The microbial loop is a concept in aquatic ecology that describes a pathway for the flow of carbon and energy through microscopic organisms in the water column. It acts as a critical intermediary step, capturing organic matter that would otherwise be lost from the main food chain and reintroducing it into the ecosystem. This process significantly increases the overall efficiency of the aquatic food web, particularly in nutrient-limited regions such as the open ocean (oligotrophic waters). By recycling organic material and regenerating nutrients, the loop supports the productivity of the entire aquatic system.
Defining the Loop’s Key Components
The engine of the microbial loop is Dissolved Organic Matter (DOM), a complex mixture of organic molecules released into the water, which serves as the primary energy source. DOM is created through various processes, including the leakage of fixed carbon from phytoplankton cells, the excretion of waste products by zooplankton, and the sudden bursting of cells due to senescence or viral activity. This DOM is generally too small or complex for larger organisms to consume directly.
Heterotrophic bacteria are the main biological components that consume this DOM, acting as microscopic decomposers that convert the dissolved material into their own particulate biomass. These bacteria are then controlled by minute predators, primarily pico- and nano-flagellates and ciliates, which graze on them.
An important parallel mechanism that influences the loop is the “viral shunt,” which involves the action of viruses lysing, or rupturing, bacterial cells. Viral lysis causes the cell contents to spill out, immediately returning organic carbon and nutrients back into the DOM pool. This process effectively short-circuits the flow of energy to higher trophic levels, keeping the material cycling rapidly at the microbial level.
The Process of Carbon Recycling
The recycling of carbon begins when heterotrophic bacteria take up the DOM from the surrounding water, converting the dissolved molecules into the organic matter that makes up their own bodies. This initial step transforms dissolved carbon, which is inaccessible to most larger organisms, into microscopic particulate biomass. This bacterial biomass is then consumed by the microbial grazers, like the heterotrophic nanoflagellates.
This predator-prey dynamic is crucial because the grazers package the scattered, tiny bacterial biomass into larger, more manageable units. The microbial grazers are generally microzooplankton, which are large enough to be consumed by the next step up the food chain. The process recovers energy that would otherwise be lost if the DOM simply sank or was respired by the bacteria as carbon dioxide.
A large fraction of the organic matter synthesized by phytoplankton becomes DOM, making the bacterial uptake step a significant pathway for carbon flow in the ocean. While a portion of the DOM assimilated by bacteria is respired and released as carbon dioxide, the remaining biomass is transferred up the food chain by the grazers. This conversion from dissolved to particulate form makes the energy available to the classic food web.
The Critical Role in Nutrient Regeneration
Beyond carbon, the microbial loop is responsible for the regeneration of inorganic nutrients, which sustains primary production in the surface ocean. When heterotrophic bacteria consume DOM, they absorb not only carbon but also other elements like nitrogen and phosphorus to fuel their growth. The bacteria often take up more carbon than they need relative to the available nitrogen and phosphorus in the dissolved material.
The subsequent consumption of these bacteria by protists drives the process of remineralization. As the protists graze and metabolize the bacterial biomass, they excrete excess inorganic nutrients, such as ammonium and phosphate, back into the water column. This release of inorganic compounds is a rapid form of nutrient regeneration, making these elements immediately available for phytoplankton to use for photosynthesis.
This continuous recycling is particularly significant in warm, stratified surface waters where nutrients are frequently scarce. The regenerated production, which is the primary component of total production in nutrient-poor ecosystems, relies heavily on the activity of the microbial loop. By efficiently recycling these elements, the loop supports the aquatic ecosystem, allowing phytoplankton to continue growing even when external nutrient supplies are low.
Connecting the Microbial Loop to the Global Food Web
The microbial loop functions as a foundational base layer that supports the classical food web of larger organisms. The flow of energy is transferred from the smallest, dissolved pool up to organisms that can be consumed by macroscopic life. This happens when the microbial grazers, which are generally microzooplankton like ciliates and nanoflagellates, are themselves consumed.
These microzooplankton are large enough to be a suitable food source for traditional meso-zooplankton, such as copepods. Copepods are small crustaceans that are a major food source for small fish, effectively linking the microscopic world to the macroscopic. Through this connection, the microbial loop acts as an energy conduit, transferring carbon and nutrients that originated as DOM up to the organisms that feed fish and other higher consumers.
The importance of this link is underscored by the fact that the entire microbial community often outweighs the combined mass of all multicellular marine organisms in the ecosystem. The microbial loop ensures that a significant portion of the energy fixed by primary producers is efficiently channeled toward supporting the entire aquatic food chain.