Abstract:

The nitrogen budget in the rotifer Brachionus rotundiformis was measured by the stable-isotope technique. The budget was estimated using the difference in the turnover time between egestion and excretion. The rotifer was fed on the algae Nannochloropsis which was labeled with N-15 as a tracer. The turnover time of egestion and excretion were 20 min and 2.5 hours, respectively. Where 77% of the ingested nitrogen was egested, and of the assimilated 23%, 18% were devoted to growth and 5% to excretion. As for the unassimilated nitrogen egested as faeces, it recycled to the rotifer through bacteriovory. When the algae provided as food were almost fully consumed, bacteriovory became dominant. The threshold occurred when the concentration of algae in the culture was between 1.5 and 0.5 million cells of Nannochloropsis per mi. In a chemostat operated with un-limited food condition, bacterial nitrogen corresponding to 20% of algal feeding was consumed by the rotifer. In a semi- continuous mass culture where food condition was limited, bacteriovory was more effective in supporting the rotifer reproduction. It contributed to the extremely high nitrogen recovery from the provided foods (algae and oil-yeast) to the harvested rotifers. The rapid and large nitrogen outflow from rotifers accelerated the propagation of edible bacteria and can explain the strange paradox observed in the culture; daily supply of foods did not cover the sum of growth and excretion. It is not too exaggerated to state that the rotifer mass culture is supported by bacteria. The future strategy for maintenance of mass cultures should consider this aspect.
(Univ. Tokyo, Fisheries Lab., 2971-4 Maisaka, Shizuoka 43102, Japan)
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