Diatoms are one of the most diverse groups of phytoplankton in the ocean. One reason for this high diversity is the evolution of multiple defence mechanisms and subsequent evolution of grazer “counter measures”. In a new study, we describe the discovery of yet another defence mechanism: when exposed to chemical cues from copepods several species become “sticky” and form rapidly sinking aggregates, allowing cells to hide in the sediment until growth conditions again become favourable.
Copepods are the main grazers of diatoms. In response to grazers diatoms have evolved a range of defence mechanisms. These include both toxin production and reduction in colony size, as well as a characteristic protective siliceous shell. In this experimental study, we demonstrate how several species of diatoms increase their stickiness when exposed to chemical cues released by copepods. The sticky diatoms form aggregates that sink rapidly and thus escape predation by copepods in the surface ocean. We demonstrate how stickiness increases with increasing concentration of copepod cues as well as duration of exposure to cues. While sinking may remove diatoms from their grazers, it also implies lost growth opportunities. We argue that increased stickiness is adaptive when grazing mortality exceeds growth rate as it implies a higher number of cells surviving at depth. This defence mechanism has obvious implications to carbon export flux.
Read the paper here: http://doi.org/10.1002/lol2.10282
Grønning J, Kiørboe T (2022) Grazer-induced aggregation in diatoms. Limnol. Oceanogr. Lett. doi: 10.1002/lol2.10282