Defence in diatoms: mechanisms and trade-offs

Defence plays an important role in shaping the planktonic food web. The combination of predation and defence allows for the co-existence of defence- and competitionspecialists and is hence an important mechanism in maintaining species diversity. For predation to promote species diversity defence must come at a cost, otherwise all species would be equally defended. Phytoplankton have developed a large array of defence mechanisms ranging from morphological to biochemical or behavioural. Many of the defence mechanisms are inducible, i.e., harnessed or intensified when grazer cues are present. This has been demonstrated for different defence strategies such as toxin production in dinoflagellates or diatoms, bioluminescent capabilities in dinoflagellates and chain-length reduction and/or shell-thickness increase in diatoms. Inducible defences are believed to evolve when defences are costly. However, the demonstration and quantification of phytoplankton defences are rare and have been notoriously difficult to establish. This PhD thesis explores defence mechanisms and trade-offs in diatoms through three different projects: (i) Inducibility of shell-thickening by grazer cues and the associated trade-offs, (ii) effect of shell-thickness on prey selection by grazers, and finally (iii) grazer-induced aggregation as a defence mechanism in diatoms. Diatoms are characterized by having a siliceous outer shell that provides protection against grazing. In fact, the shell of the diatom appears to be the strongest known biological material relative to its density. In my first project, we demonstrate for seven species of diatoms that the presence of chemical grazer cues induces thickening of their silica shells at the cost of reduced growth rates. The response is proportional to the concentration of cues, although it is highly variable, both between and within species. From previous quantification of the defensive value of thicker shells, we demonstrate that the trade-off is near neutral, i.e., that the benefits (lower mortality rate) is near equal the costs (lower growth rate), and that this may help explain why diatoms are a particularly diverse group. It is now well documented how a thicker shell decrease mortality rate in diatoms. However, the exact mechanism of how a thicker shell is beneficial has yet to be demonstrated. It is therefore unknown if the reduced mortality rate is due to the cells being de-selected by the grazer or handled for such a long time, that the general foraging time by the copepod is reduced. By using direct video-observations, we demonstrate how thick-shelled diatoms captured by a copepod are not only handled for much longer but are also far more likely to be rejected by the copepod, compared to thin-shelled diatoms. Overall, the presence of thick-shelled diatoms, whether due to light limitation, grazer presence, or both, reduces the grazing mortality of diatoms because copepods spend more time handling cells, leaving less time for actually feeding. Diatoms play an important role in the ocean carbon cycle. Rapid mass sedimentation by the end of a bloom moves great amounts of carbon from the surface to the deep. However, sinking is also of crucial survival value to diatoms and a part of their life cycle. When conditions change from replete to deplete, diatoms form aggregates that sink rapidly to the bottom. Here, the predation pressure is much less of that in the water column. This enables the transition from vegetative cells in the surface to resting stages that can survive the winter in the benthic zone. The common assumption is that aggregation is initiated by nutrient depletion; cells then become sticky and due to high cell concentrations, they collide, stick together, and sink out. In my third project, we discovered that some diatoms form aggregates in the presence of grazer cues. We explored the inducibility of stickiness in six species of diatoms. Two species increased their stickiness when exposed to copepod cues and the response was proportional to the cue concentration until saturation. In one species, the increase in stickiness was additionally proportional to the duration of exposure. We further tested the effect of nutrient limitation (Si and N) on the stickiness of five diatom species and found to our surprise that nutrient limitation did not increase the stickiness. In fact, one of the species had higher stickiness in high-nutrient conditions, compared to low-nutrient conditions.