Environmental Context. Seaweed is enjoying a revival in farming practice, in particular by organic farmers. However, seaweed accumulates arsenic, and these arsenic compounds can enter the food chain. It is known that the arsenic is present mainly as arsenosugars, but the metabolism of these compounds by ruminants needs clarification.

Abstract. Here we describe a feeding trial with Blackface sheep conducted on an organic farm in Kintyre (Scotland), which aims to prove that the metabolism of arsenic, acquired from the consumption of seaweed, is not unique to the North Ronaldsay sheep, which are adapted to a seaweed diet. Results show that the trial sheep supplemented their diet with, on average, 20 ± 9% Laminaria digitata when given the choice. The daily arsenic intake varied greatly from sheep to sheep but on average, the sheep consumed 65 μg kg^-1 b.w. Total arsenic concentrations in urine, as measured by inductively coupled plasma-mass spectrometry (ICP-MS) (m/z 75) also show significant differences between the trial and control group (P < 0.0001). HPLC coupled with ICP-MS in parallel with electrospray ionization-mass spectrometry (ES-MS) for detection was used for the identification of arsenic metabolites in urine samples. Dimethylarsinic acid (DMA^V) is the main metabolite in the control group as well as in the trial group. In addition, arsenic metabolites previously only found in the urine of North Ronaldsay sheep were successfully identified in the urine of the trial group of the seaweed-eating Blackface sheep: dimethylarsinoyl acetic acid (DMAA) and its thio-analogue dimethylarsinothioyl acetic acid (DMAAS) as well as the monosulfide of DMA^V, DMAS. However, the poor chromatographic recovery indicates that the urine contains arsenic species, which do not elute under the conditions tested.