Construction of a Daphnia-sustaining microcosm using purely inorganic medium: a system designed to investigate endogenous nutrient biosynthesis

Existing microcosms (ecosystem models) have structural limitations such as restricted element coverage in their media, exogenous vitamin supplementation, and the absence of sediment, and have not fully reproduced the biosynthesis and cycling of nutrients from inorganic substances that occurs in natural ecosystems. In the present study, an S-type microcosm (DMC: Daphnia Microcosm) was constructed by combining a medium containing all 31 essential elements as inorganic compounds (DMC medium) with a quartz-sand sediment of 0.3-0.6 mm grain size. No organic nutrients such as vitamins were added to the DMC medium. In a preliminary experiment comparing a no-sediment treatment (DMC medium only) and treatments with glass beads of different sizes, both the no-sediment and large-bead treatments failed to support Daphnia population growth and the transplanted individuals died early, whereas in the small-bead treatment dissolved oxygen within the sediment dropped below 0.1 mg L^-1 to form an anaerobic environment, the algal community diversified to include diatoms, and Daphnia reproduction was observed. In the main experiment using DMC with quartz-sand sediment, Daphnia consistently grew from three founders to a mean peak abundance of 23.8 ± 3.2 across three successive subcultures with three replicate lines each. The growth of Daphnia in a purely inorganic medium devoid of vitamins is consistent with the working hypothesis that vitamins (B12, B1, B7) are biosynthesized by microorganisms in the anaerobic sediment environment and delivered to Daphnia through the food web. However, direct analytical verification of in-system vitamin biosynthesis was not performed in this study and remains a key task for future work. As a complement and extension of existing methods, the DMC presents a new approach to constructing an ecosystem model intended to incorporate the biosynthesis and cycling of nutrients from inorganic substances.