Toward conceptual clarity: PLFA in warmed soils

Studies assessing shifts in soil nutrient cycling with warming benefit from microbial data collection with clear ecological hypotheses. However, the fact that many organisms alter their lipids to adjust to temperature complicates interpretation of the widely-used method of phospholipid fatty acid (PLFA) analysis. Changes in PLFA profiles may reflect shifts in community composition (i.e. relative abundance of microbial groups) or physiological change (i.e. the same groups after membrane optimization) or both. The various approaches presently used to interpret results from warming studies expose a need for greater conceptual clarity. We compile the various data metrics commonly used for PLFA and discuss their interpretations in the context of temperature-based studies. We propose that certain lipid ratios are likely to at least partially reflect decreased unsaturation and other warming-induced physiological shifts. A change in community composition may underlie warming results for both whole-community ordination and certain indicator lipids, provided that the effect of lipids commonly re-synthesized to maintain membrane fluidity are considered. Overall, conclusions in warming studies would be strengthened by explicit consideration of temperature-dependent lipid effects in data analysis. Further research is needed toward substantiating the interpretation of PLFA-based temperature response data as indicating community compositional or physiological change.