THE CYANOPHYTE‐HEPATIC SYMBIOSIS I. MORPHOLOGY AND PHYSIOLOGY

Abstract

SUMMARYThe Anthoceros punctatus‐Nostoc and Blasia pusilla‐Nostoc symbioses were investigated. In both associations the Nostoc colonies develop in mucilaginous cavities on the undersurface of the gametophyte. The sporophytes of Anthoceros have no Nostoc colonies; no sporophytes of Blasia were found. The symbiotic algae have been identified as Nostoc sphaericum ex. Born et Flah. The heterocyst frequency of the free‐living isolates is 3–6% but this increases to 30% (Blasia) and 43% (Anthoceros) when the algae are growing symbiotically. On infecting alga‐free gametophytes of Blasia pusilla with Nostoc, algal colonies develop in the cavities within 72 h. The developing Nostoc colonies stretch the cells of the cavity wall and there also arise from a point on the host cavity wall, usually opposite the cavity pore, septate, branched, filamentous protrusions, which increase the surface area of contact between phycobiont and host by about 30% within 4 weeks of colony formation. Such outgrowths may facilitate interchange of metabolites between the alga and liverwort. The Nostoc phycobiont of Anthoceros may infect Blasia and vice‐versa. A strain of Nostoc punctiforme isolated from Gunnera also infects Blasia, but four other Nostoc isolates, including one from cycad root nodules, do not. Five other heterocystous algae, five non‐heterocystous filamentous algae and one unicellular alga tested do not infect Blasia. The nitrogen contents of thalli with Nostoc colonies are significantly higher than those treated with algae which do not develop colonies. The symbiotic algae differ physiologically and biochemically from free‐living algae. They show higher rates of nitrogenase activity (acetylene reduction assay), are depleted of nitrogen‐storing phycobilin pigments and structured granules and although metabolically active do not evolve O2 or fix CO2 photosynthetically, but do have polyhedral bodies which contain the key CO2‐fixing enzyme ribulose‐l,5‐diphosphate carboxylase. The Blasia symbiosis grows well and remains established in the pH range 4–8, at low levels of combined nitrogen (<100 mg I−1 of nitrate‐nitrogen) but not at high levels (100–1000 mg I−1), in the temperature range 10–25°C, and under moist but not waterlogged or desiccated conditions.