A redox-regulated type III metacaspase controls cell death in a marine diatom

Programmed cell death (PCD) in marine phytoplankton was suggested as one of the mechanisms that facilitates large scale bloom demise. Yet, the molecular basis for algal PCD machinery is rudimentary. Metacaspases are considered ancestral proteases that regulate cell death, but their activity and role in algae are still elusive. Here we biochemically characterized a recombinant metacaspase 5 from the model diatom Phaeodactylum tricornutum (PtMC5), revealing calcium-dependent protease activity. This activity includes auto-processing and cleavage following arginine. PtMC5 overexpressing cells exhibited higher metacaspase activity and were more sensitive to a diatom-specific infochemical compared to WT cells. Mutagenesis of potential disulfide-forming cysteines decreased PtMC5 activity, suggesting redox regulation. This cysteine pair is widespread in diatom type III metacaspases, but was not found in any other taxa. The characterization of a cell death associated protein in marine phytoplankton will enable deeper understanding of the ecological significance of PCD in bloom dynamics.