[Coral-List] coral bleaching: response to Goreau

Andréa Grottoli grottoli.1 at osu.edu
Tue May 30 15:37:10 EDT 2006

Dear Tom,

I read your contribution on the coral list about 
coral bleaching with interest.  As the lead 
author of the recent Nature paper on 
heterotrophic plasticity in bleached corals, I 
would like to comment.  You stated:

"The recent paper that claims to have discovered "for the first time"
that corals eat zooplankton and can survive bleaching better if fed is
not new either. The 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. They kept corals completely
bleached in the dark for years, feeding them on zooplankton. So survival
of bleached fed corals has been known for over half a century and is not
a "new discovery" at all. Like so much else in the current literature."

I would like to point out that our paper showed 
that only one species, Montipora capitata, 
consumed enough zooplankton to meet all of its 
metabolic demand heterotrophically when 
bleached.   When healthy, M. capitata met less 
than 15% of its metabolic demand 
heterotrophically.  The other two species we 
studied, Porites compressa and Porites lobata, 
only met 21-35% of their daily metabolic demand 
heterotrophically when they were either healthy 
or bleached.   In all cases, our corals were 
exposed to naturally occurring zooplankton on the 
reef.  Thus under natural reef conditions, not 
all bleached corals can meet all of their 
metabolic needs heterotrophically.  Under 
artificially fed conditions (i.e., coral exposed 
to higher than ambient concentrations of 
zooplankton or brine shrimp in tanks), things can 
be quite different.   As you pointed out, the 
fact that corals do get some fixed carbon from 
zooplankton has been know for a very long 
time.  However, the fact that when bleached at 
least one species can  increase heterotrophic 
feeding to meet all of its metabolic needs while 
two others could not, is novel.  Our results 
suggest that not all species of corals would be 
able to meet their metabolic demand when 
maintained in the dark under natural 
concentrations and abundance of zooplankton (i.e, 
P compress and P lobata probably could not get 
all of their energy needs met heterotrophically 
when bleached under darkness... but this would 
need to be specifically tested).   In addition, 
bleaching induced by keeping corals in the dark 
is not necessarily the same as 
temperature-induced bleaching.  The chain of 
physiological stress responses that occur under 
high temperature include free radical and stress 
protein production, making any heterotrophic 
responses under tempreature-induced bleaching 
possibly quite different than hetertrophic 
responses under sustained darkness.


Andrea Grottoli

Andréa G. Grottoli, Assistant Professor
Ohio State University
Department of Geological Sciences
125 South Oval Mall
Columbus, OH 43210-1398
office:  614-292-5782
lab: 614-292-7415
fax: 614-292-7688
email: grottoli.1 at osu.edu
web: www.geology.ohio-state.edu/~grottoli
Office location: 329 Mendenhall Labs


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