Cellulose is a major component of the terrestrial particulate organic carbon (POCt) that enters the aquatic habitat. Although by itself it is nutritionally inadequate for Daphnia growth and reproduction, terrestrial stable isotope ratios have been shown in zooplankton. We are exploring ways in which cellulose may contribute via algal growth to an aquatic food chain in Closed Ecological Systems (CESs, isolated from the atmosphere, but receiving light). The systems are composed of a chemically defined inorganic medium, three species of green algae (Ankistrodesmus, Scenedesmus, and Selenastrum) and (in some) Daphnia magna. In several CES experiments, chemically pure cellulose served as a carbon source for algal growth and sustained Daphnia populations better than systems without an added carbon source and as well or better than those with NaHCO3 as a carbon source. We are now testing components of the CES to test the hypotheses that microbes produced extracellular enzymes that initiated the digestion of cellulose to smaller sub-molecules such as cellobiose (a double glucose connected with a beta linkage) and/or to glucose and that at least one of our algal species, probably Scenedesmus, can utilize these heterotrophically and receive an energy subsidy.
Results/Conclusions
To narrow down and help identify microbes responsible for digestion of cellulose, we treated algae with antibiotics which did not eliminate all microbes, but did inhibit the algae’s ability to grow on cellulose as a carbon source.
Using alternative carbon sources, of the three algae, Scenedesmus grew in the light and (slowly) in the dark on glucose; it will also grow in both the light and dark with cellobiose if not treated with antibiotics, but if treated with antibiotics, will only grow on cellobiose in the light.
From the CESs that showed cellulose support, we have subcultured numerous bacteria and at least three fungi that were present and are now attempting to isolate the microbial organism(s) responsible for making cellulose available as a carbon source; it is likely that it is a consortium.
These algal cells would contain the C and H stable isotope patterns of POCt, and provide the additional nutrients (e.g., fatty acids, and amino acids) required by zooplankton for growth and reproduction, thus possibly supporting terrestrial particulate matter being an energy subsidy in the aquatic food chain.