Importance of Stream Microfungi in Controlling Breakdown Rates of Leaf Litter

Abstract

Breakdown of seven leaf species covering a broad range of litter qualities (lignin: 7-31% of leaf dry mass; tannin: 0.0-6. 7%; nitrogen: 0.5-2.6%; phosphorus: 0.017-0.094%) and dynamics of fungal biomass and reproductive activity were studied in a softwater mountain stream. Litter breakdown proceeded at exponential rates k ranging from 0.0042 d-1 (evergreen oak) to 0.0515 d-1 (ash). Fungal colonization of litter was generally rapid, with the fungus-specific indicator molecule ergosterol increasing from initially negligible concentrations to 375-859 ~gig of detrital mass. Using species-specific factors relating ergosterol concentrations to mycelial dry mass, maximum fungal biomass associated with litter was estimated as 61-155 mg!g of total system mass. Minimum estimates of net mycelial production during active growth varied between 0.3 and 3.8 mg · g-1 • d -I, and maximum sporulation rates of aquatic hyphomycetes ranged from 7 60 to 7500 conidia·mg-1 ·d-1 • Initially, reproductive activity was largely synchronized with increases in ergosterol concentrations, but it declined dramatically after peak sporulation rates were reached, whereas ergosterol concentrations levelled off or decreased at considerably slower rates. Periods of highest fungal productivity were thus limited to an initial breakdown stage of ~ 2-8 wk. Strong correlations were found between the exponential breakdown coefficient and each of three parameters reflecting fungal activity in leaf litter, that is, maximum ergosterol concentration (P = 0.002, r = 0.96), net mycelial production (P = 0.02, r = 0.92), and sporulation rate (P < 0.001, r = 0.99). The initial lignin content of leaves was also significantly correlated with the rate constant k (P = 0.02, r = -0.83), suggesting that lignin was the primary factor determining litter quality and thus breakdown rate. The correlation was even stronger when data were logarithmically transformed (P < 0.01, r = -0.95). Tannin concentration was significantly correlated with k only when two high-lignin species were excluded from the analysis (P = 0.19, r = -0.56 compared with P = 0.05, r = -0.88), while initial concentrations of phosphorus (P = 0.17, r = 0.58) and particularly nitrogen (P = 0.82, r = 0.06) were poor predictors of litter decomposability. These results suggest that the initial lignin content of leaves controlled litter breakdown rate through a kinetic limitation of carbon sources for saprotrophic microfungi. The decomposer activity of these organisms, in turn, would then have governed breakdown rates. In doing this, fungi produced substantial amounts of both mycelial and conidial biomass that was potentially available to higher trophic levels of the food web.

Nutrient Uptake and Mineralization during Leaf Decay in Streams – a Model Simulation

“…FINDLAY et al (2002) found that living fungal biomass was insufficient to account for the increase in nitrogen content of decaying wetland litter, and they concluded that nitrogen sequestration in non-living microbial products or abiotic nitrogen retention must account for the high nitrogen content of the decaying litter. Several studies suggest that our model estimates of carbon and nitrogen accrual by living microbes may be fairly realistic (GESSNER and CHAUVET, 1994;SANZONE et al, 2001;FINDLAY et al, 2002;GULIS and SUBERKROPP, 2003;METHVIN and SUBERKROPP, 2003). For example, SANZONE et al (2001) estimated that living microbes could account for as much as 22% of the detrital nitrogen in Walker Branch, Tennessee.…”

No evidence for leaf‐trait dissimilarity effects on litter decomposition, fungal decomposers, and nutrient dynamics

“…However, another simple explanation for the lack of litter dissimilarity effects is that remote fungal nutrient acquisition and translocation in heterogeneous litter mixtures was ineffective. This is the more likely scenario, since stimulation of fungal growth and reproduction are expected to be positively associated with enhanced decomposition (Gessner and Chauvet 1994), which proved to be unresponsive to litter dissimilarity as well.…”

“…Litter lignin content, in particular, is a key component of litter chemistry that commonly assumes overriding importance in determining decomposition rates in a wide variety of terrestrial (Melillo et al 1982, Freschet et al 2011, Talbot and Treseder 2012 and aquatic environments (Hladyz et al 2009, Schindler and). Evidence from streams suggests that the often tight negative relationship between litter lignin content and decomposition rate also holds for lignin and fungal growth (Gessner and Chauvet 1994), which reflects the positive effects of fungal activity on leaf litter decomposition through both direct leaf degradation and enhanced nutritional quality of litter for subsequent invertebrate consumption (Webster and Benfield 1986, Grac¸a 2001, Gulis et al 2009). In addition, differences in the concentration of litter nutrients, such as nitrogen and phosphorus, also influence decomposition rates (Enriquez et al 1993), even within the same litter species (Leroy et al 2007).…”

A new technique to monitor conidia of aquatic hyphomycetes in streams using latex-coated slides

“…Aquatic hyphomycetes are the dominant mycota of freshwater streams throughout the world involved in detritus decomposition and energy flow to the higher tropic levels (Bärlocher and Kendrick 1974; Suberkropp and Klug 1976; Bärlocher 1992; Gessner and Chauvet 1994). Aquatic hyphomycetes are conventionally identified based on their conidial morphology as they produce characteristic sigmoid and multiradiate conidia as an adaptation to dispersal in flowing waters similar to plankton (Webster and Davey 1984; Webster 1987; Sridhar 2009).…”