WIDESPREAD OCCURRENCE OF THE HEMIAULUS-CYANOBACTERIAL SYMBIOSIS IN THE SOUTHWEST NORTH-ATLANTIC OCEAN

Epifluorescent examination of live Hemiaulus spp. during September 199 1 and January-February 1992 in the Bahama Islands, southwest North Atl antic Ocean, and Caribbean Sea indicated that 91-100% of the three dia tom species noted contained diazotrophic cyanobacterial symbionts. Thi s symbiosis was 5 to 254 times more abundant than the commonly reporte d Rhizosolenia-Richelia symbiosis, although the Hemiaulus symbionts we re not generally visible in transmitted light in living material. An a dditional 13 day-time series from Carrie Bow Cay, Belize found sustain ed high percentages of symbiotic Hemiaulus (average = 98%), suggesting these symbioses are persistent in tropical waters. Over 98% of the He miaulus spp. examined contained nitrogen-fixing symbionts. Additional observations on preserved material indicated the Hemiaulus symbioses w ere present at least as far north as 40-degrees-N in the Atlantic, and 31-degrees-N in the Pacific.