Autophagy Induced by Heavy Metal and Starvation Stress in Microalgae

Several genera and species of unicellular algae proved to be resistant, at different level of effectiveness, to heavy metals pollution. Some cells may arrest the metals at the wall level, by excluding the entrance into the cytoplasm; others may concentrate and precipitate the metal in a vacuole. Besides these methods, some cells appear to increase their cellular turnover by increasing autophagic activity in order to replace organelles that get damaged. Some preliminary results in Dictyosphaerium sp. may even suggest that some members of the population accumulate faster the heavy metals than the others, eventually dying and thus subtracting part of the metal from the medium. If confirmed, this activity would represent a Programmed Cell Death (PCD) case, representing the sacrifice of some cells in unicellular organisms in order to accomplish a specific task of general interest for the population. PCD, in this case, would be a method to reduce the metal availability in the medium by precipitating it in a vacuole of a dead cell. A survey of the ultrastructural morphology of effects of heavy metal-induced stress on cell organelles in unicellular algae is presented. The observed autophagic feature involved both accumulation of metal in the vacuole and formation of multilamellar bodies in consequence of organelle recycling. These two aspects of vacuolar response appear to be two different responses to toxic metal excess in the medium. The capability of some strains of species of unicellular or colonial algae to resist to high concentration of heavy metals in the environment is noteworthy, since the mechanism of resistance may provide clue about how to modify other strains known for production of useful compounds. Modified strains of species of industrial relevance could be grown even in polluted cultures and/or suggest methods to remove heavy metals from polluted water (phytoremediation). The starvation-induced autophagy in Heterosigma was preliminary to a type of PCD showing nuclear fragmentation. This specific feature is more typical of animals than algae and is probably related to the isolated phylogenetic position of Heterosigma with respect to the other microalgae here investigated, rather than to the type of PCD inducer.