title: Synchronization of cell death in a dinoflagellate population is mediated by an excreted thiol protease.
abstract: Regulated programmed cell death (PCD) processes have been documented in several phytoplankton species and are hypothesized to play a role in population dynamics. However, the mechanisms leading to the coordinated collapse of phytoplankton blooms are poorly understood. We showed that the collapse of the annual bloom of Peridinium gatunense, an abundant dinoflagellate in Lake Kinneret, Israel, is initiated by CO2 limitation followed by oxidative stress that triggers a PCD-like cascade. We provide evidences that a protease excreted by senescing P. gatunense cells sensitizes younger cells to oxidative stress and may consequently trigger synchronized cell death of the population. Ageing of the P. gatunense cultures was characterized by a remarkable rise in DNA fragmentation and enhanced sensitivity to H2O2. Exposure of logarithmic phase (young) cultures to conditioning media from stationary phase (old) cells sensitized them to H2O2 and led to premature massive cell death. We detected the induction of specific extracellular protease activity, leupeptin-sensitive, in ageing cultures and in lake waters during the succession of the P. gatunense bloom. Partial purification of the conditioned media revealed that this protease activity is responsible for the higher susceptibility of young cells to oxidative stress. Inhibition of the protease activity lowered the sensitivity to oxidative stress, whereas application of papain to logarithmic phase P. gatunense cultures mimicked the effect of the spent media and enhanced cell death. We propose a novel mechanistic framework by which a population of unicellular phytoplankton orchestrates a coordinated response to stress, thereby determine the fate of its individuals.
authors.affiliation: Department of Plant and Environmental Sciences, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
authors.forename: Assaf
authors.initials: A
authors.lastname: Vardi
authors.forename: Doron
authors.initials: D
authors.lastname: Eisenstadt
authors.forename: Omer
authors.initials: O
authors.lastname: Murik
authors.forename: Ilana
authors.initials: I
authors.lastname: Berman-Frank
authors.forename: Tamar
authors.initials: T
authors.lastname: Zohary
authors.forename: Alex
authors.initials: A
authors.lastname: Levine
authors.forename: Aaron
authors.initials: A
authors.lastname: Kaplan
mesh_terms: D000367:Age Factors; D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D005075:Biological Evolution; D018533:Biomass; D002245:Carbon Dioxide / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D053938:DNA Fragmentation; D004141:Dinoflagellida / Q000187:drug effects / Q000201:enzymology* / Q000502:physiology; D006861:Hydrogen Peroxide / Q000494:pharmacology; D007976:Leupeptins / Q000494:pharmacology; D018384:Oxidative Stress / Q000502:physiology; D015800:Protozoan Proteins / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D007976:Leupeptins; D015800:Protozoan Proteins; D002245:Carbon Dioxide; D006861:Hydrogen Peroxide; D003546:Cysteine Endopeptidases; C032854:leupeptin