Why did carbon become the pseudo-limiting factor in aquatic closed ecological systems?

Life Sciences in Space Research
Frieda B Taub

Abstract

Closure from the earth's atmosphere is a critical test of an ecosystem's ability to function. In our earlier testing of autotrophic Closed Ecological Systems (CESs), a C:N ratio of 26.4 (3.3 mM NaHCO3 and 0.125 mM NaNO3) supported algal and Daphnia populations for months, but developed extreme pH values (∼11 ungrazed, >10, grazed), suggesting that the systems were carbon-limited. Only approximately half the HCO3- (bicarbonate) would be expected to be available to green algae, the other portion becoming CO3-2 (carbonate). In an experiment described here, CESs were developed to explore a greater range of C:N ratios. To keep the medium from becoming too osmotically concentrated, NaNO3 was reduced to 0.0312 mM and NaHCO3 tested at 3.3, 13.2, and 26.4 mM, resulting in nominal C:N ratios of 105, 422, and 845. However, additional carbon was not beneficial to long-term survival of the organisms. The algal abundance was relatively insensitive to C:N ratio; greater concentrations of C were not beneficial. Daphnia populations were sensitive to C:N ratio and persisted longer at the lowest C:N ratio of 105. All of the C:N ratios tested in these CESs are outside of the expected range suggested from ecological studies, which is based on the R...Continue Reading

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