Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata and Rhizosolenia formosa with endosymbiotic cyanobacteria Richelia intracellularis was noticed and documented for the first time during a bloom of the cyanobacterium Trichodesmium erythraeum in the oligotrophic shelf waters along Kochi and Mangalore transects, southeastern Arabian Sea (SEAS), during spring intermonsoon (April 2012). Although the host is frequently seen, the symbiont is rarely reported in the Indian EEZ. The presence of nitrogen-fixing symbiotic association of Rhizosolenia-Richelia along with Trichodesmium erythraeum highlights the significance of DDAs on the nutrient and energy budgets of phytoplankton in the oligotrophic environments of the Arabian Sea during spring intermonsoon. 1. Introduction Planktonic organisms that fix atmospheric nitrogen (diazotrophs) have a critical role in oceanic production and in the marine nitrogen cycle [1]. Abundance and growth rates of such organisms depend on their ability to assimilate various sources of nitrogen [2]. The availability of fixed nitrogen (such as nitrate and ammonium) can limit the productivity of the sea [3]. Diatom-diazotroph associations (DDAs) are widely reported in oligotrophic waters and have the capacity to form episodic, largely monospecific blooms that exhibit very high rates of carbon and nitrogen fixation worldwide [4–7]. Annually, Trichodesmium contributes 1–5？mmol？N？m？2？d？1; while diazotrophic diatoms, contribute 0.4–2.4？mmol？N？m？2？d？1, which forms one quarter of the total input of nitrogen to the sea [8]. Globally, DDAs fix 4.79？Tg？N？y？1 [9], which forms almost 25% of total input of nitrogen to the sea [8]. One of the most conspicuous free-living, diazotrophic species is the colony-forming cyanobacteria, a species of the Trichodesmium that is found throughout tropical and subtropical oceans and forms large-scale surface blooms [10]. A unique group of open ocean diazotrophs is the heterocystous cyanobacteria that live symbiotically with other members of phytoplankton, primarily diatoms. Such endosymbiotic associations help most of the diatoms to fix atmospheric nitrogen in oligotrophic waters with the help of such endosymbiotic associations [11–13]. The cyanobacteria, Richelia intracellularis and Calothrix rhizosoleniae, were found in association with diatom genera such as Rhizosolenia, Hemiaulus, Bacteriastrum,？？and Chaetoceros. They are also seen as epiphyte or endosymbiont in Guinardia cylindrus in warm tropical and subtropical oligotrophic waters [6, 12, 14]. Only a few workers have
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