Theoretical Considerations Concerning Soil Microbial Growth

Microbial growth is a fundamental aspect of microbial life history, underpinning essential ecosystem functions and driving all biogeochemical cycles. While culture-based studies have provided valuable insights into microbial growth, they often fail to capture how microbes grow under natural conditions, which include complex interactions with other organisms and their physical and chemical environments.

Currently, microbial growth is typically defined as the ability of individual cells to replicate. Such a definition, however, overlooks the diverse strategies to survive and thrive in dynamic environments. These strategies reflect how microorganisms allocate carbon they take up to various pathways, including cellular replication, synthesis of storage compounds (e.g., triacylglycerides and polyhydroxyalkanoates), accumulation of osmolytes, and exudation of substances such as extracellular polymeric substances, extracellular enzymes and metabolites like short-chain fatty acids. These strategies are often accompanied by physiological shifts, such as transitioning between active and dormant metabolic states.

Despite the central importance of microbial growth, its in situ measurement remains a significant challenge. This limitation hinders our understanding of the ecological functions of soil microbiomes and our ability to accurately predict carbon use and cycling. Addressing this knowledge gap requires, a multi-faceted approach including the following key considerations:

• Expanding definitions of microbial growth: Microbial growth encompasses more than cell division and DNA replication, particularly under stress conditions, such as nutrient and water scarcity. It includes the synthesis of storage compounds, osmolytes, and extracellular material. A more flexible definition, along with a delineation of growth and activity, is urgently needed.
• Understanding and benchmarking growth methods: To isolate patterns in growth across microbial ecosystems, it is crucial to understand what different growth methods (that target various biomolecules, such as nucleic acids, proteins, and lipids) actually quantify, and how they relate to one another. Emphasis should be placed on substrate-independent methods.
• Developing and improving models: Models should prioritize the exploration of microbial growth strategies in dynamic, non-steady steady-state environments and include robust experimental validation.

By addressing these key considerations, we hope to be able to deepen our understanding of microbial growth in natural systems, enhance ecological modeling, and better predict the role of soil microbiomes in carbon cycling.