Microbial succession is the predictable, directional change in the composition of microbial communities over time within a specific habitat. This process is a fundamental concept in microbial ecology, describing how populations of bacteria, archaea, fungi, and viruses replace one another in an ordered sequence. The shifts in species are driven by the microbes’ interactions with each other and their modifications of the environment. Understanding this dynamic process is important for predicting how microbial ecosystems, from soil to the human body, respond to disturbances and environmental changes.
Defining the Ecological Process
The timeline of microbial succession is defined by distinct stages of community development. It begins with the arrival of the pioneer species, which are the initial colonizers of a new or disturbed environment. These microbes are characterized by rapid growth rates and effective dispersal, adapted to thrive in harsh, resource-poor conditions. Examples include colonizers on a fresh volcanic rock or a recently sterilized industrial tank.
As the pioneer community grows, it alters the physical and chemical properties of the habitat, creating conditions less suitable for themselves but more favorable for subsequent groups. These transitional communities are known as seral stages, where multiple intermediate microbial communities sequentially replace one another. Each seral community shifts the environment further, setting the stage for the next group until the process culminates in the climax community.
The climax community represents a stable and diverse microbial assembly in equilibrium with the prevailing environmental conditions. This final state is characterized by a complex structure, high species diversity, and more intricate food webs compared to pioneer communities. The progression is self-regulating, driven by the microbes as they consume resources and excrete metabolic byproducts that modify their surroundings.
The Mechanisms Driving Community Change
The replacement of one microbial community by the next is governed by three primary ecological mechanisms. The first is facilitation, where early colonizers make the environment more suitable for later species. For example, initial microbes may excrete organic acids that break down complex substrates, making simpler compounds available, or they might fix atmospheric nitrogen, increasing nutrient availability.
The second mechanism is inhibition, where an existing microbial species prevents the establishment or growth of others. This occurs through the release of antimicrobial compounds or by rapidly monopolizing a limiting resource. The third mechanism is tolerance, where the sequence of change is determined by the microbes’ life-history traits and their ability to compete for resources. In this model, early colonizers do not significantly help or hinder later species; success depends on survival in the increasingly crowded environment. A single successional event often involves a complex interplay of all three mechanisms, with facilitation dominating early stages and competition-based mechanisms becoming more significant as the community matures.
Classifying Successional Pathways
Microbial succession is broadly categorized into two types based on the initial state of the environment. Primary succession begins in an environment completely devoid of life and lacking pre-existing organic matter or soil. This occurs when a new, sterile substrate becomes available for colonization, such as a fresh volcanic lava flow or newly exposed glacial sediment.
The process of primary succession is slow because pioneer microbes must first break down the bare substrate to generate the initial organic material necessary for later communities. Secondary succession, by contrast, occurs where a pre-existing community has been disturbed, but structural components like soil or organic matter remain intact. Examples include the recolonization of soil after a forest fire or the recovery of the human gut microbiome following antibiotic treatment. Secondary succession is a much faster process than primary succession because the environment already contains nutrients and sometimes dormant microbial spores or cells that allow for rapid regrowth.
Real-World Applications and Examples
The principles of microbial succession are evident in numerous natural and artificial systems. In the human body, gut microbiome development in infants follows a clear successional pattern, beginning with simple communities suited to the nutrient-rich, low-oxygen environment of the newborn. As the infant’s diet diversifies and oxygen levels change, these initial communities are gradually replaced by a more complex, adult-like climax community adapted to process a wider range of food sources.
Decomposition of organic matter provides a distinct example of microbial succession, particularly in the forensic study of a deceased body, known as the thanatomicrobiome. Immediately after death, the internal environment shifts from aerobic to anaerobic, and the body’s own microbes proliferate and begin to break down tissues. This initial community is then sequentially replaced by different groups of decomposers adapted to the changing chemical environment, such as increasing acidity and gas accumulation.
Industrial processes also leverage this ecological concept to achieve specific outcomes. In wastewater treatment, engineers rely on a predictable microbial succession to purify water. Different microbial communities sequentially form on biofilms to break down organic pollutants, followed by those that consume nitrogen and phosphorus compounds. By controlling factors like oxygen levels and nutrient input, operators guide the microbial community through its seral stages to reach the desired climax community, which is the most efficient for removing contaminants.