Ronald Benner

Microbial utilization of dissolved organic matter from leaves of the red mangrove, Rhizophora mangle, in the Fresh Creek estuary, Bahamas

Abstract Dissolved organic matter was leached from [ 14 C]labeled leaves of the red mangrove, Rhizophora mangle , and used in studies to determine the rates and efficiencies of microbial utilization of the water-soluble components of mangrove leaves in the Fresh Creek estuary, Bahamas. Rates of microbial utilization (assimilation plus mineralization) of mangrove leachate ranged from 0·022 to 4·675 μg ml −1 h −1 depending on the concentration of leachate and the size or diversity of microbial populations. Microflora associated with decaying mangrove leaves utilized mangrove leachate at rates up to 18-fold higher than rates of leachate utilization by planktonic microflora. Chemical analyses indicated that tannins and other potentially inhibitory phenolic compounds made up a major fraction (18%) of the dissolved organic matter in mangrove leachate. Mangrove leachate did not appear to be inhibitory to the microbial uptake of leachate or the microbial degradation of the lignocellulosic component of mangrove leaves except at high concentrations (mg ml −1 ). The availability of molecular oxygen also was an important parameter affecting rates of leachate utilization; rates of microbial utilization of leachate were up to 8-fold higher under aerobic rather than anaerobic conditions. The overall efficiency of conversion of mangrove leachate into microbial biomass was high and ranged from 64% to 94%. As much as 42% of the added leachate was utilized during 2 to 12 h incubations, indicating that a major fraction of the leachable material from mangrove leaves is incorporated into microbial biomass, and thus available to animals in the estuarine food web.

Kinetics of microbial degradation of vascular plant material in two wetland ecosystems

SummaryVascular plant decomposition was followed during two different years in one freshwater and one marine wetland in southeastern Georgia, USA, using a modified litterbag technique. Chemical analysis of plant material revealed different rates of decomposition for different components of the plant material (soluble components, α-cellulose, hemicellulose, and lignin) and, further, that rates of decomposition of each component changed over time, such that the specific rate of decay for each fraction decreased as decomposition proceeded. Three mathematical models which differen in their treatment of the biochemical heterogeneity of vascular plant detritus were investigated with regard to their relative abilities to describe decomposition kinetics from the field incubations as well as from laboratory microcosm studies with radiolabeled plant material. A decaying coefficient model, which treats plant detritus as a single component but allows for a decreasing specific decomposition rate as material ages, was most successful in describing kinetics of decomposition. This model accomodates the changes in quality of vascular plant detritus resulting from preferential decomposition of more labile components (e.g., non-lignocellulosic material and holocellulose) and the relative accumulation of more refractory components (e.g., lignin) observed with time. The model also accomodates the potential transformation of various plant components into more refractory compounds (humification) during the decomposition process.

Temporal Relationship Between the Deposition and Microbial Degradation of Lignocellulosic Detritus in a Georgia Salt Marsh and the Okefenokee Swamp

Temperature dependence and seasonal variations in rates of microbial degradation of the lignin and polysaccharide components of specifically radiolabeled lignocelluloses were determined in sediment and water samples from a Georgia salt marsh and the nearby Okefenokee Swamp. Although temperature regimes in the two ecosystems were similar, rates of mineralization ofSpartina alterniflora lignocellulose in salt marsh sediments increased eightfold between winter and summer, whereas rates of mineralization of lignocellulose from an analogous freshwater macrophyte,Carex walteriana, in Okefenokee sediments increased only twofold between winter and summer. Temperature was the major factor influencing seasonal variations in rates of lignocellulose degradation in both environments. At any given temperature, no substantial differences in lignocellulolytic potential were observed with sediment samples collected at each season. In both ecosystems, the bulk of the lignocellulosic detritus was not degraded at the time of its peak deposition during the fall and winter. Instead, the periods of maximal decomposition occurred during the following spring and summer. These results suggest that periods of maximal nutrient regeneration from the mineralization of lignocellulosic detritus coincide with periods of highest primary production, and that, depending on hydrologic conditions, significant horizontal transport of essentially intact lignocellulosic material is possible due to the lag period between deposition and microbial degradation.

Dynamics of microbial biomass and activity in five habitats of the Okefenokee Swamp ecosystem.

A variety of freshwater marsh and swamp habitats are found interspersed in a mosaic pattern throughout the Okefenokee Swamp, Georgia, USA. We examined spatial and temporal patterns in standing stocks and activity in the microbial community of five habitats within this heterogeneous ecosystem. Standing stock dynamics were studied by measuring microbial biomass (ATP) and bacterial numbers (AODC) in both water and sediments over a 14 month period. Abundance varied temporally, being generally lower in winter months than in spring and summer months. However, a large proportion of the measured variability was not correlated with temporal patterns in temperature or with bulk nutrient levels. Spatial variability was characteristic of the Okefenokee at a variety of large and small scales. Habitat-level heterogeneity was evident when microbial standing stocks and activity (measured as [14C]lignocellulose mineralization) were compared across the five communities, although abundance differences among sites were restricted to nonwinter months when microbial biomass was high. Spatial variation within habitats was also found; patches of surface sediment with differing microbial activity or abundance were measured at scales from 30 cm to 150 m.

Modeling the Persistence of Lignocellulosic Detritus in Wetland Ecosystems

Lignocellulose derived from the biomass of marine or aquatic vascular plants constitutes the single most abundant source of organic matter in many wetland ecosystems such as marshes and swamps. The detritus that forms when the plants die and are deposited on the sediments is presumed to serve both as the basis of animal food webs and as the starting material from which sedimentary geopolymers, such as humus and peat, are made. Thus, the longterm fate of lignocellulose, which is a complex heteropolymer of lignin and the polysaccharides, cellulose and hemicellulose, is of interest to both the ecologist, from the standpoint of nutrient cycling and ecosystem trophodynamics, and the geochemist, modeling the accumulation and dissimilation of sedimentary organic matter. Central to both of these efforts is an understanding of the kinetics of lignocellulose transformation and mineralization in aquatic ecosystems.