Abstract
Mountain environments are marked by an altitudinal zonation of habitat types. They are home to a multitude of terrestrial green algae, who have to cope with abiotic conditions specific to high elevation, e.g., high UV irradiance, alternating desiccation, rain and snow precipitations, extreme diurnal variations in temperature and chronic scarceness of nutrients. Even though photosynthetic green algae are primary producers colonizing open areas and potential markers of climate change, their overall biodiversity in the Alps has been poorly studied so far, in particular in soil, where algae have been shown to be key components of microbial communities. Here, we investigated whether the spatial distribution of green algae followed the altitudinal zonation of the Alps, based on the assumption that algae settle in their preferred habitats under the pressure of parameters correlated with elevation. We did so by focusing on selected representative elevational gradients at distant locations in the French Alps, where soil samples were collected at different depths. Soil was considered as either a potential natural habitat or temporary reservoir of algae. We showed that algal DNA represented a relatively low proportion of the overall eukaryotic diversity as measured by a universal Eukaryote marker. We designed two novel green algae metabarcoding markers to amplify the Chlorophyta phylum and its Chlorophyceae class, respectively. Using our newly developed markers, we showed that elevation was a strong correlate of species and genus level distribution. Altitudinal zonation was thus determined for about fifty species, with proposed accessions in reference databases. In particular, Planophila laetevirens and Bracteococcus ruber related species as well as the snow alga Sanguina genus were only found in soil starting at 2,000 m above sea level. Analysis of environmental and bioclimatic factors highlighted the importance of pH and nitrogen/carbon ratios in the vertical distribution in soil. Capacity to grow heterotrophically may determine the Trebouxiophyceae over Chlorophyceae ratio. The intensity of freezing events (freezing degree days), proved also determinant in Chlorophyceae distribution. Guidelines are discussed for future, more robust and precise analyses of environmental algal DNA in mountain ecosystems and address green algae species distribution and dynamics in response to environmental changes.
Highlights
Green algae are unicellular, colonial or multicellular photosynthetic organisms that are ubiquitous in almost all ecosystems
We focused on the Chlorophyta phylum, an important and diverse lineage of green algae, and were interested in the Chlorophyceae class, which we expected to yield the greatest diversity of algae
Using an iteratively reweighted least squares (IRLS) procedure, a linear model explaining 25% of the variance can be established on a logarithmic scale between the relative frequencies of Chlorophyta reads estimated by Euka03 and Chlo01 (Figure 3)
Summary
Colonial or multicellular photosynthetic organisms that are ubiquitous in almost all ecosystems They have evolved in two major lineages, one referred to as Chlorophyta, what has been traditionally called green algae, another referred to as Charophyta, containing a smaller but often geographically widespread number of taxa (Lewis and Mccourt, 2004; Domozych et al, 2016). Soil may contain resting algal cysts or nonmotile spores (aplanospores), acting as a reservoir for species needing more appropriate conditions to grow. This latter context is difficult to assess, since most microbial species are noncultivable (Schmeisser et al, 2007) and the determination of life cycles are extremely challenging. Some algae species can occupy non-liquid water systems, most notably snow and ice (Remias, 2012; Lukes et al, 2014; Hisakawa et al, 2015; Holzinger et al, 2016; Lu et al, 2016; Hoham and Remias, 2019)
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