[Coral-List] Coral absorbance factor
Jeremy J. Sofonia
jeremy at sofonia.com
Thu Dec 11 15:53:29 UTC 2008
Dear Steve -
I would strongly recommend a look at Beer et al. (1998):
Beer S., M. IIan, A. Eshel, A. Weil, and I. Brickner. 1998. Use of
pulse amplitude modulated (PAM) fluorometry for in situ measurements
of photosynthesis in two Red Sea faviid corals. Marine Biology
131:607-612
Interestingly, you'll notice that they calculate the absorbance
factor (FA) by accounting for the reflectance of the coral skeleton
(Rs), subtracting this from that measured from the live coral (Rc),
and then dividing by the reflectance of the incident light by a
mirror (Rt).
AF = (Rs - Rc) / Rt
This is quite clever, and depending on the objective of the study
could result in providing some very interesting results, particularly
if one is looking at the effect of skeletal reflectance - and the
possible variations therein ( e.g. could lead to questions into
inter- and intra-specific differences in morphology etc).
On the other hand, utilising the water pick to remove the tissue is
obviously a destructive technique which prohibits repeated measure on
the same colony over time. It also may be logistically restrictive
if you are trying to assess corals in situ.
Besides, most corals (at least those that I've worked with) don't
seem to live very well without their skeletons. By subtracting this,
are we really understanding the natural process? If not, it may be
important retain this information in the FA and to consider
absorption in living corals as a 'gross' rather than 'net' function
of the coral components.
Perhaps just:
AF = Rc - Rt
One other thing to consider: There is variation in the strength of
the ambient light pluse derived from the DivingPAM. Although slight,
it may change with factors such as ambient temperature and battery
charge. From some basic trials of my own, I observed this to have a
greater effect when running the 'rapid light curves' than when
employing a single saturation pulse. I would recommend you also run
a preliminary trial using the mirror to try and assess (and account
for) how your specific machine performs.
All in all a very interesting - and I think important - part of coral
fluorometry which has a great deal of room for further investigation
and refinement. I'm certainly no expert, and would be quite happy to
hear your thoughts, and from other on the list.
Springerlink offers the .PDF of Beer et al (1998) using the following
path: http://www.springerlink.com/content/gyp1gpb955tde85h/
If the link doesn't work for some reason, try a Google Scholar search
- it should get you there.
All the best,
Jeremy
On 10/12/2008, at 7:49 PM, Steve Dalton wrote:
> Merry Christmas to all. I am looking at photoacclimation in two coral
> species using the PAM and want to determine sub-saturation
> irradiance (Ek)
> of replicate fragments exposed to different light and temperatures
> regimes.
> But, I am unable to accurately determine Ek for each treatment
> without an
> accurate determination of absorbance factor (AF) for hard corals.
> If anyone
> has been able to determine the mean AF for hard corals or know of any
> publications that indicate the absorbance factor for the PSII for
> symbiont
> algae a reply would be great. I am hoping that monitoring stressed
> coral
> through time using Ek and ETRmax as an indication of short term
> photoacclimation.
>
>
>
> Cheers
>
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