Silicified cell walls as a defensive trait in diatoms

Abstract

Diatoms contribute nearly half of the marine primary production. These microalgae differ from other phytoplankton groups in having a silicified cell wall, which is the strongest known biological material relative to its density. While it has been suggested that a siliceous wall may have evolved as a mechanical protection against grazing, empirical evidence of its defensive role is limited. Here, we experimentally demonstrate that grazing by adult copepods and nauplii on diatoms is approximately inversely proportional to their silica content, both within and among diatom species. While a sixfold increase in silica content leads to a fourfold decrease in copepod grazing, silicification provides no protection against protozoan grazers that directly engulf their prey. We also found that the wall provides limited protection to cells ingested by copepods, since less than 1% of consumed cells were alive in the faecal pellets. Moreover, silica deposition in diatoms decreases with increasing growth rates, suggesting a possible cost of defence. Overall, our results demonstrate that thickening of silica walls is an effective defence strategy against copepods. This suggests that the plasticity of silicification in diatoms may have evolved as a response to copepod grazing pressure, whose specialized tools to break silicified walls have coevolved with diatoms.

Dimensions of marine phytoplankton diversity

“…This experiment highlights that size diversity (Fig. 8b) is controlled not only by the rate of supply of the limiting nutrients, but also by size-specific grazing (Armstrong, 1994;Poulin and Franks, 2010;Ward et al, 2012).…”

“…Obviously, nutrient supply rate (a bottom up process) cannot alone lead to high size diversity. Top-down processes are essential for the build-up of size classes with higher nutrient supply (see also Poulin and Franks, 2010). Considering only correlations with productivity would lead one to miss this important biotic interaction as a control on diversity.…”

Dimensions of Marine Phytoplankton Diversity

“…There are also specific differences between functional groups in cell elemental stoichiometry, and palatability to grazers (diatoms and coccolithophores, with their hard surface covering deter grazers, see e.g. Monteiro et al, 2016, Pančić et al, 2019. The smallest phytoplankton have the highest affinity for nutrients (Edwards et al, 2012) as a result of the lowest surface to volume ratio in larger cells (Kiorboe 1993, Raven, 1994.…”

Diatom defence: Grazer induction and cost of shell‐thickening

“…The shell has the highest mechanical strength relative to its density of any known biological material (Aitken, Luo, Reynolds, Thaulow, & Greer, 2016) and the consequent force required to crush the cell wall is remarkable (Hamm et al., 2003; Wilken et al., 2011). Recent experiments have indeed demonstrated that the shell provides partial protection against copepod grazing (Liu, Chen, Zhu, & Harrison, 2016; Pančić, Torres, Almeda, & Kiørboe, 2019). Thus, a six‐fold increase in silica content in a variety of diatoms decreased the predation mortality due to a copepod grazer by four‐fold and entirely prevented small copepod nauplii from consuming the diatoms (Pančić et al., 2019).…”