Soil algae is a group of photoautotrophic organisms with unicellular, colonial or multicellular organizations of the thallus that are able to live on the surface of soil, forming accretions in the form of crusts and films, and in the soil profile. In plant communities, especially in such climatically severe regions as Russian Northeast Europe, these organisms play important role in creating organic soil matter and the main biogenic exchange cycles . The area under study has a sharp continental climate with cold winters, short and cool summers and much precipitation. This territory includes northern part of the Urals and the northeast of the East European Plain. Tundra, forest tundra and taiga vegetation occurs here. The first investigations of soil algae in Northeast Russian Europe were conducted in 1962  and continue to the present day. Huge amounts of data have been accumulated during this time about species diversity and the distribution of soil algae in different zonal communities of tundra and taiga ecosystems [3–16]. The aim of our research is to summarize the results of studies into soil algae in the tundra, mountain-tundra and boreal ecosystems of Russian Northeast Europe.
In order to summarize data on the algal diversity of terrestrial ecosystems, we analyzed literature sources and the results of our long-term research in the Bolshezemelskaya tundra, the northern part of the Urals and the northeast of the Russian Plain. Cyanoprokaryota and algae were studied using standard soil-algological methods and algological pure strains in liquid and agar media. Russian and foreign guides were used to identify algae species [17–20]. The eco-geographical characteristics of species are given according to Barinova et al. .
The algal flora of the studied area has a high level of diversity. The taxonomical list of algae includes 695 species from five divisions (Chlorophyta – 275, Cyanoprokaryota – 184, Bacillariophyta – 145, Ochrophyta – 65, Streptophyta – 25 and Euglenophyta – 1), 12 classes, 40 orders, 107 families and 245 species . This is about 20% of world soil algal flora (about 3,500 taxa). The Bolshezemelskaya tundra (251 taxa) , Pechora lowland (Vozeyskoe deposit – 208)  and mountain tundra of the Subpolar Urals (the river Kozhym basin – 206)  are well-studied areas (Figure 1).
There are less algological data from the other areas of the European northeast. Algae from the divisions Chlorophyta and Cyanoprokaryota were prevalent in most studied soil algal flora. The proportion of the other divisions varied in different areas. An exception was the algal flora of the northeast Russian Plain, where green algae prevailed in the acid soils of forests due to their resistance to acidity.
The highest algal diversity was found in the tundra ecosystems and the lowest in the forests (Table 1). A significant increase in the species diversity of soil algae in tundra and mountain tundra ecosystems was due to various forms of cryogenic relief created by permafrost thawing, frost fracturing, cryoturbation and solifluction. There is low competition from the highest plants in habitats without closed vegetation. Here, the main dominants are spore organisms, including algae that form biological soil crusts [14,15]. The algal flora of forest ecosystems was not so rich due to the closed vegetation, ash structure, overlogging of soils, high acidity and the low level of nutrient elements in the soils .
Taxonomical analysis revealed that the basis of algal flora in the studied area is formed by species from the orders: Chlamydomonadales, Oscillatoriales, Nostocales, Naviculales and Sphaeropleales. The leading families are: Chlamydomonadaceae, Chlorococcaceae, Oscillatoriaceae, Eunotiaceae, Pinnulariaceae and Nostocaceae. The highest species number was found in the genera: Chlamydomonas, Pinnularia, Eunotia, Phormidium, Leptolyngbya, Tetracystis, Nitzschia, Nostoc and Chlorococcum.
