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00925njm 22002777a 4500 dc Mata, Maria Teresa author Palma, Armando author López Parages, María author Vázquez, Victor author Cheng-Sánchez, Iván author Sarabia-García, Francisco Ramón author López-Figueroa, Félix author Jiménez-Gámez, Carlos author Segovia-Azcorra, María author 2019 Ultraviolet radiation (UVR; 280–400 nm) has a great impact on aquatic ecosystems by affecting ecophysiological and biogeochemical pro-cesses as a consequence of the global change scenario generated by anthro-pogenic activities. We studied the effect of PAR (P)+UVA (A)+UVB (B) i.e. PAB, on the molecular physiology of the unicellular green alga Dunaliella tertio-lecta for six days. We assessed the relationship between the triggered UVR stress response and metacaspases and caspase-like (CL)activities, which are proteases denoted to participate in cell death (CD) in phytoplankton. UVR inhibited cell growth and in vivo chlorophyll a fluorescence but did not cause cell death. Western blot analyses reflected that Type-II metacaspases (MCs) are present and appear to be involved in UVR induced-cell stress but not in dark-induced CD in D. tertiolecta. Enzyme kinetics revealed that cleavage of the MCs-reporter substrates RVRR, QRR, GRR, LKR, HEK, and VLK was 10-fold higher than WEHD, DEVD, IETD, and LETD CLs-substrates. The lowest apparent Michaelis-Menten constants (KMap) corresponded to RVRRase (37.5 μM) indi-cating a high affinity by the RVRR substrate. The inhibition of enzymatic activ-ities by using inhibitors with different target sites for hydrolyses demonstrat-ed that from all of the R/ Kase activities only RVRRase was a potential candi-date for being a metacaspase. In parallel, zymograms and peptide-mass fin-gerprinting analyses revealed the identities of such Rase activities suggesting an indirect evidence of possible natural physiological substrates of MCs. We present evidence of type II-MCs not being involved in CD in D. tertiolecta, but rather in survival strategies under the stressful irradiance conditions applied in this study. Mata MT, Palma A, García-Gómez C, López-Parages M, Vázquez V, Cheng-Sánchez I, Sarabia F, López-Figueroa F, Jiménez C, Segovia M. Type II-Metacaspases are involved in cell stress but not in cell death in the unicellular green alga Dunaliella tertiolecta. Microb Cell. 2019 Oct 7;6(11):494-508. doi: 10.15698/mic2019.11.696. PMID: 31799323; PMCID: PMC6859423 https://hdl.handle.net/10630/37601 10.15698/mic2019.11.696 Muerte celular Type II Metacaspases are involved in cell stress but not in cell death in the unicellular green alga Dunaliella tertiolecta.