The nutrient requirements of many algae are well documented, with most algae being found to need only one or two vitamins. In the case of their predators, only a handful of microinvertebrates have had their diets and even fewer their vitamin requirements denned. Thus, any progress in this field together with the mode of nutrient uptake, must lead to a better understanding of the component-transfer in the food chain.

Brachionus plicatilis Müller has been reported to require vitamin B12 (Droop & Scott, 1978). The work described in this paper sought first, to confirm this and then to establish the mode of uptake. The alga Brachiomonas submarina var. pulsifera Droop was used to investigate the first of these questions as the writer had previous experience with it in steady-state energetics work. The alga Dunaliella SMBA Strain 246 was used to examine the mode of uptake by the rotifer since this had also been found to be a good food organism. The possibilities considered were uptake: (a) via the algal cell; (b) directly from solution; (c) a combination of both. Mode of vitamin B12 uptake is of particular interest as it may be representative of the pathway of many of the rotifer's water soluble micronutrients.

Before considering the influence of the vitamin on Brachionus investigations were made into the relationship between the vitamin and the two food-algae. It has already been established that Brachiomonas has a definite requirement (Droop, 1961) and Dunaliella no requirement for vitamin B12 (Turner, 1979). It was of interest to see if they took-up the vitamin in the manner shown by Monochrysts, a B12 requirer, and Phaeodactylum a non-requirer (Droop, 1968).