EXCHANGES OF INFORMATION, ENERGY & MATERIALS IN SYMBIOSES

Symbiosis is important in the cell and environmental biology of algae. Some examples involving the author and numerous collaborators include: 1) chloroplasts of eukaryotic algae arose from endosymbioses. Plastids are incapable of independent existence; most of the genes of the cyanobacterial photobiont have been lost, and the majority of the rest have been transferred to the nuclear genome. Some of the genes retained by the plastid are those whose transcription is controlled by environmental cues transduced by the organelle. The general trend is for organelle genes to be transferred to the nucleus, escaping plastid redox activities generating mutagenic free radicals; 2) symbioses involving potentially free‐living photobionts include marine lichens and sponges with cyanobacterial symbionts. For the lichen, Lichina , inorganic carbon acquisition appears to involve inorganic carbon transport by the mycobiont, and for the sponge, Cymbastella , the flagellar activity of the sponge is probably important for inorganic carbon supply to the photobiont; 3) the Australasian fucalean, Notheia , is an obligate epiphyte on the fucaleans, Hormosira and Xiphophora; the four species involved all contain the hexitol, altritol. Notheia anomala is known to be phyletically‐distant from the other five altritol‐containing species. Can Notheia synthesize altritol, or is it obtained from the phorophyte?; 4) Sacoglossan gastropods retain kleptoplastids (not strictly symbionts) from ulvophycean (or rhodophycean) marine algae. Analyses of the natural abundance of stable carbon isotopes suggest significant contribution of kleptoplastid photosynthesis to the carbon and energy budget of the mollusks.