A photo of sewage fungus found in the River Crane (London, England) suspected as a consequence of airport de-icers.

Sewage fungus is a polymicrobial biofilm that proliferates in saprobic rivers^[1] and has been frequently used as a bioindicator^[2][3] of organic river pollution for the past century.^[4] Its presence has been strongly associated with discharges of untreated or inadequately treated sewage,^[5][6][7][8] yet its presence extends beyond these areas, with contributors including airport de-icers,^[9][10][11] papermill effluents,^[12] and agricultural runoff.^[13][14]

The name "sewage fungus" is somewhat of a misnomer,^[4] as these growths are not primarily fungal in nature. Instead, they are complex polymicrobial biofilms bound within a matrix of extracellular polymeric substances. Taxa most frequently associated with sewage fungus include Sphaerotilus natans, Zoogloea spp., Beggiatoa alba, and Rhodoferax spp.^[10][15][16]

Environmental Impacts

In addition to being a bioindicator of organic pollution in rivers and playing a vital role utilizing excess organic carbon in fluvial systems, sewage fungus causes significant ecological impacts through direct and indirect ecological pathways.

Sewage fungus thrives in the low dissolved oxygen (DO) environment of an organically polluted river.^{[3][16][17][18]} Whilst DO is required for sewage fungus growth, it readily outcompetes other benthic organisms at low DO,^[19][20][21] quickly smothering riverbeds, greatly altering the benthic habitat for invertebrates^[22][23] and fish spawning.^[1][24][25] The dominating growth of sewage fungus also reduces hyporheic exchange flows, an important part of a rivers self-cleaning system.^[26][27] Similar river biofilms are also reported to accumulate heavy metals^[28][29] and other toxic substances.^[30] within their matrix causing ecological impacts throughout the food web.^[31][32] As a heterotroph, sewage fungus uses considerably higher DO than an aquatic macrophyte of the same mass,^[33] it can maintain DO concentrations below thresholds required for other organisms. Once sewage fungus becomes established, it is difficult to remove,^[34] unless all sources of organic nutrients (pollution) are addressed, causing a further loss in biodiversity^[35] and other flora and fauna^[36][37] in the river. These ecological impacts and the striking visible presence of sewage fungus growth on a riverbed further affects people's perceptions and use of rivers.^[6][38]

Microbial composition

Sewage fungus is a polymicrobial biofilm and the specific composition of which is affected by the available nutrients (especially organic carbon sources) and the environmental drivers of each unique occurrence. However, several key taxa are reported as highly frequent and dominant within sewage fungus.

Sphaerotilus, and especially S. natans, has been strongly associated with sewage fungus since its inception^[4] and continues to be regarded as a key sewage fungus taxon.^[10][11] Consequently, Sphaerotilus has been used seemingly synonymously with sewage fungus and a series of laboratory studies use S. natans as sewage fungus.^[24][39][40]

Other key taxa include the bacteria Zoogloea spp., Beggiatoa spp., Thiothrix II, Flavobacterium spp., and Flexibacter spp..^[15][16] However, fungi (e.g., Leptomitus lacteus, Geotrichium candidum, and Fusarium aquaeductuum), algae (e.g., Cladophora glomerata) along with archaea and protozoa (e.g., Carchesium polypinum) also form integral and important pasts of the biofilm.

Recent genomic studies of sewage fungus composition have identified some of these taxa within airport de-icer implicated occurrences but have also identified new taxa not previously associated with sewage fungus. Exton et al (2023) identified Rhodoferax as a dominant component of sewage fungus,^[10] whereas Nott et al (2020) noted the presence of Thiothrix.^[11]

Drivers of sewage fungus

Alongside the complex nutrient utilisation requirements of sewage fungus, there are several key environmental drivers including substrate type, flow velocity, temperature, shading/sunlight, and water chemistry (e.g. pH).

Flowing water is a requirement for sewage fungus growth, to provide a constant replenishment of nutrients.^[1][3][41] However, if the velocity of the river is too fast, then growths are scoured away, especially on more readily mobilised substrates. In turn, the specific flow of the river shapes the morphotype and structure of the biofilm.^[42] Intrinsically the substrate affects the upper limit of flow as more stable riverbeds are less readily mobilised in periods of higher flows. Surfaces such as large cobbles, anthropogenic litter (e.g., bricks), and concrete channels facilitate excellent sewage fungus growth, whereas fine sediments and gravel provide a less stable substrate.

References