History

The first known record of gylletråd in Denmark is a specimen in the Danish algal herbarium in Copenhagen. It was collected in 1883 in Rude Forest, north of Copenhagen, and was labeled Ulothrix crenulata.

The species was known to the Danish phycologist Johannes Boye Petersen, a specialist in aerophytic algae. He was awarded a gold medal for his response to the Royal Danish Academy of Sciences and Letters’ botanical challenge: Danish Aerophilic Algae. His work was published in 1915 in Danish with a French summary.

Petersen illustrated and described gylletråd under the name Hormidium crenulatum. He found it, among other places, on pine trees in Strandskoven near Køge. In 1985, Hormidium crenulatum was transferred to the genus Klebsormidium. Due to the absence of a holotype, Lokhorst and Star (1985) designated a lectotype in the herbarium at Leiden, and in 2015, Michailiuk designated an epitype—a cultured alga whose DNA confirms the species’ placement within Klebsormidium.

However, gylletråd as we know it in Denmark exhibits a range of features that distinguish it from Klebsormidium and clearly associate it with the markedly different genus Apatococcus. This has been supported by various molecular techniques, including genome sequencing.

Since the previously used name, Hormidium crenulatum, belongs to a completely different group of green algae, it has been necessary to describe gylletråd under a new name: Apatococcus ammoniophilus (see Søchting et al. 2025).