[Coral-List] Thomas Goreau's comments about coral-zooxanthellae symbiosis

[Charles Booth]

Thomas Goreau wrote:

³The fact that corals don't get their carbon from zooxanthellae is also very
old knowledge, but for decades people have
ignored the old literature and have mistaken the net oxygen balance to
assume that corals are also autotrophic in carbon. This recent error has
become dogma, despite being wrong, because nowadays people don't read the
literature or ask those who know it. The first radiocarbon tracer
experiments, done by Thomas F. Goreau and Nora I. Goreau more than 50
years ago showed that very little zooxanthella carbon translocation
contributed to coral carbon, and that corals relied on zooplankton for the
vast bulk of their carbon needs.²

I plead ignorance here as I¹m not a coral specialist. But, it has been my
understanding that zooxanthellae do make a significant contribution to the
carbon requirements of the coral polyp, and I have been relating this ³fact²
to my students for many years. My sources of information have been various
textbooks and review articles, and, occasionally, original research
articles, such as the following:

 Falkowski et al (1984, BioScience 34: 705-709) described energy budgets for
Stylophora pistillata based on O2 fluxes and relative biomasses of
zooxanthellae and coral tissue, and concluded that ³carbon fixation and
translocation by zooxanthellae contributes, on average, 140% of the coral
animal¹s daily carbon requirement for maintenance respiration.²
[The original data were published by Dubinsky, Falkowsky, Muscatine et al; I
presume these values, which were derived from net O2 production, are the
ones Dr. Goreau suggests have been interpreted incorrectly. However, I have
not come across any published criticism of this method and the results;
rather, the data appear to have been widely cited by various authors, such
as:

B.G. Hatcher (1988, Trends in Ecology and Evolution, 3: 106-111), who wrote
³...zooxanthellar production is able to meet the animal¹s carbon
requirements for growth as well as respiration. Shade-adapted corals fix
less carbon..., and must depend on heterotrophy to make up the difference
(refs: McCloskey et al, 1985, Proc. Fifth Int. Coral Reef Congr, vol. 4,
Delesalle, B. ed, pp. 527-529; Muscatine, et al, 1984, Proc. R. Soc. London
Ser. B. 222: 181-222).

Even if the O2 flux-derived values for energy/carbon translocation are
incorrect, subsequent studies using 14-CO2 tracer methods appear [to me, at
least] to support a significant role of zooxanthellae-derived carbon for
coral biomass production:

In Barnes, R.S.K and K.H. Mann, eds., Fundamentals of Aquatic Ecology
(Oxford/Blackwell, publication date was early 90s, I think ­ I¹m relying on
a photocopy of the relevant pages) R.N. Hughes wrote (Chapter 11, p. 217)
 ³Incubation with 14-CO2 has shown that as much as 50% of the carbon-14
fixed daily by photosynthesis becomes incorporated into coral tissue, mostly
as lipid and protein (Muscatine and Cernichiari, 1969, Biol. Bull. 137:
506-523; Kellogg and Patton, 1983, Mar. Biol. 75: 137-149).²

In Barnes, R.S.K. and R.N. Hughes, An Introduction to Marine Ecology (3rd
ed. Blackwell Science; published ~2000, as I recall- again, I¹m relying on a
photocopy w/o the publication date), the author(s) wrote (Chapter 6, p. 137)
³Estimating these proportions [of zooplankton- vs. zooxanthellae-derived
energy] is not easy and data are rather uncertain, but the proportions of
energy ultimately derived from photosynthesis ranges from over 95% in the
Œautotrophic¹ corals down to somewhat over 50% in the more extreme
Œheterotrophic¹ species....Incubation of Pocillopora damicornis with 14-CO2
on the reef has shown that up to 50% of the 14-C fixed by the zooxanthellae
during 24 h ends up in the coral tissue, mostly as lipid and protein.
Evidently, the glycerol released by the algae is used by the animal in lipid
synthesis.² [I presume the latter statements refer to the work by Muscatine,
Cernichiari, Kellogg, and Patton cited above]

More recently still, Lesser et al (2004, Science 305: 997-1000) wrote (p.
1000)
 ³...it [glycerol] is the major carbon compound translocated from the
symbiotic zooxanthellae to the host tissues (Muscatine , L. 1990, in Coral
Reefs, Ecosystems of the World, Z. Dubinsky, ed., Elsevier).²

I may well be guilty of a cursory reading of the primary literature and an
uncritical acceptance of secondary sources ­ if so, I would like to bring
myself up to date. Perhaps Dr. Goreau, or someone else, can direct me to the
relevant literature that clarifies the current understanding of the role of
zooxanthellae in the nutrition of coral polyps.

Chuck Booth


*   *   *   *   *   *   *
Dr. Charles E. Booth
Dept. of Biology
Eastern Connecticut State University
Willimantic, CT  06226

Ph: 860-465-5260
Email: booth at easternct.edu
FAX:  860-465-5213