Oomycete Soil Diversity Associated with Betula and Alnus in Forests and Urban Settings in the Nordic-Baltic Region

Abstract

This study aimed to determine the differences and drivers of oomycete diversity and community composition in alder- and birch-dominated park and natural forest soils of the Fennoscandian and Baltic countries of Estonia, Finland, Lithuania, Norway, and Sweden. For this, we sequenced libraries of PCR products generated from the DNA of 111 soil samples collected across a climate gradient using oomycete-specific primers on a PacBio high-throughput sequencing platform. We found that oomycete communities are most affected by temperature seasonality, annual mean temperature, and mean temperature of the warmest quarter. Differences in composition were partly explained by the higher diversity of Saprolegniales in Sweden and Norway, as both total oomycete and Saprolegniales richness decreased significantly at higher longitudes, potentially indicating the preference of this group of oomycetes for a more temperate maritime climate. None of the evaluated climatic variables significantly affected the richness of Pythiales or Peronosporales. Interestingly, the relative abundance and richness of Pythiales was higher at urban sites compared to forest sites, whereas the opposite was true for Saprolegniales. Additionally, this is the first report of Phytophthora gallica and P. plurivora in Estonia. Our results indicate that the composition of oomycetes in soils is strongly influenced by climatic factors, and, therefore, changes in climate conditions associated with global warming may have the potential to significantly alter the distribution range of these microbes, which comprise many important pathogens of plants.

Keywords: Phytophthora; metabarcoding; oomycete community; plant pathogen; soil community; soil microbe.

Figure 1

Map presenting the locations of all sampled sites.

Figure 2

Numbers of oomycete OTUs classified into known (a) families and (b) genera across all samples.

Figure 3

Heatmap showing the proportion (%) of sites in each group where the specific Phytophthora species were detected.

Figure 4

Comparison of oomycete communities among countries based on relative abundances and OTU richness of prevalent orders: (a,b) alder sites, (c,d) birch sites, (e,f) forest sites, and (g,h) urban sites. Lit—Lithuania, Est—Estonia, Fin—Finland, Swe—Sweden, Nor—Norway.

Figure 5

Comparison of oomycete communities among forest and urban sites based on relative abundances and OTU richness of prevalent orders: (a,b) alder sites and (c,d) birch sites.

Figure 6

Distance-based redundancy analysis (RDA) ordination plot with climatic variables significantly affecting community composition. Blue and red indicate a higher and lower proportion of Saprolegniales OTUs, respectively. Arrow length indicates the strength of the effect.

Figure 7

Residual plots of the environmental variables significantly affecting (a,b) total oomycete OTU richness and (c,d) Saprolegniales OTU richness.