Occurrence and distribution of diverse populations of magnetic protists ina chemically stratified coastal salt pond

Chemical stratification occurs in the water columns and sediments of many a quatic habitats resulting in vertical chemical and redox gradients. Various types of microorganisms are often associated with specific depths and chem ical parameters in these situations. For example, magnetite-producing magne totactic bacteria are known to form horizontal ''plates'' of cells at the o xic/anoxic transition zone (OATZ) of such environments. Here, we report the presence of populations of diverse magnetic protists in a seasonally chemi cally stratified, coastal salt pond. The protistan types included several b iflagellates, a dinoflagellate, and a ciliate that were each associated wit h specific depths and thus, specific chemical, microbiological and redox co nditions in the water column. Most cells contained crystals of magnetite us ually arranged in chains similar to the magnetosomes in the magnetotactic b acteria. The origin of the particles is unknown. There was no evidence of t he presence of endosymbiotic magnetotactic bacteria within the protists nor were the protists observed to be engulfing magnetotactic bacteria despite the fact that high numbers of the latter were present at the same depths as the protists. However, this does not exclude ingestion of magnetotactic ba cteria as a source of the magnetic particles while others may biomineralize magnetite. Because protists play an important role in the availability of iron for phytoplankton in marine waters by ingesting and reducing colloidal iron particles, the protists described here could play a significant role in iron cycling in chemically stratified anoxic basins, either through endo genous production of magnetic iron oxide and/or iron sulfide particles, or by ingestion of magnetotactic bacteria and the subsequent reduction and dis solution of iron oxide and/or sulfide particles in their magnetosomes, (C) 2000 Elsevier Science B.V. All rights reserved.