A VAST literature on cobalamines in natural waters has accumulated since bioassays were developed1–4. About 70 per cent of all species of freshwater and marine planktonic algae require some form of vitamin B 12 more often than any other metabolite, a factor which has led to the recognition of the potential influence of this vitamin on the composition of the phytoplankton community5,6. Several reports have substantiated the assumption that vitamin B 12 is important in this respect5–7. Important sources of vitamin B 12 are in situ bacterial activity, soil runoff and clay-absorbed B 12 (refs. 7–13).