[Coral-List] HCO3- uptake by corals

[Martin Pêcheux]

Dear Craig Zievis, Coral-Listers,

There is much confusion about inorganic carbon uptake by corals and
other reef photosymbioses.
Corals and large foraminifers actively pump HCO3- for photosynthesis and

calcification (synthesis in Allemand et al., 1998, Can. J. Bot. 76,
925-941, see in forams the very good work of ter Kuile et al., 1989,
Mar. Biol., 103, 241-251), and they are fitted to their environment :
the photosynthesis is efficient even under strong light (at normal T,
water agitation and CO2), and HCO3- uptake, though costly, is
saturating. So it is of no interest to increase HCO3- concentrations (cf

Burris et al., 1983, Mar. Biol. 75, 113-116), - apart toward bleaching
conditions.

An aquatic active uptake is governed by the Hill-Whittigham equation
(see in Pecheux 1997 in www.reefbase.org), which shows a linear uptake
versus concentration up to a certain point, then a plateau (well
demonstrated in forams). Corals are, as expected, around the turning
point. They are efficient without too much cost. The position of the
turning point is strongly controlled by the diffusion in the unstirred
layer some hundreth microns thick surrounding the organism, in turn
controlled by water agitation. So increase of HCO3- may alleviate
reduction of photosynthesis under still condition compared to agitated
one.

>From the data of photosynthesis of Al-Mograbhi (in Goiran et al., 1996,
J. Exp. Mar. Biol. Ecol., 199, 207-225), one can fit already with some
sucess the more simple Michelis-Menten behavior. It seems that Vmax and
Km[HCO3] is correlated with OH- concentration. A model gave Vmax=0.011
square root(OH-) and Km[HCO3-]=0.044 square root(OH-) at normal T and
light (in ref. of Pecheux 1997). A constant Vmax/Km indicates a probable

ping-pong mecanism. But the more important, for design of interesting
experiment
and for bleaching) is the dependancy to pH, controlled by pCO2, whereas
they are
near Vmax for HCO3- concentration.
The obvious interpretation is that HCO3- uptake pumps are fuelled by
internal/external gradient of OH- or H+. Hence the importance of present

seawater global acidification (+21.6% H+) and not of HCO3- increase
(+5.8%).

Cheers,

Martin Pêcheux, Scientifique Consultant
Large Foraminifer Institute
3, allée des Elfes, 94260 Fresnes, France
martin-pecheux at wanadoo.fr


Le 29/06/2004, Cades83898 at cs.com a écrit :

Hello, I am doing research on inorganic carbon
  limitation in hard corals. Part of my research includes dosing with
  sodium bicarbonate to artificially increase inorganic carbon in a
  closed system. However, my advisor and I are concerned that large
doses
  of sodium bicarbonate will upset the ionic balance of the water. Does
  anyone out there have a strong chemistry background who might know
  something about how this will affect my water ionically? I intend to
  increase the alkalinity to 6 meq/L or ppm.
Thanks,
Craig Zievis
University of Maine