The chitin raft hypothesis for the colonization of the open ocean by cyanobacteria.

It is often assumed planktonic cyanobacteria existed in Precambrian oceans, but that their productivity was constrained. However, available evidence suggests picocyanobacteria only colonized the open ocean near the Neoproterozoic-Phanerozoic boundary, close to the start of a period of sustained atmospheric oxygenation. If earlier open oceans were devoid of planktonic cyanobacteria, we lack consensus explanations for why this was the case. Colleagues and I recently introduced the 'chitin raft hypothesis', which argues that accumulating chitin particulate waste associated with the rise of arthropods provided an essential evolutionary stepping stone in the rise of marine picocyanobacteria. According to this hypothesis, chitin particles derived from arthropod exoskeleton moults offered marine picocyanobacteria refugia from environmental stresses in the water column, allowing them to explore-and begin adapting to-the open ocean for the first time. Here, I review the context and implications of this hypothesis. One implication is that Precambrian biospheric productivity was constrained by the total global volume of benthic habitats. Hence, the rise of sub-aerial continents near the Archaean-Proterozoic boundary would have driven a major increase in biospheric productivity, with the expansion of oxygenic photosynthesis into the open ocean and onto the continents near the Proterozoic-Phanerozoic boundary driving a second major increase.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.