Biogeochemical cycling of lignocellulosic carbon in marine and freshwater ecosystems: Relative contributions of procaryotes and eucaryotes’

The relative contributions of procaryotes and eucaryotes to the degradation of the lignin and polysaccharide components of lignocellulosic detritus in two marine and two freshwater wetland ecosystems were determined. Two independent methods-physical separation of bacteria from fungi and other eucaryotes by size fractionation, and antibiotic treatments-were used to estimate procaryotic and eucaryotic contributions to the degradation of [14C-lignin]lignocelluloses and [14Cpolysaccharidellignocelluloses in samples of water and decaying plant material from each environment. Both methods yielded similar results; bacteria were the predominant degraders of lignocellulose in each of the aquatic ecosystems. Fungi and other eucaryotes contributed only minimally to the overall degradation of lignocellulose in both marine ecosystems and in the Okefenokee Swamp but did contribute significantly to the degradation of lignocellulose in the Big Cypress Swamp. These results indicate a basic difference between the microbial degradation of Iignocellulosic material in terrestrial and aquatic environments. Fungi have long been considered the predominant degraders of lignocellulose in terrestrial systems; our results indicate that in aquatic systems bacteria are the predominant degraders of lignocellulose. The importance of microorganisms as a link between primary and secondary production in detritus-based food webs of aquatic ecosystems has long been recognized (Fenchel and Jorgensen 1977; Odum and de la Cruz 1967; Teal 1962). The major portion of the organic matter entering detritus-based aquatic ecosystems such as salt marshes, mangrove swamps, and freshwater wetlands is derived from vascular plants, and therefore, consists primarily (7 5-8 5% by wt) of the highly refractory polymeric complex, lignocellulose (Benner et al. 1985; Hodson et al. 1984). Most aquatic animals do not possess the enzymatic capability to directly utilize lignocellulosic detritus as a carbon and energy source (Kristensen 1972). Instead, lignocellulolytic bacteria and fungi transform this abundant biopolymer into digestible degradation products and easily assimilable microbial biomass which then are available to animals. Despite the recognized importance of microorganisms to L This work was supported by NSF grants OCE 8 l17834, BSR 81-14823, andBSR 82-15587. Additional funding was provided from grant NA 80AA-DO0091 from the NOAA Office of Sea Grant. Contribution 536 of the Marine Institute, University of Georgia. Okefenokee Ecosystem Publication, Empirical Ser. 60. the degradation of lignified plant material, there have been few studies of the relative contributions of bacteria and fungi to the degradation of lignocellulosic detritus in aquatic ecosystems. Studies of the microbial colonization of plant detritus in aquatic environments typically indicate that fungal biomass is highest during the initial weeks of submergence, whereas bacterial biomass is initially low and increases steadily during the decay process (Federle and Vestal 1982; Lee et al. 1980; Suberkropp and Klug 1976), although in several studies bacteria were found to be the primary colonizers of plant detritus (Morrison et al. 1977; Newell 198 1; Rublee and Roman 1982). Results of experiments using antibiotics to determine the relative contributions of procaryotes and eucaryotes to the biodegradation of plant material under aquatic conditions also indicate that fungi seem to be relatively more important during the early stages of decomposition, bacteria during the latter stages of decomposition (Kaushick and Hynes 197 1; Mason 1976). Although these studies did not directly investigate the relative contributions of aquatic fungi and bacteria to the degradation of the lignocellulosic component of the detritus, the results suggest that fungi