Description: Marine fungi are an important component of pelagic planktonic communities. However, it is not yet clear how individual fungal taxa are integrated in marine processes of the microbial loop and food webs. Most likely, biotic interactions play a major role in shaping the fungal community structure. Thus, the aim of our work was to identify possible biotic interactions of mycoplankton with phytoplankton and zooplankton groups and among fungi, and to investigate whether there is coherence between interactions and the dynamics, abundance and temporal occurrence of individual fungal OTUs. Marine surface water was sampled weekly over the course of 1 year, in the vicinity of the island of Helgoland in the German Bight (North Sea). The mycoplankton community was analyzed using 18S rRNA gene tag-sequencing and the identified dynamics were correlated to environmental data including phytoplankton, zooplankton, and abiotic factors. Finally, co-occurrence patterns of fungal taxa were detected with network analyses based on weighted topological overlaps (wTO). Of all abundant and persistent OTUs, 77% showed no biotic relations suggesting a saprotrophic lifestyle. Of all other fungal OTUs, nearly the half (44%) had at least one significant negative relationship, especially with zooplankton and other fungi, or to a lesser extent with phytoplankton. These findings suggest that mycoplankton OTUs are embedded into marine food web chains via highly complex and manifold relationships such as parasitism, predation, grazing, or allelopathy. Furthermore, about one third of all rare OTUs were part of a dense fungal co-occurrence network probably stabilizing the fungal community against environmental changes and acting as functional guilds or being involved in fungal cross-feeding. Placed in an ecological context, strong antagonistic relationships of the mycoplankton community with other components of the plankton suggest that: (i) there is a top-down control by fungi on zooplankton and phytoplankton; (ii) fungi serve as a food source for zooplankton and thereby transfer nutrients and organic material; (iii) the dynamics of fungi harmful to other plankton groups are controlled by antagonistic fungal taxa.

Description: Marine fungi are prominent components of plankton communities, but to date little is known about how the different community members are integrated into trophic networks and whether fungal cell numbers/biomass reach a size that can be considered significant for the carbon cycling and food web chains. To shed light into this black box, we linked tag-sequencing based classification and CARD-FISH enumeration of mycoplankton communities sampled from a marine time series station in the North Sea in two successive studies and modeled fungal interactions with other plankton groups over complex weighted Topological Overlap (wTO) networks. The wTO analysis reported significant negative relationships for half of all fungal OTUs with phyto- and zooplankton groups, but also between fungi. Zoosporic fungi, including several newly identified clades, held a special position within the modeled food webs with highest numbers of connections. During the spring phytoplankton bloom, Rozellomycota zoospores reached values of 106 cells L-1 similar to what is reported for freshwater mycoloops. Furthermore, saprotrophic yeasts showed distant frequency patterns over the course of the year. At the end of the spring phytoplankton bloom, yeast cell number increased 11-fold within a week and reached biomass values with a maximum of 8.9 mg C m-3 comparable to values of filamentous fungi in the Chilean upwelling system. Our results suggest that marine mycoplankton is integrated into trophic networks over different planktonic interactions, causes trophic upgrading, and are part of the microbial loop.

Description: Rivers are transport systems and supply adjacent ecosystems with nutrients. They also serve human well-being, for example as a source of food. Microorganismic biodiversity is an important parameter for the ecological balance of river ecosystems. Despite the knowledge that fungi are key players in freshwater nutrient cycling and food webs, data on planktonic fungi of streams with higher stream order is scarce. This study aims to fill this knowledge gap by a fungi-specific 18S rRNA gene tag sequencing approach, investigating mycoplankton diversity in the Elbe River along a transect from shallow freshwater, to the estuary and river plume down to the adjacent marine waters (sections of 7th stream order number). Using multivariate analyses and the Quantitative Process Estimates (QPE) method, the questions of how mycoplankton communities as part of the river continuum change along the transect, what factors, spatial and environmental, play a role, and what assembly processes, such as selection or dispersion, operate along the transect were addressed. The partitioning of mycoplankton communities into three significant distant biomes was mainly driven by local environmental conditions that were partly under spatial control. The assembly processes underlying the biomes also differed significantly. Thus, variable selection dominated the upstream sections, while undominated processes like ecological drift dominated the sections close to the river mouth and beyond. Dispersal played a minor role. The results suggest that the ecological versatility of the mycoplankton communities changes along the transect as response for example to a drastic change from an autotrophic to a heterotrophic system caused by an abrupt increase in the river depth. Furthermore, a significant salinity-dependent occurrence of diverse basal fungal groups was observed, with no clade found exclusively in marine waters. These results provide an important framework to help understand patterns of riverine mycoplankton communities and serve as basis for a further in-depth work, so that fungi as an important ecological organism group can be integrated into models of e.g. usage-balance considerations of rivers