Cyanobacteria produce a high variety of hepatotoxic peptides in lichen symbiosis

Abstract

Lichens are symbiotic associations between fungi and photosynthetic algae or cyanobacteria. Microcystins are potent toxins that are responsible for the poisoning of both humans and animals. These toxins are mainly associated with aquatic cyanobacterial blooms, but here we show that the cyanobacterial symbionts of terrestrial lichens from all over the world commonly produce microcystins. We screened 803 lichen specimens from five different continents for cyanobacterial toxins by amplifying a part of the gene cluster encoding the enzyme complex responsible for microcystin production and detecting toxins directly from lichen thalli. We found either the biosynthetic genes for making microcystins or the toxin itself in 12% of all analyzed lichen specimens. A plethora of different microcystins was found with over 50 chemical variants, and many of the variants detected have only rarely been reported from free-living cyanobacteria. In addition, high amounts of nodularin, up to 60 μg g −1 , were detected from some lichen thalli. This microcystin analog and potent hepatotoxin has previously been known only from the aquatic bloom-forming genus Nodularia. Our results demonstrate that the production of cyanobacterial hepatotoxins in lichen symbiosis is a global phenomenon and occurs in many different lichen lineages. The very high genetic diversity of the mcyE gene and the chemical diversity of microcystins suggest that lichen symbioses may have been an important environment for diversification of these cyanobacteria.Nostoc | Peltigera | cyanolichen | secondary metabolites | chemical defence

Metagenomic natural product discovery in lichen provides evidence for a family of biosynthetic pathways in diverse symbioses

“…Some polyketides produced by trans-AT PKSs, such as pederin (21,22,43), bryostatin (45,46), and rhizoxin (12,47), are known to participate in host defense and pathogenicity in symbiotic associations. It has also been suggested that PKS-NRPS compounds such as the microcystins, sometimes produced by cyanobacterial symbionts, may contribute to the chemical defense of lichens against grazers (35,36). Expression of the nsp genes in P. membranacea and their presence in all Icelandic specimens tested suggest that it is a beneficial trait, although its role is unclear.…”

“…These approaches could also identify molecular variants: e.g., recently studied specimens of P. membranacea that appear negative for only one or two of the three primer sets used for nsp screening may present variants of nosperin. This possibility is akin to the situation of the microcystins and cryptophycins for which a large number of structural variants have been found (36,37). A thorough study of the >1,500 species of cyanobacteriabearing lichens and the multitude of other organisms including bryophytes, ferns, cycads, and angiosperms (54) that harbor cyanobacterial symbionts may yield many new biosynthetic pathways and metabolites to provide both alternative chemistry for potential pharmacological applications and a wealth of information on the chemical biology of symbiosis.…”

“…Two conventional cis-AT PKS-NRPS biosynthetic pathways have recently been described from cyanobacteria associated with lichens: the mcy gene cluster (35,36) involved in synthesis of microcystins, notorious hepatotoxins typical of many cyanobacteria, and the crp gene cluster, responsible for production of cryptophycins (37), anticancer agents of more limited distribution. The elements of the nosperin biosynthetic pathway, in contrast, are similar to the less common trans-AT PKS-NRPS systems responsible for a number of animalbacteria symbiosis-associated compounds including pederin, theopederins, onnamides, mycalamides, psymberin (irciniastatin A), and others (10) (Fig.…”

Molecular studies on cyanobacterial diversity in lichen symbioses

“…Summefield et al 2002, Stenroos et al 2006) C Peltigera venosa may have different Nostoc genotypes in different cephalodia (Paulsrud et al 2000) D Nostoc symbionts of bipartite Peltigera species have been identified in many studies (e.g. , O'Brien et al 2005) e Otalora et al (2010) analyzed genetic diversity of Nostoc in Collema and related cyanolichens F Many tropical Leptogium specimens were screened by Kaasalainen et al (2012) G A Nostoc strain isolated from Pannaria pezizoides produces potent hepatotoxins in culture , Kaasalainen et al 2009) H The cyanobacterial symbionts of Coccocarpia species are only distantly related to Nostoc (Lücking et al 2009). identical to that previously identified from P. membranacea in Sweden. Also the Nostoc genotype now found from Nephroma resupinatum in Oregon was quite similar to that previously found from the same lichen species in Finland.…”

Antimicrobial Activity ofCladonia verticillarisLichen Preparations on Bacteria and Fungi of Medical Importance

“…Lichens are symbiotic associations between fungus and one or more algae and/or cyanobacteria [5]. Species of the genus Cladonia have been used in folk medicine to treat respiratory diseases such as throat irritation, cough, asthma, and tuberculosis [6,7].…”