2.1 | Study species
Peltigera is a cosmopolitan genus of lichen-forming fungi; its species form large foliose lichens which mainly grow on soil and among bryophytes (O’Brien et al., 2009). Our study species Peltigera britannica (Gyelnik) Holtan-Hartwig & Tønsberg is a Holarctic species which prefers oceanic climates (Martínez et al., 2003). It forms a tripartite morph with the green alga Coccomyxa sp. and small amounts of the cyanobacterium Nostoc sp. embedded in granular structures on the upper side (cephalodia) and a bipartite morph containing only Nostoc sp. as photosynthetic partner (hereafter referred to as cyanomorph) (Pardo-De la Hoz et al., 2018; Holtan-Hartwig, 1993). The two morphs of P. britannica have a characteristic morphology (Fig. 1): when wet, the tripartite morph is apple green whereas the cyanomorph is of a bluish-gray color (Goffinet & Hastings, 1994). The photomorphs grow either as separate individuals or as a single compound thallus with cyanobacterial and tripartite sectors (Goward et al., 1995).