Nitrogen is generally considered to be a major limiting nutrient for phytoplankton growth in coastal and oceanic waters. The fixation of atmospheric nitrogen in the oligotrophic ocean is an important source of new nitrogen to surface waters, stimulating phytoplankton productivity and fueling the biological pump. In tropical waters the non-heterocystous cyanobacterium Trichodesmium is largely responsible for nitrogen fixation (Karl et al., 2002), but the heterocystous diazotroph cyanobacterium Richelia intracellularis provide quantitatively substantial inputs of nitrogen on regional scales (Carpenter et al., 1999). Symbiosis between cyanobacteria and marine organisms is abundant and wide spread among marine plants and animals. R. intracellularis Schmidt, a heterocystous cyanophyte is common in tropical and subtropical seas. It generally appears in symbiosis with phytoplanktonic species of the genus Rhizosolenia and Hemiaulus and with benthic diatoms of the genus Pleurosigma (Wood, 1965, 1967) or as an epiphyte in Chaetoceros compressus or in Rhizosolenia styliformis var. latissima (Allen and Cupp, 1935; Sournia, 1968).