An experiment in >1000 river and riparian sites found spatial patterns and controls of carbon processing at the global scale.

Microbial communities associated with submerged detritus in aquatic ecosystems often comprise a diverse mixture of autotrophic and heterotrophic microbes, including algae, bacteria, protozoa, and fungi. Recent studies have documented increased rates of plant litter mass loss when periphytic algae are present. We conducted laboratory and field experiments to assess potential metabolic interactions between natural autotrophic and heterotrophic microbial communities inhabiting submerged decaying plant litter of Typha angustifolia and Schoenoplectus acutus. In the field, submerged plant litter was either exposed to natural sunlight or placed under experimental canopies that manipulated light availability and growth of periphytic algae. Litter was collected and returned to the laboratory, where algal photosynthesis was manipulated (light/dark incubation), while rates of bacterial and fungal growth and productivity were simultaneously quantified. Bacteria and fungi were rapidly stimulated by exposure to light, thus establishing the potential for algal priming of microbial heterotrophic decay activities. Experimental incubations of decaying litter with 14C- and 13C-bicarbonate established that inorganic C fixed by algal photosynthesis was rapidly transferred to and assimilated by heterotrophic microbial decomposers. Periphytic algal stimulation of microbial heterotrophs, especially fungal decomposers, is an important and largely unrecognized interaction within the detrital microbial landscape, which may transform our current conceptual understanding of microbial secondary production and organic matter decomposition in aquatic ecosystems.

Emergent macrophytes are a major source of organic matter production in freshwater wetlands, and often represent the bulk of the plant material entering the detrital pool. We examined the decomposition and microbial dynamics associated with litter of the emergent macrophyte, Juncus effusus (soft rush), during its movement from an aerial standing dead to a submerged decay environment. Standing dead leaves of J. effusus were collected after an initial period of standing litter microbial decay, placed in 1-mm mesh litter bags, and submerged in a wetland. Litter bags were retrieved periodically over 268 d and analyzed for microbial (fungal and bacterial) biomass and production, ATP concentrations, litter mass loss, and quality (C : N : P and plant fiber). Submerged litter decay of J. effusus was slow (k ϭ 0.0010 d Ϫ1 ), with only 23% weight loss after 268 d. Both fungal and bacterial biomass and production decreased significantly soon after standing plant litter was submerged in the wetland surface waters. Despite decreases in microbial biomass and production, fungal decomposers remained the dominant microbial assemblage associated with decaying plant litter, accounting for 99% and 91% of the total microbial biomass and production, respectively. Mean fungal production ranged from 73-2,836 g C g Ϫ1 AFDM d Ϫ1 (AFDM: ash-free dry mass remaining) during the study period, whereas attached bacterial production ranged from 4-32 g C g Ϫ1 AFDM d Ϫ1. Patterns of litter ATP and nutrient concentrations (N and P) were similar to those observed for fungal and bacterial biomass, suggesting that at least a portion of the detrital N and P may have been incorporated into microbial biomass. Significant changes in microbial colonization and activity associated with emergent macrophyte litter can occur following the collapse of standing dead plant matter to the water or surface sediments. Furthermore, our findings suggest that fungi are significant contributors to the decay of coarse particulate plant matter in wetland ecosystems.In freshwater wetlands, emergent macrophytes frequently constitute a major fraction of organic matter production, with annual net above ground primary production often exceeding 2,000 g m Ϫ2 yr Ϫ1 (Wetzel 1990; Gessner et al. 1996; Wetzel and Howe 1999). The utilization of living plant matter by animal consumers as a food resource is considered negligible in most wetlands, since much of the carbon within macrophyte tissues resides in the recalcitrant cell wall structural materials (i.e., lignocellulose) which is not easily digested and assimilated (e.g., Mann 1988). Most plant matter eventually enters the detrital pool, where microorganisms, both bacteria and fungi, play an important role in litter breakdown and mineralization. Therefore, understanding the fate of vascular plant detritus and the microbial processes involved during litter decay are important aspects in elucidating energy

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This study examined the microbial dynamics associated with decomposing litter of the widespread emergent macrophyte Phragmites australis in a littoral reed stand of a large lake. Standing dead leaf and stem litter were collected, placed into fine and coarse mesh litter bags, and submerged in the reed stand. Litter bags were retrieved periodically and analyzed for fungal and bacterial biomass, fungal growth rates and production, rates of microbial respiration, litter mass loss, nutrient concentrations (N and P), and rates of dissolved organic carbon (DOC) release. Microbial biomass associated with both leaf and stem litter (12 to 85 mg C g -1 detrital C) was predominantly fungal (always ≥ 90% of the total biomass), even though bacterial biomass (0.13 to 5.6 mg C g -1 detrital C) increased and fungal biomass decreased or remained constant as litter decay proceeded. Although rates of fungal growth (0.02 to 0.08% h -1 ) and production (leaves only; 3 to 51 µg C g -1 detrital C h -1 ), and rates of microbial respiration (11 to 257 µg C g -1 detrital C h -1 ) decreased following litter submergence, fungi continued to be metabolically active in both leaf and stem litter. Significant differences in fungal and bacterial biomass, fungal production rates, and rates of respiration were observed between leaf and stem material, with leaves often having 5 times higher values than corresponding stems. Rates of mass loss differed significantly between leaf litter in fine and coarse mesh bags, with less than 10% of the initial mass remaining in coarse mesh bags after 86 d, versus nearly 60% remaining in fine mesh bags. Nitrogen and P concentrations of leaf litter enclosed in fine mesh bags increased during litter decay, whereas N concentrations of leaf litter in coarse mesh bags remained unchanged and P concentrations decreased. Both N and P concentrations of stem litter were similar among litter bags and varied little throughout the study period. Results obtained in this study indicate that significant changes in microbial colonization and activity associated with P. australis litter can occur following the collapse of standing dead plant matter to the water. Furthermore, these findings suggest that fungi are active on submerged litter and thus play a vital role in the decomposition of P. australis litter in the aquatic environment. KEY WORDS: Litter decomposition · Phragmites australis · Wetland · Microbial productivity · Fungi · Nutrients · Respiration · Growth efficiencyResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 22: 271-282, 2000 In many emergent macrophytes, such as Phragmites australis, abscission and collapse of plant material to the sediment or overlying surface waters typically do not occur immediately following shoot senescence and death. As a result, large amounts of dead plant matter remain standing within wetland habitats (Findlay et al. 1990, Lee 1990, Wetzel & Howe 1999, and are colonized and decomposed in an upright aerial position (Newell 1993, Ne...