Ectomycorrhizal fungi in Amazonian tropical forests in Colombia

Abstract

The ectomycorrhizal (EcM) symbiosis was assumed to be restricted to the temperate regions where forests are dominated by EcM host plants, and the tropics were supposed to be dominated by endomycorrhizal fungi. However, evidence of the presence of EcM symbiosis in tropical lowland ecosystems has been accumulated recently. In Neotropics, the EcM plants belong to families Fabaceae, Gnetaceae, Nyctaginaceae, Polygonaceae, and Dipterocarpaceae. Soil microorganisms, particularly fungi, are known to play a prominent role in structuring the diversity and abundance of trees in the tropical rainforest. It is thus tempting to speculate that EcM fungi also contribute to these ecological roles. One of the aims of this research was to document the fungal diversity in two types of forests in the Colombian Amazon basin. EcM fungi were studied from white sand forests (WSF) that are home to EcM trees such as Dicymbe uaiparuensis and Aldina sp. (Fabaceae, subfamilies Caesalpinioideae and Papilionoideae respectively), and terra-firme mixed forests with the tree Pseudomonotes tropenbosii (Dipterocarpaceae) that is one of the most important canopy species (PtF). Sequences of the internal transcribed spacer (ITS) regions of the ribosomal DNA was used to identify fungal species from fruiting bodies and root tips, while the chloroplast intron region trnL (UAA) to identify the host plants. A total of 114 species of EcM macrofungi were identified based on morphology-based techniques, of which 83 were identified in PtF and 49 in WSF. The most abundant families were Russulaceae (28 species), Clavulinaceae (15 species), Hymenochaetaceae and Boletaceae (14 species), Amanitaceae (12 species), and Cantharellaceae (6 species). Thirty-one species (27 %) found during this study were not reported before in Colombia, and twenty-four corresponded to undescribed species (21 %). A large number of species documented from the studied WSF have also been found in symbiotic associations with other legume and/or dipterocarp species from geographically distant forests located in Guyana, French Guyana, Venezuela, and Brazil. This result highlights the low specificity of EcM fungi in relation to their host plants in lowland Amazonian forests. The distribution of some fungal species that were previously considered restricted to the Guiana Shield was extended to P. tropenbosii forests in Colombia. We studied the soil fungal community in three types of forests in the Colombian Amazon using high-throughput DNA sequencing. The fungal diversity in WSF, PtF and terra-firme forests with the EcM trees Coccoloba, Guapira or Neea was analyzed by 454 pyrosequencing of the ITS2 region. A high fungal diversity was detected from soil samples of all three studied forests. MF and PtF shared a large number of OTUs, while the WSF presented the most unique fungal community composition. The most dominant functional groups were saprotrophs, plant pathogens, and ectomycorrhizal symbionts. Forest type, soil pH, and C/N ratio were found to be important drivers that structure the highly diverse fungal communities. . Soil pH and C/N ratio explained 14% and 12% of fungal variation, respectively. Our data highlight the high diversity and heterogeneity of fungal soil community composition in mixed forests and WSFs in Colombian Amazonia.