Inorganic carbon acquisition by eight species of Caulerpa (Caulerpaceae, Chlorophyta)

Abstract

K. Kevekordes, D. Holland, N. Haubner, S. Jenkins, R. Koss, S. Roberts, J.A. Raven, C.M. Scrimgeour, K. Shelly, S. Stojkovic and J. Beardall. Inorganic carbon acquisition by eight species of Caulerpa (Caulerpaceae, Chlorophyta).Phycologia 45: 442–449. DOI: 10.2216/05-55.1.This investigation examines the occurrence of carbon concentrating mechanisms (CCMs) in eight species of the acellular green marine macroalgal genus Caulerpa. The measurements made were of the δ13C of organic matter, extracellular carbonic anhydrase activities, pH compensation values, and the inorganic C dependence of light-saturated photosynthesis rates. The data suggest that the pyrenoid-containing C. cactoides and C. geminata, and probably C. scalpelliformis (which lacks pyrenoids) have CCMs. Net diffusive influx of CO2 fulfills the inorganic carbon requirements of the other species for which pH-drift data are available, i.e. C. flexilis, C. longifolia, C. obscura and C. brownii. No pH drift data are available for C. trifaria and no information is available as to whether it has pyrenoids, although δ13C data suggest the absence of a CCM in this species. The three species showing evidence of CCMs have the lowest affinities for inorganic C of the eight species tested. This apparently paradoxical finding has precedence for marine red-macroalgae, and requires that the selective significance of the CCMs in these organisms is not that of increased inorganic C affinity, but is perhaps associated with the ability to both suppress photoinhibition and to photosynthesize at higher seawater pH values.