Biology subjects

Mizrachi, A.

Publications and source records attributed to Mizrachi, A..

4 recordsLinked to original sources

bioRxiv2025

Metacaspases mediate thermotolerance of diatoms following marine heat waves

O_LIMarine heat waves (MHW) are extreme climate events characterized by elevated ocean temperatures lasting from days to months and spanning thousands of kilometers. Recent predictions show an increment in MHW frequency, duration and intensity worldwide along with climate change. Nevertheless, little is known about MHW impacts on marine microbial life and, specifically, on algal blooms. Recent studies in yeast and green algae suggested that thermotolerance is mediated by metacaspases (MC), cysteine proteases, structurally similar to caspases known to mediate programmed cell death. C_LIO_LIInitially we created a heatwave model of 72-hours that exposes the model diatom Phaeodactylum tricornutum to elevated temperature with a recovery phase that enables to capture the mechanisms underpinning such acclimation. We generated triple knock-out mutants of the PtMCA genes and identified a vital role for MC in heat-stress acclimation. C_LIO_LIPtMCA triple mutants exhibited increased sensitivity to the heatwave treatment, which induced cell death that peaked days after returning to initial temperatures. We further revealed that heatwave treatment induced accumulation of reactive oxygen species and the PtMCA mutants were hypersensitive to oxidative stress as compared to WT cells. C_LIO_LIWe propose that metacaspases have a pivotal role in diatoms acclimation to elevated temperatures a trait vital for algal survival considering climate change. C_LI

microbiology↗

bioRxiv2024

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

Phytoplankton play a crucial role in global primary production and can form vast blooms in aquatic ecosystems. Bloom demise and the rapid turnover of phytoplankton are suggested to involve programmed cell death (PCD) induced by diverse environmental stressors. However, fundamental knowledge of the PCD molecular components in algae and protists in general remains elusive. Previously, we revealed that early oxidation in the chloroplast predicted subsequent cell death or survival in isogenic subpopulations that emerged following H2O2 treatment in the diatom Phaeodactylum tricornutum. Here, we performed transcriptome analysis of sorted sensitive oxidized cells and resilient reduced cells, to discover genes linked to their contrasting fates. By cross-comparison with a large-scale mutant screen in the green alga Chlamydomonas reinhardtii, we identified functionally relevant conserved PCD gene candidates, including the cysteine protease cathepsin X/Z (CPX). CPX mutants in P. tricornutum CPX1 and C. reinhardtii CEP12 both exhibited profound resilience to oxidative stress, supporting a conserved function in algal PCD. P. tricornutum cpx1 mutants, generated using CRISPR-Cas9, also exhibited resilience to the toxic diatom-derived infochemical cyanogen bromide. Phylogenetic and predictive structural analyses show that CPX is highly conserved in eukaryotes, and algae of the green and red lineages exhibit strong structural similarity to human cathepsin CTSZ. CPX is expressed by diverse algae across the oceans and during toxic Pseudo-nitzschia blooms, supporting its ecological importance. 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.

microbiology↗

bioRxiv2018

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.

microbiology↗

bioRxiv2018

Phenotypic variability in chloroplast redox state predicts cell fate in a marine diatom

Diatoms are photosynthetic microorganisms of great ecological and biogeochemical importance, forming vast blooms in diverse aquatic ecosystems. Current understanding of phytoplankton acclimation to stress is based on population-level analysis, masking cell-to-cell variability. Here we investigated heterogeneity within Phaeodactylum tricornutum populations in response to oxidative stress, which is induced by environmental stress conditions. We combined flow cytometry and a microfluidics system for live imaging to measure redox dynamics at the single-cell level using the roGFP sensor. Chloroplast-targeted roGFP exhibited a light-dependent, bi-stable oxidation pattern in response to H2O2, revealing distinct subpopulations of sensitive oxidized cells and resilient reduced cells. Subpopulation proportions depended on growth phase, linking the bi-stable phenotype to proliferation. Oxidation of chloroplast-targeted roGFP preceded commitment to cell death and was used as a novel cell fate predictor. We propose that light-dependent metabolic heterogeneity results in differential stress responses that regulate cell fate within diatom populations.

microbiology↗