PRESENCE OF THE CO2-CONCENTRATING MECHANISM IN SOME SPECIES OF THE PYRENOID-LESS FREE-LIVING ALGAL GENUS CHLOROMONAS (VOLVOCALES, CHLOROPHYTA)

Physiological and morphological characteristics related to the CO2-con centrating mechanism (CCM) were examined in several species of the fre e-living, unicellular volvocalean genus Chloromonas (Chlorophyta), whi ch differs morphologically from the genus Chlamydomonas only by lackin g pyrenoids. The absence of pyrenoids in the chloroplasts of Chloromon as (Cr.) rosae UTEX 1337, Cr. serbinowii UTEX 492, Cr. clatharata UTEX 1970, Cr. rosae SAG 26.90, and Cr. palmelloides SAG 32.86 was confirm ed by light and electron microscopy. In addition, immunogold electron microscopy demonstrated that ribulose-1,5-bisphosphate carboxylase/oxy genase (Rubisco; EC 4.1.1.39) molecules were distributed almost evenly throughout the chloroplasts in all five Chloromonas strains. However, Chloromonas exhibited two types of physiological characteristics rela ted to the CCM depending on the species or strains examined. Chloromon as rosae UTEX 1337 and Cr. serbinowii had high photosynthetic affiniti es for CO2 in cells grown in culture medium bubbled with air (low-CO2 cells), compared with those grown in medium bubbled with 5% CO2 (high- CO2 cells), indicating the presence of the low-CO2-inducible CCM. In a ddition, these two Chloromonas strains exhibited low-CO2-inducible car bonic anhydrase (CA; EC 4.2.1.1) activity and seemed to have small int racellular inorganic carbon pools. Therefore, it appears that Cr. rosa e UTEX 1337 and Cr. serbinowii possess the CCM as in pyrenoid-containi ng microalgae such as Chlamydomonas reinhardtii. By contrast, Cr. clat harata, Cr. rosae SAG 26.90 and Cr. palmelloides showed low photosynth etic affinities for CO2 when grown under both CO2 conditions. Moreover , these three strains exhibited an apparent absence of intracellular i norganic carbon pools and lacked low-CO2-inducible CA activity. Thus, Cr. clatharata, Cr. rosae SAG 26.90 and Cr. palmelloides, like other p yrenoid-less algae (lichen photobionts) reported previously, seem to l ack the CCM. The present study is the first demonstration of the CCM i n pyrenoid-less algae, indicating that pyrenoids or accumulation of Ru bisco in the chloroplasts are not always essential for the CCM in alga e. Focusing on this type of CCM in pyrenoid-less algae, the physiologi cal and evolutionary significance of pyrenoid absence is discussed.