Normally, it has been considered that the only organisms capable of photosynthesis have been cyanobacteria, algae and plants. It is partly true, they are the only organisms capable of synthesizing chlorophyll, an essential molecule for the performance of this metabolic function.
In cyanobacteria , prokaryotic cells without a nucleus, the molecular machinery that performs photosynthesis is found free in the cell body. Algae and plants, however, are organisms made up of cells with a nucleus and have the molecular machinery for photosynthetic function in cellular organelles called chloroplasts.
Traditionally, it was thought that of the organisms made up of eukaryotic cells, only those that have the ability to reproduce their own chloroplasts and their own chlorophyll could carry out photosynthesis. Until the process of kleptoplasty or kleptoplasty was discovered. This is the name given to the process by which an organism —such as a protozoan or an animal— captures the chloroplasts or the whole cells of the plants or algae it consumes, and preserves and maintains them in its body. Since all the photosynthetic machinery is located in the chloroplast, it continues to photosynthesize.
Strictly speaking, photosynthesis is carried out by the chloroplast, and not by the animal itself, which only reaps its benefits. Of course, if we assume the process in the strict sense, it is not the plants that carry out photosynthesis, but rather the chloroplasts they contain, descendants of ancestral cyanobacteria.
Indeed, in evolutionary terms, these chloroplasts are, originally, photosynthetic bacteria that at some point, billions of years ago, associated with eukaryotic cells in an internal and permanent symbiosis. This endosymbiosis not only explains the bacterial origin of chloroplasts, but also that of mitochondria. But that is another topic.
But going back to the animals that carry out kleptoplasty, unlike plants and algae, they do not usually have the biochemical machinery that allows the captured chloroplasts to reproduce, and maintenance is not indefinite; Over time they end up degrading. So the chloroplast scavengers are forced to have a more or less constant supply.
The process was discovered in Elysia chlorotica , a sea slug from the Sacoglossa group, by researcher Hillary H. West, in 1979, and was the focus of her doctoral thesis. But, although since then, this curious animal has been taken as an example to talk about kleptoplasty, today we know many other animals with that ability.
Pteraeolidia ianthina , the blue dragon
The blue dragon ( Pteraeolidia ianthina ) is also grouped into what we commonly call “sea slugs” or nudibranchs , among which there are several groups. Those that practice kleptoplasty are usually nudibranchs that are members of the Sacoglossa group , such as Elysia chlorotica . However, the blue dragon is a Cladobranchia, a distinct group of sea slugs.
It is, therefore, an evolutionarily distant species from Elysia chlorotica . Also, unlike this one, the blue dragon is not green. Its body is translucent, and it has expansions, called cerata, that can vary from an almost golden brown to an electric blue.
In this case, kleptoplasty does not apply to the chloroplasts of the plants or of the algae that it consumes, but to complete unicellular algae of the group of dinoflagellates, specifically, of the genus Symbodinium . They are brown algae, which are captured and enclosed in vacuoles of the endoderm, where they are preserved, continue to photosynthesize, and give the animal its brown hue.
Baicalellia solaris , the photosynthesizing flatworm
Beyond sea slugs it is possible to find photosynthetic animals.
We found this behavior in animals as evolutionarily distant as flatworms . That is the case of Baicalellia solaris, a marine flatworm with a round shape and less than a millimeter in diameter. It feeds on diatoms, which it distributes throughout its transparent body. Aside from the captured algae, the only thing that is pigmented on this worm is its eyes.
A particularity of Baicalellia solaris is that it is the only known species of its genus that performs kleptoplasty, and therefore performs photosynthesis. Being such a small and flat animal, with very little volume and a large surface area exposed to light, it has the advantage that photosynthetic efficiency can approach, in magnitude, that of some algae. However, there is much about these animals that we still do not know. Experimenting with these animals is very complicated, because at the moment they cannot be cultivated , experiments can only be carried out with flatworms captured directly from the natural environment and kept alive.
Ambystoma maculatum , the only photosynthetic vertebrate
Most of the animals that practice kleptoplasty are invertebrates. All except one: the yellow-spotted salamander ( Ambystoma maculatum ) . This amphibian captures and retains in its skin cells of the green algae Oophila amblystomatis, with which it establishes a symbiotic relationship.
This mutually beneficial relationship is observed especially during embryonic development. The embryo growing inside the egg needs to breathe, and it produces ammonia as a waste, which is toxic to it. On the other hand, ammonia is an optimal source of nitrogen for the algae that form this symbiosis, which, through photosynthesis, produce oxygen that the embryo of the salamander can use to breathe.
In this case, it is the algae that penetrate the body of the embryo through the digestive system during its formation. They remain in the organism while it develops, remaining in your body throughout your life; they reproduce inside it, something exceptional that does not usually happen in animals with kleptoplasty; and are transmitted to the eggs. It is a form of endosymbiosis that, as far as we know, is unique in the world.
REFERENCES:
Burghardt, I. 2008. Symbiosis between Symbiodinium (Dinophyceae) and various taxa of Nudibranchia (Mollusca: Gastropoda), with analyses of long-term retention. Organisms Diversity & Evolution, 8(1), 66-76. DOI: 10.1016/j.ode.2007.01.001
Van Steenkiste, N. W. L., Stephenson, I., et al. 2019. A new case of kleptoplasty in animals: Marine flatworms steal functional plastids from diatoms. Science Advances, 5(7), eaaw4337. DOI: 10.1126/sciadv.aaw4337
West, H. H. 1979. Chloroplast symbiosis and development of the ascoglossan opisthobranch Elysia chlorotica [Northwestern University]. https://www.proquest.com/openview/5d4fe23e5a134bd768c8ff4481829c12/1?pq-origsite=gscholar&cbl=18750&diss=y
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