Deep-water rhodolith productivity and growth in the southwestern Atlantic

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.