Detrital Carbon and Nitrogen Relations, Examined with Degrading Cellulose
Authors: Biddanda Bopaiah , Riemann Franz 
Published: May 2008

Abstract. In the current conceptual framework of aquatic detritus food webs, several aspects of carbon and nitrogen dynamics remain unresolved, in particular the issue of nitrogen enrichment during the course of detritus degradation. Perhaps one of the reasons for this is the use of complex detritus originating from plant sources in most previous studies. Using nitrogen‐free pure cellulose as a simple and defined model substrate, we have addressed the detritus nitrogen problem by following the changes in microbial and chemical parameters during its degradation in a semi‐continuous flow of fresh North Sea water over a period of 12 weeks. Initial colonization and growth of heterotrophic bacteria and associated mucus was followed by heterotrophic protozoans. Closely attached or loosely bound cells that were embedded in voluminous mucus constituted the bacterial community. Protozoans comprised heterotrophic nanoflagellates and large euglenoid flagellates. The euglenoids produced plentiful faecal pellets 3 to 4 μm in diameter ‐ a novel source of microscopic detrital particles. After about 12 weeks, patches of cellulose fibres had been totally converted to microbial biomass and associated mucus. The carbon content decreased from 40% to 25% of the initial cellulose weight, while nitrogen increased from 0% to 0.5% as a result of net uptake of dissolved nitrogen from the ambient water. Elutriation experiments with dilute acetic acid showed that, after 12 weeks, 4.2% of the total nitrogen in degrading cellulose was present as amino acids; these are presumably weakly bound in some way to the mucus secreted by microorganisms, indicating a potential sequestering capability of microbial slime, which is ubiquitous in the aquatic environment. The results suggest that degrading detritus may gain an increasing pool of labile nitrogen compounds other than microbial protein; this may be of considerable importance for animal nutrition.

Using nitrogen‐free, pure cellulose filter paper pieces as a simple and defined model substrate, we have addressed the issue of detrital nitrogen enrichment by following the changes in microbial and chemical parameters during the degradation in a semi‐continuous flow of North Sea water over a period of 12 weeks. Bacteria associated with voluminous amounts of mucus and faecal pellet‐producing euglenoid flagellates were the most conspicuous organisms; fungi were absent. After 12 weeks, patches of cellulose fibres had been totally converted to microbial biomass and associated mucus.

The carbon content decreased from 40% to 25% of the initial cellulose weight, while nitrogen increased from 0% to 0.5%. Contrary to previous experiments with nitrogen‐containing natural detritus particles, where nitrogen conservation relative to carbon could influence the eventual C/N ratio, the present experiments with a nitrogen‐free substrate clearly demonstrate the amount of nitrogen originating from the ambient water.

Elutriation experiments with dilute acetic acid showed that, after 12 weeks, 4.2% of the total nitrogen in degrading cellulose was present as amino acids, presumably weakly bound in some way to the mucus secreted by microorganisms. It is suggested that the extracellular, slimy envelopes of microbes thriving on detritus particles increase the adsorptive capacity for amino acids by virtue of their fibrillar, surface‐rich substructure. Since previous studies estimate that bacterial cells account for only 6% or less of the particulate nitrogen in detritus, the present demonstration of 4.2% elutriable amino acid nitrogen in degrading cellulose fibres is relevant. It suggests that for animal nutrition there may be an important supplement of labile nitrogen compounds in addition to microbial protein.