Soil bacterial neutral lipid fatty acids: Markers for carbon storage or necromass?

Intracellular carbon storage is a common strategy of soil microbes to cope with resource fluctuations. While fungal neutral lipid fatty acids (NLFAs) are widely accepted indicators of carbon storage, bacterial NLFAs are more ambiguous: they have been interpreted either as storage compounds or as degradation products of phospholipids, suggesting necromass. These conflicting interpretations limit the use of bacterial NLFAs in understanding microbial physiology and carbon cycling in soils. In this perspective, we revisit the functional origin of bacterial NLFAs by synthesising findings from microbial culture studies, screening of soil bacterial genomes, and a ¹³C-labelling soil experiment. Our results suggest that many soil bacteria possess the genetic and physiological capacity to synthesise storage lipids, and actively allocate excess carbon into NLFAs under carbon-rich conditions—supporting their interpretation as storage rather than necromass markers. We also highlight assumptions about phospholipid degradation and note that its products are not necessarily recovered as NLFAs. We conclude that soil bacterial NLFAs are mainly derived from storage compounds, although contributions from degraded phospholipids and possibly other intracellular lipids need further validation. We propose targeted experiments to clarify their biochemical origin, and highlight bacterial NLFAs as underused but promising markers in soil science.