Factors contributing to the potential expansion of Limnomonas gaiensis (Chlamydomonadales, Chlorophyta) in freshwater lakes in Northern Europe

Limnomonas gaiensis is a recently described green microalga inhabiting freshwater lakes in northern Europe (England, Sweden). There is little information on the species’ tolerance to environmental factors and its capacity to spread to unconnected lakes. Using a barcoding approach, we investigated the phylogeography of L. gaiensis along a latitudinal gradient spanning 54–60°N. Experimental long-term exposure to a gradient of water temperatures, mimicking natural conditions, was performed to assess its thermal tolerance. Genetic analyses showed that this limnic species is easily identified at the species level. Phylogeographic analyses revealed the existence of two genetically similar haplotypes over the mitochondrial cytochrome C oxidase subunit 1 gene in different countries, suggesting that the range of L. gaiensis has recently expanded from a common source lineage, despite geographic isolation. Heterogeneity in the Helix III of the nuclear Internal Transcribed Spacer 2 region of the English strain suggested past hybridization. Physiological analyses showed that L. gaiensis can survive and reproduce asexually, potentially year-round. Optimal growth was observed at 15°C, with a doubling time of 1.1 days. The species is able to survive short periods of desiccation. The lack of connectivity between the lakes where the species was sampled rules out aquatic dispersal of this species. However, its physiological features make L. gaiensis a good candidate for propagule dispersal by either wind or human/animal-mediated transportation. Our study calls for further investigation into limnic systems, using rapid barcoding, with concurrent atmospheric and physiological analyses to establish its distribution over wider geographic scales and to understand how L. gaiensis disperses.