Thoughts On The Evolution And Ecological Niche Of Diatoms

Diatoms are the most recent major algal lineage added to the geological record, appearing more than 200 million years ago. They are stramenopile protists resulting from a secondary endosymbiotic event that yielded the only photosynthetic protistan lineage expressing external siliceous cell wall structures called frustules. Many diatoms also have large internal vacuoles, and a common assumption in the literature is that success of the diatoms is largely attributable to these two morphological inventions: the frustule for defense and vacuole for luxury nutrient uptake. Here, we revisit the evolution of these inventions, propose sequential steps in frustule development, replace luxury nutrient uptake with predator defense and buoyancy control as the driver of vacuole expansion, and suggest that perhaps the greatest significance of the frustule for diatom evolution is the secondary consequence of enhancing sexual reproduction. In this synthesis, we emphasize a distinction between the "general" success of diatoms and the success of "bloom-forming" species, as the physiological and morphological drivers of these successes differ. Importantly, the bloom-forming species are responsible for the major role of diatoms in aquatic biogeochemical cycles. The bloom-forming habit we ascribe to specific physiological attributes that, at their core, revolve around influencing the balance between diatom growth and losses to predators. We propose that these physiological adaptations are linked to size-dependent maximum division rates in bloom-forming diatoms, because of size scaling of predator-prey interactions. The existence of these bloom-forming species yields an apparent allometric relationship that has previously been interpreted in terms of nutrient acquisition. Our analysis yields insights into species successions during blooms, considers the fundamental benefit of blooming (and subsequent sinking) from a reproductive standpoint, and provides some reinterpretation of diatoms success over geologic time and in the modern ocean.