Microcystin/Nodularin Profiles

Although many toxigenic strains simultaneously produce several microcys- tin variants (Puddick et ah, 2014), usually only one to three of them are dominant in any particular strain. It appears that some microcystin variants are more abundant within a certain genus than within others, though this may be biased in some cases by the limited availability of standards as well as the analytical methods used.

Globally, Microcystis strains and field samples dominated by Microcystis spp. are reported to contain chiefly microcystin-LR, -RR and -YR in varying proportions (Sivonen & Jones, 1999; Vasconcelos, 2001; Gkelis et ah, 2005; Kemp & John, 2006; Faassen & Liirling, 2013; Mowe et ah, 2015; Beversdorf et ah, 2017). Demethylated variants of, for example, [Dha7]MC-LR and -RR are also observed in Microcystis strains, but are less frequently dominant compared to their methylated forms (Vasconcelos, 2001; Gkelis et ah, 2005). More hydrophobic microcystins (e.g., MC-LA, MC-LW, MC-LF) can also be found regularly in Microcystis strains and field samples; however, high proportions are reported only infrequently (Cuvin-Aralar et ah, 2002; Wood et ah, 2006; Graham et ah, 2010; Faassen & Liirling, 2013; Beversdorf et ah, 2017). This picture of a high diversity combined with an abundance of genotypes with a certain microcystin profile has been confirmed in situ for some natural Microcystis populations from central Europe. Typing of single Microcystis colonies from nine European countries by mass spectrometry revealed a high abundance of genotypes producing microcystin-LR, -RR and -YR, while clones with demethylated variants or other microcystins were less abundant (Via-Ordorika et ah, 2004). Flowever, exceptions from this overall pattern occur; for example, in Microcystis strains and colonies from Finland, demethylated MC-LR and -RR have been seen frequently as dominant variants (Luukkainen et ah, 1994; Via-Ordorika et ah, 2004), and in one Australian bloom of Microcystis, 23 microcystins were detected, none of which was microcystin-LR (Jones et ah, 1995).

Planktotbrix and some strains of Dolichospermum seem to produce only demethylated microcystins (Puddick et ah, 2014). In European Planktotbrix agardbii and P. rubescens isolates, cultured strains and field samples primarily produce demethylated variants of microcystin-RR like [D-Asp3] MC-RR and [D-Asp3, Dhb7] MC-RR that have been found as major microcystins (Sivonen et ah, 1995; Fastner et ah, 1999; Briand et ah, 2005; Kurmayer et ah, 2005; Cerasino et ah, 2016). Various other demethylated microcystins such as [D-Asp3] MC-LR or [D-Asp3] MC-HtyR are also found in Planktotbrix isolates but are rarely the dominant variants (Kosol et ah, 2009). As with Microcystis, multiple clones with different microcystin profiles exist in natural populations of Planktotbrix (Welker et ah, 2004; Flarustiakova Sc Welker, 2017).

Only few data exist on microcystin congeners produced by benthic species, and detected variants comprise MC-LR, MC-RR, MC-YR, MC-LA,

[Asp5] MC-LR as well as unidentified microcystins (Aboal 8c Puig, 2005; Jungblut et al., 2006; Izaguirre et al., 2007; Fetscher et ah, 2015).

Nodularin-R seems the major nodularin present in samples from the Baltic Sea, Turkey and Australia, while other nodularin variants usually seem less abundant (Sivonen et ah, 1989; Jones et ah, 1994a; Lehtimaki et ah, 1997; Mazur-Marzec et ah, 2006b; Ak^aalan et ah, 2009).

Biosynthesis

Knowledge of the biosynthesis of microcystins and nodularins has increased since the turn of the millennium. Complete sequences of biosynthesis gene clusters are available for several species, and biochemical pathways are largely understood (Pearson et ah, 2016).

Microcystins and nodularins are synthesised by a combined nonribo- somal peptide synthetase (NRPS) and polyketide synthase (PKS) pathway, which is well known for the synthesis of peptide antibiotics in bacteria and fungi, including penicillins (Kleinkauf 8c Dohren, 1996; Dittmann 8c Borner, 2005). Microcystins are produced by large multienzyme complexes consisting of peptide synthetases, polyketide synthases and tailoring enzymes. These enzymes activate specific amino acids and condense them to peptides. The genes encoding for microcystin synthetases (mcyA-mcyJ) have been characterised for Microcystis, Dolichospermum, Fischerella, Nostoc and Planktothrix (Tillett et ah, 2000; Christiansen et ah, 2003; Rouhiainen et ah, 2004; Fewer et ah, 2013; Shih et ah, 2013). The biosynthesis of nodularins is encoded by homologous genes (ndaA-ndal) that have been characterised from Nodularia (Moffitt & Neilan, 2004). Both the microcystin and nodularin gene clusters comprise around 50 kb pairs in all investigated species, but differences in the gene order as well as DNA sequence variation in the same modules have been observed. Based on coding nucleotide sequences, Rantala et ah (2004) concluded that microcystin synthetase genes have already been present in an early stage of cyanobacte- rial evolutionary history.

Microcystin and nodularin production appears to be constitutive in genotypes which have the complete microcystin gene clusters, while it is absent in genotypes lacking the whole or relevant parts of the gene cluster (Christiansen et ah, 2008; Tooming-Klunderud et ah, 2008). Smaller mutations in single mcy genes can lead to genotypes unable to synthesise microcystin (Kurmayer et ah, 2004; Christiansen et ah, 2006; Fewer et ah, 2008).

 
Source
< Prev   CONTENTS   Source   Next >