Grazer-induced aggregation in diatoms

Sinking is part of diatom life history. Typically, by the end of a bloom, diatoms form aggregates that sink rapidly to the bottom. This opportunistic lifestyle can be considered an adaptation to reduce predation losses, because predation pressure in the sediment is less than that in the water column. In this study, we demonstrate that two out of six examined diatoms become sticky and coagulate into aggregates when exposed to chemical copepod cues. Cell stickiness increases proportionally to the concentration of copepod cues and duration of exposure until saturation. Surprisingly, nutrient limitation (Si and N) did not increase stickiness. Furthermore, we demonstrate a chain length reduction of Skeletonema marinoi, when exposed to copepod cues. We argue that the increase in stickiness is adaptive when grazing mortality exceeds growth rate. This ensures that the maximum number of cells survive in the sediment, ready to utilize the next window of growth opportunity.