Porellales

Porellales is an order of lichen-forming fungi within the class Lecanoromycetes. The order is monogeneric, containing the single family Porellaceae, which is represented primarily by the genus Porella and a few related genera. Members of Porellales are predominantly foliose lichens whose thalli are typically thin, leafy, and loosely attached to their substrate. They possess a unique photobiont association, usually a cyanobacterium from the genus Nostoc, though some species also associate with green algae. The fungal partner (mycobiont) is characterized by a simple, unbranched hyphae structure and a smooth, crystalline cortex. Apothecia, when present, are often small, sessile, and racemose, with a disc-bearing hymenium that bears hyaline ascospores.

Ecologically, Porellales lichens are widespread in tropical and subtropical regions, often occupying humid, shaded forest understories or rocky outcrops in montane environments. Their cyanobacterial symbionts allow them to fix atmospheric nitrogen, making them important contributors to nutrient cycling in ecosystems where soil fertility is limited. They are also one of the few lichen groups that can form sizable, dense mats, providing microhabitats for invertebrates and contributing to soil formation on exposed rock surfaces.

The taxonomy of Porellales has been refined through molecular phylogenetic studies, which have confirmed its placement within Lecanoromycetes and confirmed its close relationship to the order Lecanorales. Despite its ecological significance, many species remain poorly studied, and further taxonomic work is needed to resolve interspecific differences. Current databases such as Index Fungorum and MycoBank provide authoritative listings of Porellales species, while the International Association for Lichenology publishes periodic reviews of lichen taxonomy and ecology that frequently include updates on this order.