Energy flux controls tetraether lipid cyclization in Sulfolobus acidocaldarius

Abstract

12 Significance Statement 13Microbial lipid membranes protect and isolate a cell from its environment while regulating 14 the flow of energy and nutrients to metabolic reaction centers within. We demonstrate that 15 membrane lipids change as a function of energy flux using a well-studied archaeon that 16 thrives in acidic hot springs and observe an increase in membrane packing as energy 17 becomes more limited. These observations are consistent with chemostat experiments 18 utilizing a low temperature, neutral pH, marine archaeon. This strategy appears to regulate 19 membrane homeostasis is common across GDGT-producing lineages, demonstrating that 20 diverse taxa adjust membrane composition in response to chronic energy stress. 21 Summary 22 23 Microorganisms regulate the composition of their membranes in response to environmental 24 cues. Many archaea maintain the fluidity and permeability of their membranes by adjusting 25 the number of cyclic moieties within the cores of their glycerol dibiphytanyl glycerol 26 tetraether (GDGT) lipids. Cyclized GDGTs increase membrane packing and stability, which 27 has been shown to help cells survive shifts in temperature and pH. However, the extent of 28 this cyclization also varies with growth phase and electron acceptor or donor limitation. 29 These observations indicate a relationship between energy metabolism and membrane 30 composition. Here we show that the average degree of GDGT cyclization increases with 31 Zhou et al. 2019 20190817 2 doubling time in continuous cultures of the thermoacidophile Sulfolobus acidocaldarius 32 (DSM 639). This is consistent with the behavior of a mesoneutrophile, Nitrosopumilus 33 maritimus SCM1. Together, these results demonstrate that archaeal GDGT distributions can 34 shift in response to electron donor flux and energy availability, independent of pH or 35 temperature. Paleoenvironmental reconstructions based on GDGTs thus capture the energy 36 available to microbes, which encompasses fluctuations in temperature and pH, as well as 37 electron donor and acceptor availability. The ability of Archaea to adjust membrane 38 composition and packing may be an important strategy that enables survival during episodes 39 of energy stress. 40 41

Multiple environmental parameters impact core lipid cyclization in Sulfolobus acidocaldarius

“…In gas fed-batch experiments, separate clusters can be seen based on both growth phase and oxygen concentration (Figure 6A). In chemostat experiments from Zhou et al (2019) clusters seem to arise based on doubling times (Figure 6B). Stress values for all conditions are plotted on each NMDS plot, and were all below 0.05, indicating a good fit of the original data to the two-dimensional ordination.…”

“…Oxygen flux caused variation in all observed GDGTs, except GDGT-0, -3, -3 isomer, and -4. We also reanalyzed data from a recent chemostat experiment we performed with DSM639 (13), finding that growth rate significantly correlated with the relative abundance of all GDGTs, except GDGT-3 isomer. Considering in aggregate our atmospheric batch, gas-fed batch, and chemostat experiments with DSM639, our results demonstrate that each physical variable can independently influence different subsets of GDGT core lipids.…”

“…In gas-fed batch experiments (D), lipid samples taken for GDGT analysis during each of the two sampled growth phases are identified by color, and 0.2* represents the serially transferred 0.2% O 2 experiment. Data from constant-rate experiments performed with the same strain (E) are provided for comparison (data from Zhou et al 2019). Linear regressions for each experiment have grey shaded regions showing 95% confidence interval (A: slope = −0.10 ± 0.01, p < 0.0001, R 2 =0.64; B: slope = −0.147 ± 0.03, p < 0.001, R 2 =0.62; C: slope = −0.011 ± 0.0017, p < 0.0001, R 2 =0.53; D: slope = 0.120 ± 0.0015, p < 0.0001, R 2 =0.68; E: slope = 0.031 ± 0.0031, p < 0.0001, R 2 =0.82).…”

“…Biomass samples were pelleted by centrifugation, frozen at −80°C, and freeze-dried prior to lipid extraction. Core lipid fractions were obtained and analyzed as previously detailed (13, 21). Specific growth rates were calculated by a modified implementation of the algorithm in Hall et al (2014).…”

“…Rate calculation script is available on GitLab (see Supplement). GDGT abundances were measured by ultra-high-performance liquid chromatography (UHPLC) coupled to an Agilent 6410 triple-quadrupole mass spectrometer (MS) as previously described (13). Ring indices (RI) were calculated using Eq 1.…”