|dc.description.abstract||DIC acquisition mechanisms and the use of DIC in photosynthesis were
examined in the temperate red macroalga, Palmaria palmata (Linnaeus) O.
Kuntze. Photosynthetic O2 evolution was unsaturated at the ambient DIC
concentration in seawater. Experiments with inhibitors of the mammalian
-/Cl- anion exchanger AE1, and the HCO3
- -hydroxylating enzyme carbonic
anhydrase (CA), showed that HCO3
- uptake is mediated mainly by a
bicarbonate transporter protein. Photosynthesis measured at small increments
of DIC addition showed biphasic kinetics, with a hiatus located at 0.625 mM
DIC. Inhibitor experiments indicated that extracellular CA is active at low DIC,
therefore the biphasic kinetics of photosynthesis may have been caused by the
presence of two DIC acquisition mechanisms. Culturing thalli for 6 d in artificial
seawater containing 8 mM DIC induced a bicarbonate transporter in the CAuser
Chondrus crispus, but no change in activity was detected in P. palmata.
Presence of a bicarbonate transporter mechanism was also confirmed by using
the polymerase chain reaction. Primers based on the mammalian AE1 gene
sequence were used to amplify a 1500bp fragment of bicarbonate transporter
gene from genomic DNA extracted from P. palmata and the calcifying
microalga, Emiliania huxleyi. Comparison of the algal and mammalian
sequences revealed a high amino acid sequence homology, indicating that they
encoded structurally and functionally similar proteins.
F.F. Blackman’s precept, which implies that carbon limitation does not occur at
subsaturating light, was investigated by measuring induced chlorophyll
fluorescence at different DIC concentrations and irradiances. Photosynthetic
electron transport rate was was stimulated at saturating light, but only slightly at
subsaturating light. Thus, Blackman’s theory was upheld.
Photosynthesis and bicarbonate transporter activity was monitored in P.
palmata growing at St. Margaret’s Bay, Kent, UK. Distinct peaks in
photosynthesis occurred in March and October, and coincided with equinoctial
tides. Bicarbonate transporter activity was maximal in spring, and decreased
during summer. Although it was not possible to show what determined the
seasonal patterns of photosynthesis and bicarbonate transporter activity, the
possibility that exposure to springwater containing high concentrations of DIC
and nitrate is discussed.||en_US