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Subject: Carbon Dioxide/Bicarbonate Specificity in Algae
From: "T.J. Evens" <[log in to unmask]>
Reply-To:T.J. Evens
Date:Mon, 20 Nov 1995 16:08:07 -0800
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Thanks to all of you who responded to my query on carbon
dioxide/bicarbonate specificity in algae.  It seems that this topic is
still as controversial as ever.  I have to admit that my question arose
from not considering one obvious parameter (see below), and was not as
involved as some you seemed to think.  I will attempt to give a brief
synopsis of some of the salient points I garnered from your replies
(forgive the length, but there were quite a few replies):
 
>It seems that if various species have different affinities for either CO2
>or HCO3, then measuring >photosynthesis by the 14C-uptake technique should
>also give species-specific results.
 
        John Beardall gets the credit for clearing up this one.  The
affinity only applies if you are measuring carbon uptake on time scales of
less than about 1 min (the time required for H14CO3 to equilibrate).  After
that all of the radiolabeled carbon is then found in the SAME PROPORTIONS
AS THE NON-LABELED CARBON.  A few researchers have shown that the uptake
rates for CO2 and HCO3 are different (which, like most elegant exercises,
makes complete intuitive sense AFTER it's pointed out to you), but most of
their research has been conducted on these very short time scales.
        The caveat:  In very dense suspensions in which the initial CO2
supply is exhausted, "CO2-users" must then rely on the equilibration rate
of HCO3 to CO2 to meet their carbon demands.  In this case you could
probably start seeing some effect in your activity measurements.
Strickland and Parsons (A Practical Handbook of Seawater Analysis, 1972)
indicate that the 14C method is good for Chl concentrations of up to 100
mg/m^3.  Does anybody have any experience with biomass concentrations this
high?
 
Other interesting points:
        It is EXTREMELY important to be aware of the total inorganic carbon
concentrations of your media.  This factor is used in equating specific
14C-activity to total fixed carbon, and can have a profound effect on your
derived fixation rates.  It is also important to be aware of how much
unlabeled carbon is in your stock solution (Raymond Ritchie says up to 18
mM, which is not an insignificant number).
 
        There are also a host of factors involved with stock solutions of
H14CO3.  In the interest of brevity, if you would like to know some of the
problems involved with using and storing H14CO3 please contact me directly,
and I'll outline them for you (if enough people want to know I may post
it).
 
        The uptake affinity for 12C over 14C is generally regarded as 5% in
favor of 12C.  Even the pioneers of this technique stated that this number
had a bit of "uncertainty" about it, and things are not any better today.
Unfortunately, it's all we've got to go on...
 
       Some relevant papers:
 
                Riebesell et al (1993) Nature 361: 249-251
                Imamura et al (1983) Plant & Cell Physiol 24(3) 533-40
                Aisawa et al (1986) Plant & Cell Physiol. 27(1) 37-47
                Miyachi et al (1983) Plant & Cell Physiol. 24(3) 441-51.
                Chen and Durbin (1994) MEPS 109: 83-94.
 
                Most anything by J. Raven or J. Beardall.
 
                Canadian J. of Botany vol. 69 (1991) has an entire issue
dedicated to these questions.
 
*******************************************************************
                                     t.j. evens
                           department of biology
                          university of california
                           santa barbara, ca 93106
                               (805) 893-4319
                           [log in to unmask]
*******************************************************************

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