Zoochlorella

Abstract: The intertidal sea anemone Anthopleura elegantissima contains two symbiotic algae, zoochlorellae and zooxanthellae, in the Northern Puget Sound region. Possible nutritional advantages to hosting one algal symbiont over the other were explored by comparing the photosynthetic and carbon translocation rates of both symbionts under different environmental conditions. Each alga translocated 30% of photosynthetically fixed carbon in freshly collected anemones, although zoochlorellae fixed and translocated less carbon than zooxanthellae. The total amount of carbon translocated to the host was equivalent because densities of zoochlorellae were two to three times greater than were densities of zooxanthellae. In A. elegantissima maintained under high and low irradiance (100 and 10 {mu}mol photons/m2/s) at 20{deg}C and 13{deg}C for 21 days, both algae fixed and translocated carbon at greater rates at 20{deg}C (translocation rates: 0.38 pg C /zoochlorella/h; 1.12 pg C /zooxanthella/h) than at 13{deg}C (translocation rates: 0.06 pg C /zoochlorella/h; 0.37 pg C /zooxanthella/h). However, zoochlorellate anemones received 3.5 times less carbon at 20{deg}C than at 13{deg}C because the higher temperature caused a significant reduction in the density of zoochlorellae. Environmental variables, like temperature, that influence the densities of the two symbionts will affect their relative nutritional contribution to the host. Whether these differences in carbon translocation rates of the two algal symbionts affect the ecology of their anemone host awaits further investigation.

Abstract: Both zooxanthellae and zoochlorellae are found in the cerata of Aeolidia papillosa after it has ingested symbiotic Anthopleura elegantissima containing these algae. High rates of photosynthesis were found in algae present in the cerata and in algae isolated from nudibranch feces. For algal cells present in the cerata of nudibranchs collected in June 1991, carbon fixation by zooxanthellae (1.18 +/- 0.36 pg C/cell/h) was significantly greater than carbon fixation by zoochlorellae (0.55 +/- 0.32 pg C/cell/h). Algal densities within the cerata of laboratory fed nudibranchs were significantly greater for zoochlorellae (175 +/- 82 cells/μg protein, light treatment; 131 +/- 106 cells/μg protein, dark treatment) than for zooxanthellae (38 +/- 18 cells/μg protein, light; 53 +/- 30 cells/ μg protein, dark). Ceratal densities of zooxanthellae (16 +/- 8 cells/μg protein) in the field during January 1992 were low in comparison to ceratal densities in the laboratory--several of the nudibranchs in the field lacked any symbiotic algae, and zoochlorellae were always absent. Nudibranch algal densities were not stable and dropped rapidly if the nudibranchs were starved. Both zoochlorella and zooxanthella densities dropped to 0 cells/μg protein within 11 days of starvation. While these results show that the relationship between A. papillosa and the two algae is not a stable symbiosis, the photosynthetic activity of the algae in the cerata suggests that the nudibranch and/or the algae may benefit from the association while it lasts.

Abstract: Of several pioneers in the field of algal culture in the last decade of the 19th century, none was more influential than the Dutch scientist, Martinus Willem Beijerinck (1851-1931). Even those present-day researchers who regard history as an esoteric irrelevancy are reminded of Beijerinck by the culture solution which bears his name and by the frequently encountered citation of his name as the founder of the ubiquitous green algal genus Chlorella. In attempting to determine the cause of the dark green colour of some ponds near Delft, Beijerinck (l.c.) discovered countless minute living green cells. He immediately recognised their similarity to those endozoic cells that impart a green colour to certain protozoans and invertebrate metazoans, particularly those described from Hydra (a freshwater hydroid) and Spongilla (a freshwater sponge) by Brandt (I.c.), who established the genus Zoochlorella to accommodate them. Beijerinck initially thought that the cells in the pond were free-living forms of Z. conductrix (symbiont of Hydra) and Z. parasitica (symbiont of Spongilla), but his attempts to isolate the symbiotic algae failed. He concluded that the pond-dwellers and symbiotic algae were congeneric, but not conspecific. He did not explain why he changed the generic name to Chlorella. Presumably, he thought that Zoochlorella was inappropriate for a genus that included algae not associated with animals. The fact that Chlorella is an illegitimate substitute name was appreciated by Shihira & Krauss (Chlorella: 2. 1965), who stated their intention to propose its conservation but failed to do so. Contrary to Beijerinck's opinion, the symbiotic and free-living forms are now considered to be congeneric and possibly conspecific (Reisser, Algae & Symbioses, 1992). Chlorella is retained as the name of the genus while "zoochlorella" is used informally to apply to a symbiotic Chlorella. The "zoochlorella" from Hydra (Z. conductrix) is considered to be closely related to C. vulgaris. Beijerinck (1890) characterised Chlorella as follows: cells occurring singly, spherical, ellipsoidal, or flattened, 1-6 pm diameter, with only one chloroplast in the shape of a segment of a sphere; pyrenoid indistinct or lacking; photosynthate stored as starch; reproduction by successive cleavage of cell contents to form autospores; zoospores absent. He recognised two free-living species, C. vulgaris and C. infusionum, the former with spherical cells 2-6 ,um diameter, the latter with flattened or short-cylindrical cells 2-4 pm diameter. To these two species were added C. parasitica (K. Brandt) Beij. and C. conductrix (K. Brandt) Beij.