Do algae use arsenic in membrane lipids?

Arsenic, a famous poison, has also been ascribed beneficial properties when administered in small doses. This has led to the presumption that arsenic might be useful, or even essential, to organisms at trace levels. Results from two recent independent lines of research have suggested experiments to test this hypothesis. The first, performed by scientists at the Woods Hole Oceanographic Institute, showed that, in oceanic waters with low phosphate levels, algae use nitrogen and sulfur in place of phosphorus for membrane lipid synthesis as a way of conserving their essential phosphorus reserves. The second study, performed by our group in Graz, identified a novel group of arsenic-containing lipids (arsenolipids) from marine samples. We propose to test the hypothesis that in phosphate- limiting waters, algae can utilise arsenic, ever present in seawater as arsenate, to biosynthesise arsenolipids for use in membranes. The study will be undertaken in collaboration with the Woods Hole oceanographic group and an Australian research group which will perform complementary laboratory experiments with axenic algal cultures exposed to arsenic at varying arsenate/phosphate ratios. If the hypothesis is correct, the study will provide the first evidence of arsenic serving a valuable biological role under common environmental conditions, and open a new area of arsenic biological chemistry.

The project investigated the biological significance of arsenolipids, especially in connection with phosphate and phospholipids, and possible involvement in membrane chemistry. Our laboratory and field studies with unicellular algae and varying arsenate/phosphate regimes have revealed a fascinating connection between the arsenate/phosphate ratio and the type of arsenolipid produced by the algae. In addition, the research has shown that algae can produce a totally novel type of arsenolipid, which has been identified based on mass spectrometry and chemical transformations. During the course of the project 20 new arsenolipids were identified most of which belonged to three new groups of arsenolipids. The most recently identified group comprises phosphatidylcholines and phosphatidylethanolamines, which are compounds integral to membrane structure. The last named compounds were found in fish roe, chosen because they have high content of membranes. The discovery of these compounds supports their involvement in membrane chemistry. Additionally, arsenic lipids bound into an arsenosugar were found in marine algae thereby providing the link between the major arsenic compounds in algae (arsenosugars) and the lipids. The arsenosugars are considered to be key compounds in the transformation of toxic arsenate into the usually harmless organoarsenic compounds found in marine organisms. We also developed an analytical method for quantifying some of the arsenolipids in a certified reference material. Such measurements have become increasingly important because of human health issues related to arsenic in seafood. Another highlight of the study was the development of highly sensitive methods for measuring organoarsenic compounds in seawater. Future application of the methods will allow exploration of the role of arsenic and of arsenolipids in algal ecology.