Fungi in water

Fungi are traditionally considered to be dominant in organic matter cycling in terrestrial environments, while bacteria correspondingly are superior in aquatic areas. However, fungi are commonly found in sea and lake water, and their importance in nutrient cycling may be higher than expected. Fungal biomass in freshwater is typically dominated by filamentous hyphomycetes. Hyphomycetes belong to the phylum Deuteromycota, also known as ‘‘fungi imperfecti’’ because no sexual forms have been observed.

Most research on aquatic fungi have focussed on either taxonomy and morphology, e.g. of spores, and on degradation of leaves and plant debris. Little is known about their abundance and growth physiology. The abundance of pelagic fungi, determined as the content of ergosterol (component of fungal cell walls), has been measured in 49 Baltic rivers, see figure at left. The biomass correlated positively with concentrations of dissolved organic matter DOM) and inorganic nutrients, as well as with spectral DOM properties, indicating that the fungi most likely had a terrestrial origin. The fungal biomass was 12- to 100-fold lower than the biomass of pelagic bacteria, suggesting that fungi in the water column of the rivers probably had a minor importance in the riverine ecosystems at the sampling time.

Reference: Jørgensen NOG & Stepanauskas R (2009). Biomass of pelagic fungi in Baltic rivers. Hydrobiologia 623: 105-112

Geographical distribution of ergosterol concentrations in the Baltic rivers. Width of the bubbles indicates concentrations of ergosterol
Map source: Link to Wikipedia

Biology of aquatic fungi

Presence of fungi in natural waters raise interesting questions such as
(i) is occurrence of fungi in water unintended (transport into the water by rain, groundwater, ect)
(ii) do aquatic fungi form hyphae like terrestrial species and if yes, how do they cope with water currents
(iii) can the fungi grow on minute surfaces such as dead planktonic algae - and
(iv) how do the fungi survive in an environments crowed by bacteria that are expected to have a high uptake capacity for organic matter?

In an attempt to answer some of these questions, an array of preliminary experiments were conducted.Aquatic fungi were isolated from estuarine and marine environments by spreading a small volume of water onto petri dishes with potato-starch agar. About 10 colonies/species with a acceptable growth potential were reisolated and used for experiments.

Suspensions of the fungi (whirl-mixed hyphae in water) were added to cultures of dead Chlorella sp. algae in natural lake or sea water. After a week the fungal biomass had increased up to 20-fold (based on ergosterol content, figure at left), while the density of the bacteria only increased slightly.

Analysis of the proteolytic, extracellular activity (leu-MCA assay) in the water demonstrated an up to 6-fold higher activity when fungi were present.

These preliminary tests show that aquatic fungi in lake water with a moderate algal biomass can compete with bacteria for degradation of dead algae. This suggests that the organic matter content - at least in eutrophic environments - can support a successful growth of aquatic fungi.