Cyanobacteria-dominated biofilms: a high quality food resource for intertidal grazers

Hong Kong rocky shores are dominated by cyanobacterial biofilms composed of a diversity of species. Thirteen common species, belonging to seven genera, were isolated in pure culture in MN+ and MN- media under defined growth conditions from a semi-exposed shore in Hong Kong. The nutritional values (i.e., protein, carbohydrate and calorific value) of these 13 species were determined. All species showed high nutritional quality in terms of protein, carbohydrate and calorific value, however, overall nutritional value varied between the species. Species of Spirulina and Phormidium were most nutritious (highest nutritional values) whereas species of Calothrix and Lyngbya were the least nutritious. Microphagous molluscan grazer density and diversity were relatively high at the study site, despite the seemingly low biomass (as assessed. by chlorophyll a concentration) of the biofilm. It is suggested that the high nutritional quality of cyanobacteria, together with their fast turnover rates can support high levels of secondary production (biomass of grazers). The high nutritional quality of cyanobacteria on tropical, cyanobacteria-dominated, rocky shores is therefore of great importance in the benthic food web.