RESUMEN
Photosynthetic O2 evolution by the upper littoral lichen, Lichina pygmaea (Lightf.) C.Ag., under light-saturated conditions at 5 °C is saturated by the 2 mol m-3 inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m-3 inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO3 - . It is concluded that HCO3 - is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO2 enters the photobiont (Calothrix) cells even during HCO3 use. pH drift experiments support the notion of HCO3 - use. Emersed photosynthesis at 5 °C is more than half-saturated by 35 Pa (normal atmospheric) CO2 ; the light- and CO2 -saturated emersed photosynthetic rate is not significantly different from the light and inorganic C-saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO3 - fluxes to supplement CO2 movement. The CO2 compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark 14 C-inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM-like variation in intracellular titratable acidity. The high value (-11.5%) of δ13 C, the low CO2 compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free-living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.
RESUMEN
The literature, and previously unpublished data from the authors' laboratories, shows that the δ13C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from -3 to -35. While some marine macroalgae have δ13C values ranging over more than 10 within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5 (e.g. the red alga Plocamium cartilagineum which has values between -30 and -35). The organisms with very negative δ13C (lower than -30) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ13C values (higher than -10) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ13C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ13C values lower than -30 have pyrenoids. Previous work showed a good correlation between δ13C values lower than -30 and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ13C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ13C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ13C values more positive than -10 (i.e. more positive than the δ13C of CO2 in seawater) must involve HCO3- use.
RESUMEN
Much evidence suggests that life originated in hydrothermal habitats, and for much of the time since the origin of cyanobacteria (at least 2.5 Ga ago) and of eukaryotic algae (at least 2.1 Ga ago) the average sea surface and land surface temperatures were higher than they are today. However, there have been at least four significant glacial episodes prior to the Pleistocene glaciations. Two of these (approx. 2.1 and 0.7 Ga ago) may have involved a 'Snowball Earth' with a very great impact on the algae (sensu lato) of the time (cyanobacteria, Chlorophyta and Rhodophyta) and especially those that were adapted to warm habitats. By contrast, it is possible that heterokont, dinophyte and haptophyte phototrophs only evolved after the Carboniferous-Permian ice age (approx. 250 Ma ago) and so did not encounter low (