Cathepsin X is a conserved cell death protein involved in algal response to environmental stress.

Phytoplankton are responsible for half of the global photosynthesis and form vast blooms in aquatic ecosystems. Bloom demise fuels marine microbial life and is suggested to be mediated by programmed cell death (PCD) induced by diverse environmental stressors. Despite its importance, the molecular basis for algal PCD remains elusive. Here, we reveal novel PCD genes conserved across distant algal lineages using cell-to-cell heterogeneity in the response of the diatom Phaeodactylum tricornutum to oxidative stress. Comparative transcriptomics of sorted sensitive and resilient subpopulations following oxidative stress revealed genes directly linked to their contrasting fates of cell death and survival. Comparing these genes with those found in a large-scale mutant screen in the green alga Chlamydomonas reinhardtii identified functionally relevant conserved PCD gene candidates, including the cysteine protease cathepsin X/Z (CPX). CPX mutants in P. tricornutum CPX1 and C. reinhardtii CYSTEINE ENDOPEPTIDASE 12 (CEP12) exhibited resilience to oxidative stress and infochemicals that induce PCD, supporting a conserved function of these genes in algal PCD. Phylogenetic and predictive structural analyses show that CPX is highly conserved in eukaryotes, and algae exhibit strong structural similarity to human Cathepsin X/Z (CTSZ), a protein linked to various diseases. CPX is expressed by diverse algae across the oceans and correlates with upcoming demise events during toxic Pseudo-nitzschia blooms, providing support for its ecological significance. Elucidating PCD components in algae sheds light on the evolutionary origin of PCD in unicellular organisms and on the cellular strategies employed by the population to cope with stressful conditions.