Zooxanthella
Encyclopedia : Z : ZO : ZOO : Zooxanthella
Zooxanthellae are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans. They are typically dinoflagellate algae, although other algae such as diatoms can also be zooxanthellae. They are often acquired by direct ingestion, and subsequently multiply in the host's tissues, providing it with various nutrients. Most are autotrophs and provide the host with energy from photosynthesis. Their population in the host tissue is limited by controlling the amount of food and light they receive and by expulsion of excess cells. There are also zooxanthellae transmitted by the coral eggs.
Hermatypic (reef-building) corals have zooxanthellae and are largely dependent on them, limiting their growth to the photic zone. The symbiotic relationship is probably responsible for the phenomenal success of corals as reef-building organisms in tropical waters. However, when corals are subjected to high environmental stress, they can lose their zooxanthellae by either expulsion or digestion and die, changing from their normal colour to their white 'skeletons' in a process known as coral bleaching.
Other organisms which may have zooxanthellae include jellyfish, clams, sea slugs, and radiolaria. There are several different species of zooxanthellae, typically grouped together as the genus Symbiodinium, which appears to be monophyletic.
To: "Coral-list"
Greetings all,
I can't remain on the sidelines of this discussion.
There is a solid body of literature that supports the flux of energy
from symbiotic dinoflagellates to animal hosts, including the seminal
work by Muscatine, McCloskey and Porter (e.g. Muscatine et al. 1981,
Limnol Oceanogr. 26:601; Muscatine et al. 1984, Proc. r. Soc Lond
B:222:181-202), as well as that of numerous others. Yes, these studies
do rely on respirometry, but in conjunction with other measurements that
allow determination of the flux of carbon from symbiont to host. These
peer-reviewed papers in major journals cannot be simply dismissed.
There is also a substantial literature, most of it pre-1980, in which
14C (radiocarbon) has been used to show organic fluxes to the host.
Three examples: Bob Trench's Ph. D. thesis, in which translocated
glycerol was shown to be incorporated into host lipids (PRS
177B:225-235), Muscatine's in situ 14C work with corals in Hawaii (Biol.
Bull. 137:506-523), and Jim Battey's paper indicating rapid catabolism
of translocated glycerol by host tissue (Mar. Biol. 79, 27-38).
These studies bring up the point of the use of autoradiography using
histological sections, as Tom (F.) and Nora Goreau did in the 1960
Science paper that has been cited in this discussion. Given that a
substantial amount of the translocated carbon may be rapidly respired by
the host (as suggested by Jim Battey), and that translocated products
such as glycerol are alcohol-soluble and are likely to be leached out by
the procedures used in routine histological preparation, one would
expect that such studies will underestimate translocation.
Having said this, let me make these two points. (1). The discussion
sounds as if corals and other hosts of symbiotic dinoflagellates are
either autotrophic or heterotrophic. They are probably best viewed as
being polytrophic, using both ingested and translocated carbon as energy
sources. (2). There is now a wealth of information on the diversity
of Symbiodinium, but we have very little information on how this
diversity relates to energy or nutrient fluxes between the partners. It
seems quite likely that this diversity will affect the degree of
"autotrophy" or "heterotrophy" in various associations.
My apologies for the lengthy discourse!
Regards,
Clay
Clayton B. Cook, Ph. D.
Senior Scientist
Harbor Branch Oceanographic Institution
Ft. Pierce, FL 34946 USA
Ph. 772-465-2400 x 301
Mobile 772-579-0599
Fax 772-468-0757
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