Fungi and bacteria are the major organic matter (OM) decomposers in aquatic ecosystems. While bacteria are regarded as primary mineralizers in the pelagic zone of lakes and oceans, fungi dominate OM decomposition in streams and wetlands. Recent findings indicate that fungal communities are also active in lakes, but little is known about their diversity and interactions with bacteria. Therefore, the decomposer niche overlap of saprotrophic fungi and bacteria was studied on pollen (as a seasonally recurring source of fine particulate OM) by performing microcosm experiments with three different lake types. Special emphasis was placed on analysis of fungal community composition and diversity. We hypothesized that (I) pollen select for small saprotrophic fungi and at the same time for typical particle-associated bacteria; (II) fungal communities form specific free-living and attached sub-communities in each lake type; (III) the ratio between fungi or bacteria on pollen is controlled by the lake's chemistry. Bacteria-to-fungi ratios were determined by quantitative PCR (qPCR), and bacterial and fungal diversity were studied by clone libraries and denaturing gradient gel electrophoresis (DGGE) fingerprints. A protease assay was used to identify functional differences between treatments. For generalization, systematic differences in bacteria-to-fungi ratios were analyzed with a dataset from the nearby Baltic Sea rivers. High abundances of Chytridiomycota as well as occurrences of Cryptomycota and yeast-like fungi confirm the decomposer niche overlap of saprotrophic fungi and bacteria on pollen. As hypothesized, microbial communities consistently differed between the lake types and exhibited functional differences. Bacteria-to-fungi ratios correlated well with parameters such as organic carbon and pH. The importance of dissolved organic carbon and nitrogen for bacteria-to-fungi ratios was supported by the Baltic Sea river dataset. Our findings highlight the fact that carbon-to-nitrogen ratios may also control fungal contributions to OM decomposition in aquatic ecosystems.

Fungal metabarcoding of substrates such as soil, wood, and water is uncovering an unprecedented number of fungal species that do not seem to produce tangible morphological structures and that defy our best attempts at cultivation, thus falling outside the scope of the International Code of Nomenclature for algae, fungi, and plants. The present study uses the new, ninth release of the species hypotheses of the UNITE database to show that species discovery through environmental sequencing vastly outpaces traditional, Sanger sequencing-based efforts in a strongly increasing trend over the last five years. Our findings challenge the present stance of some in the mycological community – that the current situation is satisfactory and that no change is needed to “the code” – and suggest that we should be discussing not whether to allow DNA-based descriptions (typifications) of species and by extension higher ranks of fungi, but what the precise requirements for such DNA-based typifications should be. We submit a tentative list of such criteria for further discussion. The present authors hope for a revitalized and deepened discussion on DNA-based typification, because to us it seems harmful and counter-productive to intentionally deny the overwhelming majority of extant fungi a formal standing under the International Code of Nomenclature for algae, fungi, and plants.

Lake sediments harbor diverse microbial communities that cycle carbon and nutrients while being constantly colonized and potentially buried by organic matter sinking from the water column. The interaction of activity and burial remained largely unexplored in aquatic sediments. We aimed to relate taxonomic composition to sediment biogeochemical parameters, test whether community turnover with depth resulted from taxonomic replacement or from richness effects, and to provide a basic model for the vertical community structure in sediments. We analyzed four replicate sediment cores taken from 30-m depth in oligo-mesotrophic Lake Stechlin in northern Germany. Each 30-cm core spanned ca. 170 years of sediment accumulation according to 137Cs dating and was sectioned into layers 1–4 cm thick. We examined a full suite of biogeochemical parameters and used DNA metabarcoding to examine community composition of microbial Archaea, Bacteria, and Eukaryota. Community β-diversity indicated nearly complete turnover within the uppermost 30 cm. We observed a pronounced shift from Eukaryota- and Bacteria-dominated upper layers (14 cm) dominated by enigmatic Archaea that typically occur in deep-sea sediments. Taxonomic replacement was the prevalent mechanism in structuring the community composition and was linked to parameters indicative of microbial activity (e.g., CO2 and CH4 concentration, bacterial protein production). Richness loss played a lesser role but was linked to conservative parameters (e.g., C, N, P) indicative of past conditions. By including all three domains, we were able to directly link the exponential decay of eukaryotes with the active sediment microbial community. The dominance of Archaea in deeper layers confirms earlier findings from marine systems and establishes freshwater sediments as a potential low-energy environment, similar to deep sea sediments. We propose a general model of sediment structure and function based on microbial characteristics and burial processes. An upper “replacement horizon” is dominated by rapid taxonomic turnover with depth, high microbial activity, and biotic interactions. A lower “depauperate horizon” is characterized by low taxonomic richness, more stable “low-energy” conditions, and a dominance of enigmatic Archaea.

