Benthic Bacterial and Fungal Productivity and Carbon Turnover in a Freshwater Marsh

Abstract

Heterotrophic bacteria and fungi are widely recognized as crucial mediators of carbon, nutrient, and energy flow in ecosystems, yet information on their total annual production in benthic habitats is lacking. To assess the significance of annual microbial production in a structurally complex system, we measured production rates of bacteria and fungi over an annual cycle in four aerobic habitats of a littoral freshwater marsh. Production rates of fungi in plant litter were substantial (0.2 to 2.4 mg C g ؊1 C) but were clearly outweighed by those of bacteria (2.6 to 18.8 mg C g ؊1 C) throughout the year. This indicates that bacteria represent the most actively growing microorganisms on marsh plant litter in submerged conditions, a finding that contrasts strikingly with results from both standing dead shoots of marsh plants and submerged plant litter decaying in streams. Concomitant measurements of microbial respiration (1.5 to 15.3 mg C-CO 2 g ؊1 of plant litter C day ؊1 ) point to high microbial growth efficiencies on the plant litter, averaging 45.5%. The submerged plant litter layer together with the thin aerobic sediment layer underneath (average depth of 5 mm) contributed the bulk of microbial production per square meter of marsh surface (99%), whereas bacterial production in the marsh water column and epiphytic biofilms was negligible. The magnitude of the combined production in these compartments (ϳ1,490 g C m ؊2 year ؊1 ) highlights the importance of carbon flows through microbial biomass, to the extent that even massive primary productivity of the marsh plants (603 g C m ؊2 year ؊1 ) and subsidiary carbon sources (ϳ330 g C m ؊2 year ؊1 ) were insufficient to meet the microbial carbon demand. These findings suggest that littoral freshwater marshes are genuine hot spots of aerobic microbial carbon transformations, which may act as net organic carbon importers from adjacent systems and, in turn, emit large amounts of CO 2 (here, ϳ870 g C m ؊2 year ؊1 ) into the atmosphere.

Experimentally Simulated Global Warming and Nitrogen Enrichment Effects on Microbial Litter Decomposers in a Marsh

“…A possible explanation is that fungi were relatively inactive at the time when samples were taken 4.5 and 7.5 months after the submergence of the leaf litter in enclosures. This interpretation is in accordance with (i) the relatively low fungal biomass at the time of sampling (for comparison, see reference 21), (ii) declines in fungal biomass observed after the detachment of leaves from emergent macrophyte shoots (as described above), and (iii) the finding that bacterial productivity on submerged plant litter in freshwater marshes appears to assume great importance (9,45). Thus, it is conceivable that at the time of sampling in spring and summer, when leaf litter was in an advanced stage of submerged decomposition, fungi contributed relatively little to the overall activity of the litter-associated microbial communities.…”

“…This includes wetlands dominated by emergent aquatic vegetation (21) where plant production often is high (1.5 kg shoot dry mass m Ϫ2 year Ϫ1 or more [23,29]), and only a small fraction is consumed by herbivores (11). In these systems, both bacteria and fungi colonize plant litter and produce a substantial biomass and/or play an important role in the decomposition process (9,21,36,41). The microbial communities colonizing decomposing plant litter can be characterized by means of bulk measures such as total bacterial and fungal biomass and by activity measurements such as respiration, which provide a broad integrative assessment of aerobic microbial metabolism (e.g., see reference 26).…”

“…The column temperature was set at 33°C (Waters temperature control module; Waters AG, Baden-Dät-twil, Switzerland (34). Ergosterol was quantified based on comparison to a commercial standard (Ͼ97% purity; Fluka, Sigma-Aldrich, Schnelldorf, Germany) and converted to fungal biomass and carbon, respectively, based on a fungal ergosterol content of 7.3 mg g Ϫ1 dry mass and 43% carbon in fungal dry mass (9).…”

Microbial abundance, composition and enzymatic activity during decomposition of copepod carcasses

“…Carlson et al 2002, Kritzberg et al 2006. Fungal community structure, however, seems to be more dependent on a combination of environmental factors including substrate availability, temperature and competition with bacteria (Buesing & Gessner 2006). The high diversity of obviously different types of fungi on the freshly killed copepods compared to that of decomposing copepod carcasses indicates that even live copepods may have been naturally colonized by presumably parasitic fungi (Carman & Dobbs 1997, Decaestecker et al 2005.…”

“…Fungi play a key role in decomposing leaf litters and other allochthonous detritus in freshwater ecosystems (e.g. Buesing & Gessner 2006), and fungal infection of copepods and their eggs in eutrophic lakes has been frequently observed (Redfield & Vincent 1979, Burns 1980, 1985, but their role in decomposing plankton carcasses has to date not been reported. In a study with the marine copepod Acartia tonsa, Tang et al (2006b) also observed that copepod carcasses were decomposed by unidentified epiphytes in the presence of antibiotics.…”