Changes in intact membrane lipid content of archaeal cells as an indication of metabolic status

Archaea build their cellular membrane with glycerol dialkyl glycerol tetraether (GDGT) core lipids, bound to polar head groups to form intact GDGTs. These lipids are commonly used as indicators of archaeal biomass in the environment. Here, we measured both intact and core GDGTs and archaeal 16S rRNA gene copies (a proxy for cell abundance) in order to determine cellular GDGT content in a coastal fjord at different locations, seasons and depths. We found that the abundance of both intact and core GDGTs relative to 16S rRNA gene copy numbers varied by two orders of magnitude, were generally higher in surface than deeper water and were elevated during periods associated with seasonal phytoplankton blooms. Core and intact GDGTs followed similar trends, but the ratio of core GDGT/16S rRNA gene copy number reached a maximum in October, following a peak in the ratio of intact GDGT/16S rRNA gene copy number that occurred in August. Our data suggest that spatial and temporal variability in these ratios must be considered when attempting to quantify cell numbers in the environment using either GDGTs or 16S rRNA genes. We used optimized extraction methods for both GDGTs and DNA. However, there is still potential for bias in qPCR methodology that could influence the GDGT/16S rRNA gene copy ratio. The apparent variability in cellular membrane lipid concentration could serve to evaluate the status of archaeal populations as it relates to: (i) varying cell size resulting from changes in either community structure or nutritional status, (ii) population turnover times and (iii) changing environmental factors that affect the lifetime of intact GDGTs after cell death.