments showed higher mortality rates of grazers fed toxic diatoms, supporting the hypothesis that DA production is an induced defense mechanism.

Effects of predator lipids on dinoflagellate defence mechanisms - increased bioluminescence capacity.

Short flashes of blue light (bioluminescence) from dinoflagellates can reduce copepod grazing of light-emitting cells. Other protective strategies against grazing are toxicity, reduced cell chain length and altered swimming patterns in different phytoplankton. Both toxicity and bioluminescence capacity in dinoflagellates decrease in copepod-free cultures, but toxin production can be restored in response to chemical alarm signals from copepods, copepodamides. Here we show that strains of the dinoflagellates Lingulodinium polyedra and Alexandrium tamarense, kept in culture for 14 and 9 years respectively, are capable of increasing their total bioluminescence capacity in response to copepodamides. The luminescence response to mechanical stimulation with air bubbles also increases significantly in L. polyedra after exposure to copepodamides. Effects on size, swimming speed and rate of change of direction in L. polyedra and A. tamarense were not detected, suggesting that post-encounter mechanisms such as bioluminescence and toxin production may constitute the dominating line of defence in these taxa. To our knowledge, this study provides the first evidence of changes in bioluminescence physiology as a response to chemical cues from natural enemies and emphasizes the importance of bioluminescence as an anti-grazing strategy.