APOPTOTIC‐LIKE CELL DEATH PATHWAY IS INDUCED IN UNICELLULAR CHLOROPHYTE CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE) CELLS FOLLOWING UV IRRADIATION: DETECTION AND FUNCTIONAL ANALYSES1

Chlamydomonas reinhardtii (Ehrenberg) cells exhibited cell death process akin to that of apoptosis when exposed to ultraviolet (UV)‐C irradiation (1–100 J/m²). We observed typical hallmarks of apoptosis including cell shrinkage, associated nuclear morphological changes, flipping of phosphatidylserine, and DNA fragmentation detected by the terminal deoxynucleotidyl transferase‐mediated dUTP nick end‐labeling assay and oligonucleosomal DNA laddering assay. Interestingly, fluorescence imaging of DNA changes in UV‐C exposed cells, following PicoGreen staining, revealed that extra‐nuclear DNA disintegrates before that of nuclear changes, where the latter extensively diffuses out of the nuclear compartment, spreading into the whole cell and reaching the periphery of dying cells. Antibodies against a mammalian caspase‐3 shared epitopes with a protein of 28 kDa; whose pattern of expression correlated with the onset of cell death. Moreover, growth experiments indicate that spent medium recovered from UV‐C exposed cells exhibit a protective effect against cell killing of fresh cultures of C. reinhardtii cells by UV irradiation. The protective effect of UV‐spent medium is not a general growth promotional response on normal cells, but rather, is specific to UV‐exposed cells. We propose a model that C. reinhardtii cells exposed to UV elicit apoptotic‐like changes, which in turn lead to an adaptive response in neighboring cells against fresh rounds of UV exposure, thereby promoting survival of the cell population.     

LIGHT‐INDUCED RESPONSES OF OXYGEN PHOTOREDUCTION,REACTIVE OXYGEN SPECIES PRODUCTION AND SCAVENGING IN TWO DIATOM SPECIES1

Diatoms are frequently exposed to high light (HL) levels, which can result in photoinhibition and damage to PSII. Many microalgae can photoreduce oxygen using the Mehler reaction driven by PSI, which could protect PSII. The ability of Nitzschia epithemioides Grunow and Thalassiosira pseudonana Hasle et Heimdal grown at 50 and 300 μmol photons · m^?2 · s^?1 to photoreduce oxygen was examined by mass spectrometric measurements of ¹⁸O₂. Both species exhibited significant rates of oxygen photoreduction at saturating light levels, with cells grown in HL exhibiting higher rates. HL‐grown T. pseudonana had maximum rates of oxygen photoreduction five times greater than N. epithemoides, with 49% of electrons transported through PSII being used to reduce oxygen. Exposure to excess light (1,000 μmol photons · m^?2 · s^?1) produced similar decreases in the operating quantum efficiency of PSII (F_q′/F_m′) of low light (LL)‐ and HL‐grown N. epithemoides, whereas HL‐grown T. pseudonana exhibited much smaller decreases in F_q′/F_m′ than LL‐grown cells. HL‐grown T. pseudonana and N. epithemioides exhibited greater superoxide and hydrogen peroxide production, higher activities (in T. pseudonana) of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and increased expression of three SOD‐ and one APX‐encoding genes after 60 min of excess light compared to LL‐grown cells. These responses provide a mechanism that contributes to the photoprotection of PSII against photodamage.     

EFFECT OF ELEVATED TEMPERATURE,DARKNESS, AND HYDROGEN PEROXIDE TREATMENT ON OXIDATIVE STRESS AND CELL DEATH IN THE BLOOM‐FORMING TOXIC CYANOBACTERIUM MICROCYSTIS AERUGINOSA1

This study assessed the implication of oxidative stress in the mortality of cells of Microcystis aeruginosa Kütz. Cultures grown at 25°C were exposed to 32°C, darkness, and hydrogen peroxide (0.5 mM) for 96 h. The cellular abundance, chl a concentration and content, maximum photochemical efficiency of PSII (F_v/F_m ratio), intracellular oxidative stress (determined with dihydrorhodamine 123 [DHR]), cell mortality (revealed by SYTOX‐labeling of DNA), and activation of caspase 3–like proteins were assessed every 24 h. The presence of DNA degradation in cells of M. aeruginosa was also assessed using a terminal deoxynucletidyl transferase‐mediated dUTP nick end labeling (TUNEL) assay at 96 h. Transferring cultures from 25°C to 32°C was generally beneficial to the cells. The cellular abundance and chl a concentration increased, and the mortality remained low (except for a transient burst at 72 h) as did the oxidative stress. In darkness, cells did not divide, and the F_v/F_m continuously decreased with time. The slow increase in intracellular oxidative stress coincided with the activation of caspase 3–like proteins and a 15% and 17% increase in mortality and TUNEL‐positive cells, respectively. Exposure to hydrogen peroxide had the most detrimental effect on cells as growth ceased and the F_v/F_m declined to near zero in less than 24 h. The 2‐fold increase in oxidative stress matched the activation of caspase 3–like proteins and a 40% and 37% increase in mortality and TUNEL‐positive cells, respectively. These results demonstrate the implication of oxidative stress in the stress response and mortality of M. aeruginosa.