Evidence for membrane permeability as the mechanism for a salinity effect in lipid D/H ratios.

A positive correlation between lipid δD values and growth medium salinity has been recently reported, both for pure cultures of microalgae and for cyanobacterial lipids from lacustrine sediments (Shouten 2006, Sachse 2008). This relationship could be used to develop a lipid-based paleosalinity proxy if it is a widespread and consistent phenomenon in microbes. However, the biochemical basis for such a correlation is not yet understood. To help understand the mechanism leading to this effect, we measured δD values of extractable lipids, culture medium, and intracellular water in cultures of the halotolerant cyanobacterium Cyanothece PCC 7418, spanning a salinity range from 2.5% to 10%. Lipids were extracted from cells harvested in mid-log growth phase, and δD values of alkanes and fatty acid methyl esters (FAMEs) were measured by GC/IRMS. All lipids showed an average increase in δD value of approximately 40‰ over the salinity range tested, or 0.53‰/PSU. This value is slightly below, but consistent with the correlation previously observed (0.8 to 3.0‰/PSU). Intracellular water from the same cultures was collected via filtration and vacuum extraction of cell cakes, and its D/H ratio measured by absorption spectroscopy. Preliminary data for intracellular water also indicate a D enrichment at higher salinity, with an increase in δD of 0.33 ± 0.08‰/PSU. This result suggests that much of the change in δD of lipids can be attributed to corresponding changes in intracellular water. Our experimental data thus support the hypothesis of Sachse and Sachs (2008), that the biochemical mechanism leading to the observed salinity effect is one of decreased membrane permeability at higher salinity, which results in larger fractionations of intracellular water by metabolic processes.