Structure and composition of the photochemical apparatus of the antarctic green alga,Chlamydomonas subcaudata

The green alga, Chlamydomonas subcaudata, collected from a perennially ice-covered Antarctic lake, was able to grow at temperatures of 16°C or lower, but not at temperatures of 20°C or higher, which confirmed its psychrophilic nature. Low temperature (77 K) Chl a fluorescence emission spectra of whole cells of the mesophile, C. reinhardtii, indicated the presence of major emission bands at 681 and 709 nm associated with PS II and PS I, respectively. In contrast, emission spectra of whole cells of C. subcaudata exhibited major emission bands at 681 and 692 nm associated with PS II, but the absence of a major PS I emission band at 709 nm. These results for C. subcaudata were consistent with: (1) low ratio of Chl a/b (1.80); (2) low levels of PsaA/PsaB heterodimer as well as specific Lhca polypeptides as determined by immunoblotting, (3) decreased levels of the Chl-protein complexes CP1 and LHC I associated with PS I; and (4) an increased stability of the oligomeric form of LHC II as assessed by non-denaturing gel electrophoresis in the psychrophile compared to the mesophile. Furthermore, immunoblotting indicated that the stoichiometry of PS II:PS I:CF₁ is significantly altered in C. subcaudata compared to the mesophile. Even though the psychrophile is adapted to growth at low irradiance, it retained the capacity to adjust the total xanthophyll cycle pool size as well as the epoxidation state of the xanthophyll cycle. Despite these differences, the psychrophile and mesophile exhibited comparable photosynthetic efficiency for O₂ evolution regardless of growth conditions. P^max for both Chlamydomonas species was similar only when grown under identical conditions. We suggest that these photosynthetic characteristics of the Antarctic psychrophile reflect the unusual light and low temperature regime to which it is adapted.