Diatom defence: Grazer induction and cost of shell-thickening

1. Diatoms account for 40 % of the ocean primary production and play a key
role in the oceans’ ability to sequester carbon. The evolutionary success
of diatoms and their role in ocean biogeochemistry are related to the
siliceous shell that provide partial protection against grazing. 2. The
structure and function of phytoplankton communities are governed by
environmental constraints and organismal trade-offs. Defence mechanisms
may help explain the high diversity of phytoplankton (incl. diatoms) in
the ocean, but only if the defence comes at a cost. Defence costs have
been notoriously difficult to demonstrate and quantify in marine
phytoplankton. 3. Here, we demonstrate for seven species of planktonic
diatoms that their shell thickens and their growth rate declines when
cells are exposed to chemical cues from copepods, important predators of
diatoms. The responses are proportional to the concentration of grazer
cues, but are also highly variable, both between and within species. 4. At
our standard experimental condition, the typical decline in growth rate is
10 %, and the typical increase in cellular biogenic silica is 16 %. The
latter value corresponds to a decline in grazing mortality due to small
copepods of 11 %. Thus, silification in response to grazers is exactly
warranted. 5. The similar magnitude of the costs and benefits of
silification suggests a flat fitness landscape along the
competition-defence axis. This may help explain the high diversity of
coexisting diatoms in the ocean. 6. The significant but variable
contribution of diatoms to the downward flux of organic carbon in the
ocean depends to a large extent on the silica content of the cells. This
is due less to the ballasting effect of silica, but mainly to the
different life histories of more or less defended cells that are governed
by evolutionary adaptations and – as demonstrated here - plastic responses
to grazers.