[Coral-List] Heat;light, food, and bleaching, continued

[Esther Peters]

This has been a most interesting thread, but no one has mentioned
looking at the composition and structure of the tissues of corals under
different conditions and in different species to help explain the gross
appearances and physiological and biochemical patterns observed
(structure = function).  I have had many students now who have learned
the basics of histology and histotechniques (the study of cells and
tissues). Most of them have continued with these tools to study coral
diseases, with most focusing on those caused by microbial pathogens.

Yet more data can be collected using these tools to help answer some of
the questions posed in this thread.  To begin, Thomas F. Goreau did some
of the first work in using histology and immunohistochemistry to examine
the location of enzymes in corals (I must apologize to Tom if I don't
get this correct as I'm many miles from the copy I have of his
father's dissertation).  I have not had the opportunity to pursue this
kind of work, but from the tissue sections I've made of many species of
corals I have many questions.  For example, the gastrodermal cells of
stony coral species that never contain symbiotic algae (such as
Balanophyllia) and cold-water corals that may acquire the zooxanthellae
(such as Astrangia) are often loaded with eosinophilic (probably)
proteinaceous vesicles, suggestive of zymogens (precursors to digestive
enzymes), in addition to the eosinophilic granular gland cells of the
mesenterial filaments.  If so, this might be a good indicator of species
that depend more heavily on zooplankton for nutrition.  Tropical
species, such as Montastraea, lack such granules in the gastrodermis.
Another unanswered question (to my mind, anyway) is whether some corals
might be able to degrade/digest zooxanthellae.  This is based on my
observations of zooxanthellae debris in the gastrodermal cells of some
species (nonbleached, normally pigmented), which are not found in others.

Histology (using both light and electron microscopy procedures) can also
provide visual records of the kinds of cell and tissue damage associated
with bleaching, which probably provide insights into whether the
bleached corals can recover.  For example, I have found zooxanthellae
expulsion by exocytosis, in which cellular integrity remained and even a
few algal cells in good condition in the gastrodermis (but too few to
color the coral tissue) and the coral tissue in fair condition.  On the
other hand, I have also seen damaged zooxanthellae remaining in
gastrodermal cells (probably loss of photosynthetic pigments so the
tissue now was translucent) with necrosis of coral cells as well or
corals which had sloughing of intact gastrodermal cells and algae (the
epidermis and mesoglea were still intact).  In these latter cases, I
suspect the coral is not going to recover, but I have not done the
long-term sampling to confirm this.

We (myself and my students) also have more evidence that just because
there is a layer of apparently normally pigmented tissue covering the
coral skeleton, it does not mean that the coral is in good health.
Subcellular to cellular changes can be present that might be either
reversible (recovery can occur) or irreversible (death will occur).
Thus, percent coral cover data can mean many things!  More field and
laboratory studies should take advantage of our growing knowledge of
coral histology (through collaborations with a growing group of coral
histologists and the Coral Disease and Health Consortium) to provide the
visual records of tissue condition to aid in interpretation of gross
observations, physiological measurements, and biochemical analyses.

Esther Peters, Ph.D.

Michael Risk wrote:

  > Hi Tom.
  >
  > Thank you for those kind words-although telling the -list that "most of
  > the papers are garbage except those by Mike Risk" (sic) is perhaps an
  > exaggeration, and is without doubt a comment that will come back to
  > haunt us both. I insist I am quite capable of producing garbage.
  >
  > Your main point, I think (as one whose hair long since turned grey), is
  > well-taken. Rather than stand on the shoulders of giants who have gone
  > before, coral reef ecology sometimes seems to prefer reinventing
  > several wheels. I have no easy solution. In an ideal world,
  > grant-review panels would be objective, and sort for originality;
  > manuscript reviewers would be familiar with pre-existing literature,
  > and would reject work that was not original. Perhaps some of the blame
  > may be laid at the feet of an academic system in which one's income
  > depends on quantity over quality.
  >
  > You may be interested in a little paper from 2003 (Marine Geology 202:
  > 71-78, "Smoke signals from corals.")
  >
  > I was in Indonesia during the 1997 "haze" event, which was caused by
  > the Suharto government (plus some small land-holders) setting fires on
  > Borneo to clear canefields and "vacant" land. The smoke, which
  > spin-doctors quickly morphed into "haze", covered much of Indonesia,
  > Singapore and Malaysia, creating a public health problem of some
  > magnitude. I recall taking the train from Bogor to Jakarta, and the
  > horizontal visibility was sometimes <100 metres. Everybody except the
  > unprepared "belander" was wearing masks.
  >
  > It occurred to me that this plume of smoke would shade the corals. Make
  > a long story short: corals under the haze shifted their metabolism
  > towards more heterotrophy, then shifted back again to autotrophy when
  > the haze had passed. (One needs a zillion analyses to show this.)
  >
  > Another example of the plasticity of these critters. Of interest is the
  > possibility of retrospective determination of similar metabolic shifts,
  > via skeletal analyses.
  >
  > Mike
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