Haloarchaeal genomes encode the complete set of enzymes

of the TCA cycle (Falb et al., 2008). Furthermore, activity of all enzymes of the cycle was detected in Hbt. salinarum (Aitken & Brown, 1969). Field studies on a hypersaline cyanobacterial mat have shown metabolic interactions between haloarchaea and the primary producer Coleofasciculus (Microcoleus) chthonoplastes. This cyanobacterium excretes acids of the citrate cycle into the medium, and aerobic halophilic CFTR activator Archaea further utilizes these as the major carbon and energy source (Zvyagintseva et al., 1995). The existence of a functional glyoxylate cycle has been demonstrated in Haloferax volcanii (Serrano et al., 1998) and in Natronococcus occultus (Kevbrina & Plakunov,

1992). Inquiries effectuated on the 13 complete halophilic genomes present in the HaloWeb data base (DasSarma et al., 2010) did not find any simultaneous positive matches for the glyoxylate cycle key enzymes: isocitrate lyase and malate synthase (with the exception of previous mentioned species Hfx. volcanii). A blastp (Altschul et al., 1997) search made on NCBI using the amino acid sequences of the Hfx. volcanii isocitrate lyase and malate synthase showed that PD-0332991 datasheet these enzymes are present also in Haladaptatus paucihalophilus strain DX253. Recently, a novel pathway for the synthesis of malate from acetyl-CoA was discovered

in Hfx. volcanii and in Har. marismortui, in which acetyl-CoA is oxidized to glyoxylate via methylaspartate as key intermediate (Khomyakova et al., 2011). Although most halophilic Archaea preferentially use amino acids as carbon and energy source, there are carbohydrate-utilizing species such as Haloarcula marismortui, Halococcus saccharolyticus, and Hfx. mediterranei. These species have the capacity to metabolize pentoses (arabinose, xylulose), hexoses (glucose, fructose), sucrose, and lactose (Rawal et al., 1988; Altekar & Rangaswamy, Chloroambucil 1992; Johnsen et al., 2001). Comparative analysis of ten haloarchaeal genomes showed that Halorhabdus utahensis and Haloterrigena turkmenica encode over forty glycosyl hydrolases each and may break down complex carbohydrates. Hrb. utahensis has specialized in growth on carbohydrates and has few amino acid degradation pathways. It uses the nonoxidative pentose phosphate cycle and a transhydrogenase instead of the oxidative pathway, giving it a great deal of flexibility in the metabolism of pentoses (Anderson et al., 2011). Hrb. utahensis degrades xylan and can grow on xylose (Wainø & Ingvorsen, 2003). Many species of Halobacteriaceae also produce exoenzymes such as proteases, lipases, DNAses, and amylases to degrade organic polymeric substances extracellularly, making small organic molecules available as carbon and energy source.