Bryopsis corticulans Morphology
Cellular structure and composition
Bryopsis corticulans is a beautiful feather-like siphonous green algae. Siphonous algae (orders Caulerpales, Cladophorales and Dasycladales) lack tranverse cell walls (dividing up their cytoplasm, organelles and nuclei) in their thalli, thus Bryopsis is unicellular, but multinucleate (or coenocytic).
B. corticulans is blackish-green in color with axes (thalli) that are naked above and pinnately branched above. The thallus of Bryopsis has vacuole (fluid-filled region) that transverses through the center of the thallus. The vacuole is membrane bound and supported by a cytoskeleton of actin and microtubule filaments. Thus the chloroplasts and organelles are forced to travel in a narrow region between the membrane and the vacuole (stream-lined movement). This is probably why you can see the chloroplasts so clearly in the pictures below (at 200X) becuase the chloroplasts have all been pushed to the outside in a narrow space between the vacuole and the membrane.
Bryopsis is a green algae and therefore has chlorophylls a and b that give it its luscious green color, as well as several accessory pigments including carotenes and xanthin. Like other greens, it stores its food mainly as starch (carbohydrate). Its chloroplasts can have pyrenoids (outpockets in in the thylakoid membranes of chloroplasts), though the function of pyrenoids is not clear. Bryopsis has a very thin external membrane that is composed of pectin and cellulose.
Because Bryopsis is unicellular, damage to the plant (which would be the entire cell) could be potentially fatal, e.g. all the cytoplasm leaking out of the cell when it is cut. A multicellular organism could localize the wound to one or a few cells, but Bryopsis does not have that option. Luckily, Bryopsis as well as other members in its family have evolved a way to "heal" themselves when damaged (tears, cuts, dessication, etc.). When the membrane is punctured, "clotting factors" (specific proteins, organelles and chloroplasts) will actually aggregate at the site of the wound and plug it up. WIthin 15-20 minutes, a gelatinous envelope composed of polysaccharides and lipids develops around the aggregates. A cell membrane and cell wall are subsequently formed around the aggregate as well [6, 7]. This process somewhat resemebles the blood clotting process humans undergo when we get cut.
This clotting process in Bryopsis is also involved in vegetatively reproduction through abscission and fragmentation of the plant. Sometimes, a separated fragment can become a plant of its own (complete with ability to reproduce and everything) because clotting will plug up the openings of the fragment. From there, a rhizoid will develop at the base and the thallus will continue to grow apically like the parent plant [6, 7].