Rhodolith beds are an important source of marine calcium carbonate worldwide. Dense aggregations are found in deep water in the tropical southwestern Atlantic. In order to understand the distribution of coralline algae that build rhodoliths, algae responses were measured in light levels over their depth range. Qualitative samples were obtained by dredging at 90-100 m depth 80 km offshore of Cabo Frio Island, southeastern Brazil. Histological sections indicate that Mesophyllum engelhartii (Foslie) Adey was the most frequent coralline algae in 22 sampling stations. Its lumpy and thin thallus is characterized by raised multiporate sporangial conceptacles, with cells of similar size and shape around the pore canal. Accretion growth rates were below detectable levels for two rhodolith species. Photosynthetic peak was reached at 0.5-1.5 % of the maximum surface irradiance (10-30 μmol photons m −2 s −1 ) showing an extremely narrow P-I curve of net primary production. Readings of primary production at irradiance close to saturation and lower light levels (5 μmol photons m −2 s −1 ) showed no significant difference, suggesting that rhodoliths are acclimated to restricted light ranges in deep water.

The taxonomy of the rhodolith-forming species of non-geniculate coralline algae from the subfamilies Neogoniolithoideae and Hydrolithoideae (Rhodophyta, Corallinales) found from Espírito Santo State, Brazil, was evaluated based on extensive subtidal sampling. A comparative analysis of the Brazilian material to other recently described species lead us to conclude that one relatively well-known species, namely Hydrolithon rupestre, a conferatum species Neogoniolithon cf. brassica-florida and a probably new species of the genus Hydrolithon, are present in the area. All three species can be distinguished by their tetrasporangial or gametangial conceptacle anatomy. This study strengthens the hypothesis that Brazilian rhodolith beds are comparatively more diverse in numbers of rhodolith-forming species than other areas of the world.