From the blue algae to the symbiotic one
Given that at that time there were also “predators”, the blue algae were eaten. These devouring enemies are thought to have maybe been a sort of “arche” bacteria, like those that managed to live in the warmest springs. Even today, in fact, these bacteria continue to exist at depths of more than 1.000 meters or in some containers of the purification plant. However, at a certain point it happened that a photosynthetic bacterium (a blue algae) was not digested by its “predator” and attempted with its enemy a synchronized cellular division, or rather they multiplied together. The “predator” therefore became a “hosting organism”, managing then to use the products of the photosynthesis, since the bacterium enclosed found satisfying vital conditions. A chloroplast originated from the blue algae. The blue algae had not only chlorophyll, but also a further system of antennae for the light in the yellow-green area: phycobilins of a blue or red aspect. The blue algae contain phycocyanin, while the red ones contain phycoerythrin. Other predators, which are probably mono-cellular on the organizational scale of the present protozoa, devoured the small red algae without digesting them. This allowed the creation of algae that are easily identifiable under the microscope in almost any type of water. The first symbiont produced, the chloroplast, presents two membranes, its one ant the reduced one of the red alga. The blue algae however, not only became chloroplasts. Other bacteria, with other characteristics, developed too. In fact there were uncolored bacteria not only able to use the sugar, but also to completely “breath” it. As product, like at the beginning, there were still water and carbon dioxide. These bacteria today are called mitochondria, and have been continually transferred so that even we humans, with the help of these endosymbionts in our body cells, are able to completely burn the sugars. The difference between the animals and the successors of the algae, or rather the superior plants, consists in the fact that the latter have photosynthetic bacteria. Whether our predecessor animals did not devour these bacteria, did not manage to maintain them or were lost during the evolution, it is something unknown. Still today there are uncolored algae. They are able to devour bacteria or other algae, using them for a certain period and then digesting them. Probably these species have lost their chloroplast and now feed in this way. Such strategies are found in the dinoflagellates. Some contain bacteria that produce toxins, like it happens for the Gonyaulax water flowerings.
The “programmed” death begins
Today among the mono-cellular organisms it is no longer possible to distinguish between vegetable and animal protozoa, but they are all together defined “Protista”. In this way it is possible to indicate all the mono-cellular organisms with a nucleus, which multiply by nuclear fission. For the first time in the evolution history we encounter an interesting phenomenon, which can only be observed among the superior algae and the animal multi-cellular organisms: the programmed death of the organism itself. The animal or vegetable mono-cellular organisms are potentially immortal, since they divide in a continuous way. A cell absorbs the nutrient substance, then it grows up to a certain size and divides into two identical daughter cells. In theory, on hearth there might be billion years old mono-cellular organisms, if they managed to avoid predations or other causes of mortality. The same happens for a part of the threadlike algae, in which still all cells divide. Since their origin, at a certain point of the evolution, after the doubling of the algal cells, no fission has taken place, and this “strange” cellular division must have brought some advantages. For examples the filters are not able to take threadlike nutritious particles, and in such way these floating algae must had less predators. During a further evolution step the cells of the algae no longer doubled on a sole level, but in all the directions. Such development led to more complex algae like the thallus (creeping) ones, and others that formed new tissue, like for example the focus ones. The development of the multicellularity also led to a distribution of the tasks of the single components and a cellular differentiation took place. Only few of them still maintained the task of breeding. The remaining ones took on other tasks and after a certain period died, limiting the lifespan of the alga. Some of them, later, also developed the sexuality, through the appearance of egg cells and movable swarms from the half chromosome content. Some examples among the green algae are the Ulva (sea lettuce), Caulerpa and Chara. Algae related to the latter, then, originated the mosses and the other superior plants, in which the egg cell after the fertilization remained protected by the mother plant, evolving until the embryo stage that was separated only later. The superior plants improved this breeding process, “wrapping” this embryo with a certain quantity of energetic reserve and producing the seed. In this way it is possible to explain the definition of the algae as “oxidative-photosyntetic organisms without embryo”.