Intracellular carbon storage by microorganisms is an overlooked pathway of biomass growth

The concept of microbial biomass growth is central to microbial carbon (C) cycling and ecosystem nutrient turnover. Growth is usually assumed to occur by cellular replication, despite microorganisms’ capacity to increase biomass by synthesizing storage compounds. Here we examined whether C storage in triacylglycerides (TAGs) and polyhydroxybutyrate (PHB) contribute significantly to microbial biomass growth, under contrasting conditions of C availability and complementary nutrient supply. Together these compounds accounted for 19.1 ± 1.7% to 46.4 ± 8.0% of extractable soil microbial biomass, and revealed up to 279 ± 72% more biomass growth than observed by a DNA-based method alone. Even under C limitation, storage represented an additional 16 – 96% incorporation of added C into microbial biomass. These findings encourage greater recognition of storage synthesis and degradation as key pathways of biomass change and as mechanisms underlying resistance and resilience of microbial communities.