Light is required for cellulose to support Daphnia populations in closed ecological systems

Background/Question/Methods

Terrestrial inputs to aquatic environments have been shown to contribute to community metabolism and zooplankton biomass.  Much of terrestrial input includes cellulose, which is usually considered to be unavailable to most organisms until degraded by cellulolytic microbes, thought to be a slow, months to years process.   In Closed Ecological Systems (CESs), the initial dissolved inorganic carbon is controlled, unlike natural systems in which there are many sources of dissolved inorganic carbon. Earlier results in our laboratory showed that chemically pure cellulose, as the major carbon source, supported algal and Daphnia populations exposed to light:dark cycles.   Three new experiments were undertaken to confirm those findings, and to determine if light was necessary for this food chain support. The CESs are composed of chemically defined nutrients, three algae, Ankistrodesmus, Scenedesmus and Selenastrum, (in some) the grazer Daphnia magna, and microbes associated with each culture.  Experiment 1 used 12:12 hour Light:Dark, CESs with and without Daphnia, with and without cellulose.  Experiment 2, all with Daphnia, 12:12 Light:Dark, with and without cellulose. Experiment 3, 12:12 hour L:D or continuous darkness, all with Daphnia, open to the atmosphere or sealed. Algal abundance was estimated from in vivo fluorescence; Daphnia were counted by eye.   Statistics are two-tailed, two-way ANOVAs, with P<0.05 significance.

Results/Conclusions

Experiments 1 and 2 confirmed that cellulose, with light:dark cycles,  supported algal population increases (from day 7) and supported more Daphnia (from day 21or 28) as compared to no-added-carbon controls.  Experiment 3 demonstrated that CESs with cellulose, incubated in continuous darkness did not sustain algal or Daphnia populations either open or closed, while CES that received light developed dense Daphnia populations. Of the CESs receiving light, peak Daphnia populations were greater in the open than closed systems (days 21-28), suggesting that atmospheric CO₂ could have made some contribution.   More research is needed to determine if the cellulose is degraded to CO₂, or if organic intermediates are used by the algae. In conclusion, the support of the aquatic food chain by cellulose involves photosynthesis and this provides an additional route by which terrestrial stable isotope ratios can influence zooplankton, and bridges the algae vs. allochthonous debate. Cellulose should not be considered recalcitrant and inactive in aquatic communities.