Hydra viridis and Chlorella are inseparable; indeed, they are bound by strange circumstances. Hydra viridis, the fresh water hydra, is only a few millimeters long. It looks like a tube defined on one end by a foot, with which it secures itself to a rock, and on the other end by a mouth crowned with tentacles. Hydra viridis is really a solitary creature yet it has clinched a deal with the microscopic green alga Chlorella. Chlorella is unicellular and resides in the hydra’s cells. Such an association bestows upon the alga not only protection from the outside world but also the means to survive. In exchange and in perfect symbiosis, the alga supplies energy which the hydra uses in the event of reproduction when conditions are not so favorable.
What exactly unites the two? Chlorella stimulates the synthesis of a number of proteins in the fresh water hydra, namely the HvAPX1 enzyme. HvAPX1 is a peroxidase which traps free radicals otherwise toxic to the cell. However, this particular peroxidase is only synthesized when the hydra develops a reproductive cell: the ovum. It is likely that the role of HvAPX1 is to protect the ovum so that it may complete its development successfully. Consequently, the hydra’s progeny is ensured and that of the algae also since they are readily transferred into the ovum as it forms.
From where could a protein - which has a role in the survival of two very different organisms - possibly originate? The answer is surprising. Despite the fact that it is produced by the hydra’s machinery, the protein itself is not of animal origin… In fact, it has never been found in any animal before. Which is hardly surprising since it is in effect characteristic of plants! So, how on earth did it end up in the hydra? Well the gene coding for HvAPX1 must have been borrowed from a plant on a rather permanent basis…not from Chlorella however but most probably from a plant - currently unknown to us - and that may well have been an ex-inhabitant of the hydra. Be that as it may, Hydra viridis and Chlorella are now bound by a diverted protein in their quest for mutual survival and the finding that a hydra can make such good use of a protein to which it was initially foreign, gives a novel insight into the mechanisms of evolution within the realm of symbiotic organisms.