Microfungal oasis in an oligotrophic desert: diversity patterns and community structure in three freshwater systems of Cuatro Ciénegas, Mexico

Abstract

The Cuatro Ciénegas Basin (CCB) comprises several oligotrophic aquatic ecosystems limited by phosphorus. These aquatic systems are dominated by a high prokaryotic diversity, shaped by the stress of low nutrient supplies and interspecific competition. Although fungi constitute a diverse and important component of microbial diversity, the microfungal diversity in the CCB remains to be unveiled. With the aim to explore microfungal diversity and ecological patterns in this area, we present the first investigation analyzing cultivable taxa from sediment and water, as well as lignocellulolytic taxa obtained from incubated submerged plant debris, and wood panels in three contrasting freshwater systems in the CCB: Churince, Becerra and Pozas Rojas. We chose a culture-based approach to analyze sediment and water samples in order to obtain fungal cultures, providing opportunities for a posteriori studies, and the possibility of ex situ preservation of the diversity. We evaluated sequence data from the nuclear ribosomal internal transcribed spacer including the 5.8 rDNA region for 126 isolates, revealing 37 OTUs. These OTUs were phylogenetically affiliated to several genera in the fungal phyla: Zygomycota, Basidiomycota, and Ascomycota. We recorded two OTUs with saline affinity, agreeing with previous findings on the prokaryotic communities with ancestral marine resemblances. All the studied systems showed moderate diversity levels, however discrepancies among the diversity indexes were observed, due to the occurrence of abundant taxa in the samples. Our results indicated that lignocellulolytic microfungal communities are dominated by transient fungal taxa, as resident species were not recorded perhaps as a result of the long-term strong competition with the highly adapted prokaryotic community. Moreover, the obtained microfungal taxa occurred mostly on the resident plant debris, rather than submerged wood panels, perhaps as a result of the high adaptation to specific environmental conditions. In conclusion, the CCB possess a moderate taxonomical diversity compared to other arid environments, probably as a result of high selective pressures. Nonetheless, due to high spatial and temporal heterogeneity, the functional fungal diversity was considerable as predicted by the intermediate disturbance hypothesis. Decisively, the assessment of microfungal diversity freshwater systems is relevant, since this ecological group of microorganisms represents an important indicator of trophic complexity and biotic interactions among microbial communities, having important implications for understanding eukaryotic survival at the oligotrophic limit for life.

Keywords: Ascomycota; Biodiversity; Facultative aquatic fungi; Oasis mycobiota; Plant debris; Submerged wood baits.

Figure 1. Location of the sampling sites in the Cuatro Ciénegas Basin, Coahuila, Mexico.

(A) Geographical position of the studied freshwater systems within the CCB, and (B) detail of the sampling sites in the Churince system.

Figure 2. Bayesian tree.

Consensus tree of ITS region phylotypes recovered from 126 fungal isolates from water and sediment samples from Cuatro Ciénegas Basin and additional 100 reference sequences from the Genbank, sequence nomenclature corresponds to Table S1. Tree topologies are supported by Bayesian posterior probabilities (first number) and bootstrap values for 1,000 replications (second number). For clarity of the presentation, branches with sequences belonging to the same OTU have been collapsed and are shown as triangles.

Figure 3. Cultivable OTUs frequency.

Microfungal communities inhabiting the studied hydrologic systems in the Cuatro Ciénegas Basin, including cultivable OTUs relative abundance and their distribution in the studied sites. (A) Frequency of the OTUs in the overall sample, and (B) occurrence of the OTUs at a site level.

Figure 4. Rarefaction plot.

Relationship of sampling effort and OTU richness for cultured fungi from three freshwater systems in the CCB. Dark line indicates the accumulation of taxa as a function of number of isolates, whereas the grey line indicates the accumulation of taxa as a function of number of samples sites.

Figure 5. Microfungal diversity in the Cuatro Cienegas Basin.

Rényi and Tsallis diversities for each freshwater system. Green dashed lines represent the maximum and minimum diversity values. Pink dashed lines correspond to the median diversity observed for the overall sample. Blue circles indicate the observed diversity values of cultivable fungi in for each system. The x-axis refers to the scale used to denote entropy values.

Figure 6. Dissimilarity indexes among sites.

Ward’s distance trees based on the (A) Bray-Curtis and (B) Jaccard dissimilarity indexes among the five assessed freshwater systems.

Figure 7. DCA plot.

Detrended correspondence analysis of the dominant cultivable microfungal taxa in Cuatro Ciénegas Basin. Dominant OTU names are presented in black, and site names are displayed in red, see Table 1 for nomenclature.

Figure 8. Transient lignocellulolytic aquatic fungi.

Diversity of lignocellulolytic transient aquatic fungi occurring on plant wreckage in some water systems in CCB. Phoma sp. 1 (A, B), Phoma sp. 2 (C, D), Stachybotrys sp. 1 (E), Stachybotrys sp. 2 (F). Pilose pycnidia growing on wood (A). Pigmented and generally septated conidia (B). Glabrous pycnidia on wood (C). Hyaline, aseptated conidia (D). Conidiomata on Phragmites debris (E). Conidiophore and phialoconidia (F). Scale bar: A = 20 µm, B = 27 µm, C = 20 µm, D = 20 µm, E = 20 µm, F = 20 µm.

Figure 9. Transient lignocellulolytic aquatic fungi.

Diversity of lignocellulolytic transient aquatic fungi occurring on plant wreckage in some water systems in CCB. Volutella sp. (A, B), Venturia sp. 1 (C, D), Venturia sp. 2 (E, F). Sporodochia on substrate (A). Detail of the conidiophores aggregated into sporodochia hyaline with setae around margin of conidiomata, and aseptated hyaline conidia (B). Ascoma erumpent through host bark (C). Squashed subglobose ascoma revealing asci and lightly pigmented 1-septated thick-walled cells (D). Superficial globose ascoma on wood (E). Eight-spored and bitunicated asci, and detail on the morphology of 1-septated, asymmetrical, heavily pigmented thick-walled cells (F). Scale bar: A = 20 µm, B = 20 µm, C = 20 µm, D = 20 µm, E = 20 µm, F = 20 µm.

Figure 10. Soft rot cavities produced by lignocellulolytic fungi.

Soft rot cavities on local plant wreckage supporting freshwater transient ascomycetes. One-month incubation (A). Three-month incubation (B). Six-month incubation (C). Scale bar = 7 mm