Breaking the paradigm-Prototheca algae occur only sporadically in soils under a temperate climate

Prototheca are unicellular, achlorophyllous, yeast-like microalgae that have been implicated as the causative agents of opportunistic infections in animals and humans, collectively referred to as protothecosis. Still, the algae are typically saprophytic in nature and mostly occur in areas of high humidity and high organic matter content. Only a few recent studies have explored the environmental reservoir of Prototheca. This work is the first study since the early 1980s to investigate the prevalence of Prototheca spp. in natural terrestrial sources. A total of 226 soil samples were collected, with only 10 yielding Prototheca growth. The overall isolation rate of the algae was 4.4%, with the predominant species being Prototheca bovis. These results show that Prototheca algae only sporadically occur in soils of the temperate climatic zone. Consequently, this work contradicts the ubiquity of Prototheca in soils, breaking the paradigm that, despite the lack of serious experimental appraisal, has long been perpetuated in the literature.IMPORTANCEThere is a scarcity of studies exploring the environmental habitat of Prototheca, a rare and poorly studied genus of microalgae, comprising opportunistic pathogens of humans and animals. This work focuses on the occurrence of Prototheca algae in soil environments. Given the potential hazard to human and animal health from exposure to environmental pathogens such as Prototheca, the prevalence of the microalgae in soils deserves an insightful examination. The study is the first since the 1980s to assess the prevalence of Prototheca spp. in natural terrestrial sources and the first ever performed to approach this topic by molecular methods. Soils have been considered to be ubiquitously inhabited by Prototheca. This view has been perpetuated in the literature despite the lack of any serious experimental appraisal. Surprisingly, we found that the algae are very rare soil inhabitants, clearly breaking the prevailing paradigm of their ubiquity in this environment.