Sunsheds Or what if you are a plant in the shower

    Hi Habr! I recently came across this miracle, a “photosynthetic” mollusk, and decided to write about photosynthesis not in plants. Immediately I warn you, the text with a small amount of humor and frivolous places.

    For simplicity, let's start with the definition. Photosynthesis is the process of transformation of the light energy absorbed by the body into the chemical energy of organic (and inorganic) compounds.

    Plants and cyanobacteria have the ability to photosynthesis, but animals do not, which makes them strictly dependent on food sources, i.e., ready-made organic compounds. And how attractive it would be for animals to learn how to photosynthesize ... Imagine a flock of green chickens, cows ... students, hurrying after the sun, bathing in its rays ... They need almost nothing except sunshine. Fantastic, isn't it?
    However, in the process of evolution these organisms have already appeared. The process of introducing cells of another organism into the cells of one organism (including photosynthesizing), which took root there and became endosymbionts, took place. Endosymbiosis between the eukaryotic cell and cyanobacteria originated approximately 1.5 billion years ago. Then the first analogues of modern chloroplasts appeared.

    Funny moment. The ancestors of some photosynthetic organisms never entered directly into endosymbiosis with cyanobacteria - they assimilated algae cells, while taking on photosynthesis from them. This phenomenon is called "secondary endosymbiosis". They explain the presence in the shell of chloroplasts of some organisms more than two membranes and a strongly reduced nucleus (nucleomorph) that has lost its significance. Typical representatives of such organisms are ochrofitovye algae (Ochrophyta). Endosymbiosis can also be tertiary and less often quaternary. But for us it is not important now.

    Photosynthesis in animals

    Endophotosymbionts (cyanobacteria, microalgae or their functionally active chloroplasts) are found in representatives of mollusks, sponges, corals, anemone, hydras, worms, and ascidians. And we begin ascending. From minimally using their symbionts, to those whom they feed all their life.

    Attempts to create

    For more than a decade, they have been trying to create such a symbiosis. In the described by Yu.S. In experiments, chloroplasts were introduced into mouse fibroblasts by pinocytosis, were not attacked by the immune system and, while remaining intact, retained the ability to photosynthesis for five generations of cells. Attempts were made to cultivate chloroplasts in artificial nutrient media: chloroplasts carried out photosynthetic processes and RNA synthesis, were intact for about 100 hours, and retained their ability to divide for about 24 hours.

    However, the use of chloroplasts of higher plants in order to create a mixotrophic organism should be considered an unpromising approach. After all, they have lost most of their genome (transferred to the nucleus) and cannot exist for a long time in isolation from the host cell.
    For example, take the enzyme RuBisKo (ribulose-bisphosphate carboxylase), which catalyzes the addition of carbon dioxide to ribulose-1,5-biphosphate and starts the Calvin cycle. This is the most important enzyme in the dark stage of photosynthesis. It consists of two subunits, large and small. In green algae and higher plants, large subunits are encoded by the plastid genome, and small ones by nuclear. So without the nucleus of the host cell, the chloroplast cannot be stretched.

    However, there is one near successive Istria. These guys managed to inject the thermophilic cyanobacterium Synechococcus elongatus into the cells of macrophages and even run necrotic processes in its cells. In the same way, they introduced them into Danio rerio fish embryos and raised these fish to adulthood. The visual experience was ensured by the fact that a gene encoding the synthesis of a fluorescent protein was pre-introduced into cyanobacteria, and Danio fry are transparent and survive well in aquarium conditions. The bacteria
    did not die, but they could not give any significant amount of energy and substances to change the development of the fish itself or its nutrition.

    Cyanobacteria fluorescent cells are marked in red.

    Natural examples

    Yellow Spotted Ambystoma

    The green color of caviar of salamander Ambystoma maculatum is due to the presence of the endosymbiotic alga Oophila ambistomatis . The alga absorbs the products of the nitrogen metabolism of the embryo, giving it the oxygen produced during photosynthesis. In addition, carbohydrate metabolism can occur between them, as indicated by the proximity of the cells of the algae imitochondria in the eggs. [here is a link to the article, who are interested in the details ]

    Hydra green

    Hydra viridissima, enters endosymbiosis with zoochlorella. Zoochlorella living inside its cells are divided 32 times slower than free-living organisms, which is associated with inhibition of their growth. Hydra can digest or exorcise an excessive amount of symbionts from its cells, however, this is observed so far only in laboratory conditions.
    It was shown that non-photosynthetic hydras divide faster and grow larger than photosynthesizing. This is due to the need for green hydras to coordinate their growth and the growth of zoochlorella, as well as to control their activity. They are characterized by a relatively small number of growth activators compared to the number of inhibitors, thereby increasing the size of the host cells and creating more space for the growth and development of algae. Internal factors interact in such a way that stable endosymbiosis in large hydra is impossible.


    And here we come to the most interesting comrades. In fact, not a single slug "can" photosynthesize. Representatives of various taxa, such as Conchoidea, Stiligeroidea, and Elysioidea, are able to do this.
    Milaha in the photo at the beginning of the page - 5 mm Costasiella kuroshimae . And if you want to meet him - you are in Okinawa. But he is not the only one or the most studied slug of its kind. There is one more - Elysia chlorotica . He is a little bigger than his relative (up to 6 cm) and lives on the Atlantic coast of the USA and Canada. And on it I will stop in more detail.

    He, like his predecessor, is a proud sea slug.
    Chloroplasts he receives during kleptoplastia - sucking organelles from algae cells. Young slugs independently feed on the algae of Vaucheria litorea for about two weeks, while the chloroplasts are not digested by them, and deposited in the cells of the branched intestine. Then the slug stops feeding and completely switches to using the energy of light. Under laboratory conditions on the "neck of chloroplasts" it can hang all his life. This is approximately 8-9 months (if there is light and carbon dioxide available).
    In the event of an unfavorable period, or in the dark, it can digest chloroplasts, then it again has to extract them from algae.
    Thanks to horizontal gene transfer ...
    So, there is a horizontal transfer, and there is a vertical one. Vertical is the transfer of genes from the organism of the ancestor to the organism of the descendant, the horizontal - the transfer to the non-help organism. That is, from dad and mom to you - vertical. The bacterium ate someone else's DNA and built it in - horizontal.

    ... so Elysia chlorotica is able to maintain the functioning of the absorbed plastids for a long time just due to the horizontal transfer. For example, it synthesizes chlorophylls a and b independently .
    Usually 80-90% of plastid and mitochondrial proteins are encoded in the nuclear genome. Therefore, they do not survive for a long time without the nucleus of the host cell. However, the cunning slug, apparently, "borrowed" the necessary genes from the alga genome and quite express itself in them. At least, this is evidenced by the high level of transcription and translation of the light-absorbing components even at the 8th month after the “migration” of chloroplasts (which is not possible without the participation of the plant cell nucleus).
    In conclusion about the sad. His life is not at all as happy as it seems. After reproduction, they die. What caused the activity of the virus living in its cells.

    Here is such a story. All good week and warm last autumn days.
    Glory to Photosynthesis!

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