Molecular analyses of soil and water commonly reveal large proportions of fungal taxa that cannot be assigned to any taxonomic or functional groups. Some of these so-called dark taxa have been encoded alphanumerically, while others have remained completely overlooked. Using long-read sequencing that covers much of the ribosomal RNA operon, we shed light on the phylogenetic and ecological distribution of fungal dark taxa and formally describe 30 of the most prominent phylum- to order-level lineages based on their characteristic DNA features. This increases the known large-scale fungal phylogenetic diversity by roughly one-third. Formal names will enhance taxonomic reproducibility, facilitate communication among researchers, and enable the estimation of conservation and quarantine needs for uncultivable species and higher-ranking taxa. The new species in the respective highest-level novel taxonomic groups include Pantelleria saittana (Pantelleriomycetes), Paraspizellomyces parrentiae (Paraspizellomycetales), Aquieurochytrium lacustre (Aquieurochytriomycetes), Edaphochytrium valuojaense (Edaphochytriomycetes), Tibetochytrium taylorii (Tibetochytriomycetes), Tropicochytrium toronegroense (Tropicochytriomycetes), Algovorax scenedesmi (comb. nov.) and Solivorax pantropicus (Algovoracomycetes), Aquamastix sanduskyensis (Aquamastigomycetes), Cantoromastix holarctica (Cantoromastigomycetes), Dobrisimastix vlkii (Dobrisimastigomycetes), Palomastix lacustris (Palomastigomycetes), Sedimentomastix tueriensis (Sedimentomastigomycetes), Terrincola waldropii (Terrincolales), Curlevskia holarctica (Curlevskiomycota), Mycosocceria estonica (Mycosocceriales), Maerjamyces jumpponenii (Maerjamycetes), Ruderalia cosmopolita (Ruderaliomycetes), Bryolpidium mundanum (Bryolpidiomycetes), Chthonolpidium enigmatum (Chthonolpidiomycetes), Savannolpidium raadiense (Savannolpidiomycetes), Gelotisporidium boreale (Gelotisporidiomycetes), Sumavosporidium sylvestre (Sumavosporidiomycetes), Parakickxella borikenica (Parakickxellomycetes), Aldinomyces tarquinii (Aldinomycota), Borikenia urbinae (Borikeniomycota), Mirabilomyces abrukanus (Mirabilomycota), Nematovomyces vermicola (comb. nov.) and N. soinasteënsis (Nematovomycota), Viljandia globalis (Viljandiomycota), Waitukubulimyces cliftonii (Waitukubulimycota), and Tartumyces setoi (Tartumyceta).

The internal transcribed spacer (ITS) is used often in DNA metabarcoding of fungi. One disadvantage of its high variability may be a failure to classify OTUs when no similar reference sequence exists. We tested whether the 5.8S region, often sequenced with ITS2 but discarded before analysis, could provide OTU classifications when ITS2 fails. We used in silico evaluation to compare classification success of 5.8S and ITS2 from the UNITE database when reference sequences of the same species, genus, or family were removed. We then developed an automated pipeline for a combined 5.8S ‐ ITS2 analysis and applied it to mixed environmental samples containing many lineages that are underrepresented in databases. ITS was clearly superior for species‐level classifications with a complete reference database, but 5.8S outperformed ITS at higher level classifications with an incomplete database. Our combined 5.8S‐ITS2 pipeline classified 3x more fungal OTUs compared to ITS2 alone, particularly within Chytridiomycota (10x) and Rozellomycota (3x). Missing reference sequences led to the failure of ITS to classify many fungal OTUs at all, and to a significant underestimation of environmental fungal diversity. Using 5.8S to complement ITS classification will likely provide better estimates of diversity in lineages for which database coverage is poor.