A high occurrence frequency (more than 60% from all sites under study) was found for (alphabetical order): Botrydiopsis eriensis, Bracteacoccus aggregatus, B. minor, Bumilleriopsis terricola, Chlamydocapsa lobata, Chlorella vulgaris var. vulgaris, Chlorococcum infusionum, C. lobatum, Coenochloris signiensis, Cylindrocystis brebissonii, Elliptochloris bilobata, Eunotia fallax, Eustigmatos magnus, Hantzschia amphioxys, Klebsormidium flaccidum, Leptolyngbya foveolara, Leptosira terricola, Macrochloris dissecta, Microcoleus autumnalis, Mychonastes homosphaera, Myrmecia bisecta, M. incisa, Nitzschia palea, Nostoc commune f. ulvaceum, N. punctiforme, Parietochloris alveolaris, Phormidium ambiguum, P. corium, Pinnularia borealis, P. subcapitata, Pseudococcomyxa simplex, Scenedesmus rubescens, Scotiellopsis levicostata, S. terrestris, Stichococcus bacillaris, S. minor, Spongiochloris excentric, Stenomitos frigidus, Stigonema minutum, S. ocellatum, Tetracystis aeria, Tolypothrix tenuis, Ulothrix variabilis and Vischeria helvetica.
The dominant complex of species was different in tundra, mountain and boreal ecosystems. In tundra, the most prevalent species were: Nostoc commune, Stigonema minutum, S. ocellatum, Gloeocapsopsis magma, Scytonema hofmannii, Leptolyngbya foveolarum, Pseudococcomyxa simplex, Sporotetras polydermatica, Palmellopsis gelatinosa, Myrmecia bisecta, M. incisa, Klebsormidium flaccidum, Chlamydomonas debaryana var. atactogama, Nostoc punctiforme, Chlorella vulgaris, Microcoleus autumnalis, Tolypothrix tenuis, Phormidium ambiguum, Scotiellopsis levicostata, Desmonostoc muscorum, Mychonastes homosphaera and Pseudophormidium edaphicum. In mountain tundra ecosystems, the most prevalent species were: Stigonema minutum, S. ocellatum, Nostoc commune, Tolypothrix tenuis, Elliptochloris bilobata, Pseudococcomyxa simplex, Sporotetras polydermatica, Eustigmatos magnus, Gloeocapsopsis magma, Leptolyngbya foveolara, Microcoleus autumnalis, Myrmecia bisecta, Chlamydocapsa lobata, Phormidium corium and Stenomitos frigidus. In forests, the dominant species were: Bracteacoccus minor, Chlamydomonas elliptica, C. gloeogama, Chlorella vulgaris, Chlorococcum infusionum, C. lobatum, Eustigmatos magnus, Myrmecia incisa, Stichococcus bacillaris, Klebsormidium flaccidum and K. nitens .
Eco-geographic analysis of soil algal flora by type of habitat revealed the prevalence of edaphophilic and typical soil species. The proportions of amphibian (water–air) and hydrophilic (water) species were about the same (Figure 2). Most species were indifferent to acidity (Figure 3) .
Geographical analysis revealed the prevalence of cosmopolite species typical for the northern algal florae . Species indicating the severe climate of the study area count for more than 20% of the flora: boreal, arcto-alpine and holarctic taxa (Figure 4).
In order to find the most important factors affecting the structure of species diversity in the ecosystems of the European northeast, we performed PCA ordination  using the occurrence of species in the algal florae under study. As a result, three groups of algal communities typical for mountain tundra, forests and plain tundra were revealed (Figure 5). Their distribution along axis 2 correlates with species richness (number of species). It was not possible to reliably interpret the dependence of the location of the distinguished groups on axis 1. However, the dependence of this distribution on the humidity and vegetation density in the habitats under study is very likely.
The algal flora of the Russian European Northeast is well studied. The revealed species diversity is close to that in other regions. In the soils of Kirov region (the River Vyatka basin), 581 species were found , while in Siberia (Irkutsk region, the Republic of Buryatia, Transbaikal region) 638 species (705 with interior taxa) of algae were found . Some extra data on the algae of the study area may be obtained from studies of less comprehensively investigated algal complexes in forest, meadow, mountain forest and mire ecosystems. The collection of living cultures of Cyanoprokaryota and algae SYKOA (http://ib.komisc.ru/sykoa registered in the World Catalogue of microorganism culture collections GCM No 1125) contains more than 200 strains of microalgae, isolated from the terrestrial habitats of different plain and mountain areas of the European Arctic and Subarctic.