Popular Science: A green parasite of two faces
Phyllosiphon is a very special representative of the green algal group from class Trebouxiophyceae. These are frequently coccoid in shape, with size of several micrometers, and are found for instance on the bark of trees, soil, rocks or walls. Here they form communities resembling “green dust”, called biofilms. Phyllosiphon is also present in biofilms, but even the trained eye is unable to identify it. It is very difficult, if not impossible, to distinguish algal genera of this class from each other due to the lack of morphological traits. But Phyllosiphon is the only one of this class which lives as a parasite. It lives in the leaves of Araceae in form of branched siphonal filaments. It can be found on the leaves of infected plants with the naked eye as pale green spots that remain green even after the leaves dry.
Kateřina Procházková and her colleagues had previously shown that in the Mediterranean region, there are both parasitic form of Phyllosiphon in host plants and free living form on tree bark. From molecular data it is clear that the genus Phyllosiphon is quite diverse and more widespread than previously thought.
The question remains whether there are indeed Phyllosiphon species that live entirely without a host, or are only able to survive for a long period of time in the form of spheres and when conditions are favorable to infect host again. Even the parasitic species are apparently able to survive for a long time without a host, as their good growth in culture indicates. The way they infect the host is unclear so far, as scientists failed to infect the host experimentally.
Five species of Phyllosiphon were described so far. The only European one was Phyllosiphon arisari, the others had been reported from the tropics. Kateřina Procházková isolated a number of samples from infected plants, both Arisarum vulgare and Arum italicum, including biofilms growing on nearby trees. An interesting thing came to light: each of these two genera of plants has its own species of the parasitic alga. The same species that were found in the leaves of infected plants, occurred also in biofilms on nearby tree bark, only in the form of coccoid cells, not filaments. Phycologists of the Department of Botany therefore described a new species, Phyllosiphon ari. The fact that the diversity of the genus Phyllosiphon copies the diversity of the host plant species may imply co-evolution of host and parasite. Given that Araceae are richly represented in the tropics, it is possible that a vast number of yet undescribed species of this unique alga is waiting to be discovered.
Procházková, K., Němcová, Y. & Neustupa, J. (2016): Phyllosiphon ari sp. nov. (Watanabea clade, Trebouxiophyceae), a new parasitic species isolated from leaves of Arum italicum (Araceae). Phytotaxa 283: 143–154.