Md. Iglesiasrodriguez et Mj. Merrett, DISSOLVED INORGANIC CARBON UTILIZATION AND THE DEVELOPMENT OF EXTRACELLULAR CARBONIC-ANHYDRASE BY THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM, New phytologist, 135(1), 1997, pp. 163-168
The presence of extracellular carbonic anhydrase (CA) in relation to m
edium composition was investigated using cultures of the marine diatom
Phaeodactylum tricornutum Bohlin. Large-volume cultures, with low ini
tial cell inocula were grown on ASP-2 (no dissolved inorganic carbon (
DIC), 550 mu M NO3-), f/2 (2.0 mM DIC, 880 mu M NO3-) and modified f/2
(2.0 mM DIC, 20 mu M NO3- media. Cells growing on ASP-2 showed extrac
ellular CA in the early stages of growth, whereas extracellular CA was
not detected until partial depletion of total DIC in the stationary p
hase for cultures on f/2 or modified f/2. Both HCO3- and CO2 were impo
rtant in carbon limitation, extracellular CA being present when the fr
ee-CO2 concentration fell below 5 mu M, but the HCO3- concentration ne
eded to be below 1 mM. When carbon-replete cells were transferred to c
arbon-limited conditions, extracellular CA was recorded within minutes
, the process being light-dependent and completely inhibited by 3,3,4-
dichlorophenyl-1,1-dimethylurea (DCMU). The addition of DIC to carbon-
limited cells resulted in a rapid decrease in extracellular CA activit
y. The membrane-impermeable inhibitor of carbonic anhydrase, dextran-b
ound sulphonamide (DBS) was used to inhibit extracellular CA activity
in relation to photosynthetic rate in carbon-replete and carbon-limite
d cells. At the lowest DIC concentration (0.10 mM), for cells with max
imum external CA activity, DBS gave over 80% inhibition of the photosy
nthetic rate, demonstrating the key role of external CA in maintaining
high photosynthetic rate under conditions of carbon limitation. It is
proposed that the key factor in the regulation of extracellular CA ac
tivity is the total flux of inorganic carbon (C-i) into the cell. This
determines the C-i flux into the chloroplast and when this is inadequ
ate to support the photosynthetic rate attained by a carbon-replete ch
loroplast at optimum photon flux density, extracellular CA is activate
d. This mechanism would explain the observed interaction of CO2 and HC
O3- in the regulation of extracellular CA activity.