, 1997; Sandh et al., 2009; Berman-Frank find more et al., 2001). Heterocystous cyanobacteria including Nostocales and Stigonematales (true branching) separate CO2 and N2 fixation spatially. Heterocysts are terminal, intercalary or both, differentiated cells specialized for nitrogen fixation, which lack the oxygen-producing photosystem II and have thick cell walls that are less permeable to gases, efficiently protecting the oxygen-sensitive nitrogenase and allowing nitrogen fixation to

occur during the daytime (Haselkorn, 2007). Morphological and molecular-based classifications verify that heterocyst-forming cyanobacteria constitute a monophyletic group (Honda et al., 1998; Tomitani et al., 2006; Gupta & Mathews, 2010). Cyanobacterial orders that form heterocysts are usually intermingled in terms of their genealogies, and it has been difficult to precisely establish their phylogenetic check details affiliations (Rajaniemi et al., 2005; Sihvonen et al., 2007; Berrendero et al., 2008). Tomitani et al. (2006) suggested, based on genetic distances and fossil calibrations, that heterocyst-forming cyanobacteria arose within the age range of 2450–2100 MYA. Later, molecular clock dating confirmed the age of the appearance of heterocystous cyanobacteria to 2211–2057 MYA (Falcón et al., 2010). These time frames coincide with

the Great Oxidation Event (∼2450 MYA), the time period when free oxygen starts to be traced in the fossil record (Holland, 2002). Although heterocyst-forming cyanobacteria are important players at an evolutionary and an ecological scale, our knowledge is also scant with regard to their natural

history and phylogenetic affiliations. Attempts have been made to unravel life history patterns of certain heterocystous cyanobacteria, including those pertaining to the multigenera Order Nostocales (Anabaena, Aphanizomenon, Aulosira, Trichormus, Nostoc, Nodularia, Mojavia, Calothrix, Gloeotrichia, Tolypothrix, Rivularia, Sacconema, Isactis, Dichothrix, Gardnerula, Microchaete, Cylindrospermopsis and Raphidiopsis) (Lehtimäki et al., 2000; Castenholz, 2001; Henson et al., 2004; Lyra et al., 2005; Rajaniemi et al., 2005; Sihvonen et al., 2007; Depsipeptide mw Berrendero et al., 2008; Lukesováet al., 2009; Stucken et al., 2010; Thomazeau et al., 2010). Nevertheless, sequences available for the Rivulariaceae 16S rDNA gene are restricted to the four genera Rivularia, Calothrix, Gloeotrichia, and Tolypothrix (Narayan et al., 2006; Tomitani et al., 2006; Sihvonen et al., 2007; Berrendero et al., 2008), which has hindered the advancement of our knowledge with regard to their evolutionary relationships. The aim of this study was to advance our knowledge on the phylogenetic affiliations of heterocyst-forming cyanobacteria within the Rivulariaceae (order Nostocales), specifically including representatives of the genera Calothrix, Rivularia, Gloeotrichia and Tolypothrix collected from different environments.