Bacterivory, the consumption of bacteria as a primary food source, is a fundamental trophic interaction underpinning nearly every natural environment. Bacteria are ubiquitous, forming massive populations in soil and aquatic ecosystems, providing an enormous and constant food supply. A single gram of soil, for example, can contain hundreds of millions of bacterial cells. The global ocean harbors an estimated \(3 times 10^{26}\) bacteria, demonstrating the sheer volume of this microscopic biomass available for consumption by diverse animal groups.
Tiny Animals That Actively Consume Bacteria
The smallest animals engage in direct bacterivory by actively hunting and ingesting individual bacterial cells. Free-living protozoa, such as amoebas, flagellates, and ciliates, are dominant bacterivores in aquatic and soil environments. They often engulf bacteria through phagocytosis, selectively grazing on larger, rapidly dividing cells. This process effectively crops the bacterial population and transfers organic matter up the food chain.
Rotifers use a specialized structure called the corona—a crown of beating cilia—to create a powerful current that sweeps bacteria and other particles into their mouths. The food is then ground by a muscular pharynx known as the mastax before digestion. These microscopic freshwater animals consume bacteria and detritus, acting as a crucial intermediate link between the microbial world and larger invertebrates like copepods and insect larvae.
Nematodes, particularly free-living species found in the soil, are also voracious grazers. They use a muscular pharynx to suck in bacteria, directly linking the rich soil microbial community to the rest of the terrestrial food web.
Aquatic Animals That Filter Bacteria
Larger, often sessile, aquatic animals process massive volumes of water to capture microscopic prey through filter feeding. This strategy is exemplified by phyla like sponges and bivalves. Sponges are efficient filter feeders, using specialized collar cells (choanocytes) to create water currents that draw water through their porous bodies and capture bacteria as small as \(0.1\) micrometers.
Bivalves, including clams, oysters, and mussels, are prodigious bacterivores, utilizing their gills as sophisticated sieves. They draw water in through an incurrent siphon and pass it over ciliated gills, which trap bacteria and particles before moving them to the mouth. An individual oyster can filter over a gallon of water per hour, demonstrating the enormous scale of bacterial removal from the water column. Certain marine worms, such as tube-dwelling polychaetes, also employ filtration, extending specialized appendages to passively capture suspended bacteria and other organic matter.
Animals That Rely on Symbiotic Bacteria for Sustenance
A specialized form of bacterivory involves animals that host bacteria internally in a mutualistic relationship rather than consuming them externally. This is most often seen in the deep ocean, where life is supported by chemosynthesis instead of sunlight.
Giant tube worms (Riftia pachyptila) found at hydrothermal vents possess no mouth or digestive tract. They rely entirely on billions of symbiotic bacteria housed within a specialized organ called the trophosome. These bacteria use chemicals like hydrogen sulfide, abundant in the vent fluid, to synthesize organic compounds, providing the worm with all its nutritional needs.
Deep-sea mussels (Bathymodiolus) and various clams similarly harbor chemosynthetic bacteria in their gill tissues, converting methane or sulfide into food for their hosts. On land, specialized terrestrial organisms, such as the “zombie worm” (Osedax), feed on the bones of dead whales. They use internal bacteria to break down complex fats and collagens, illustrating the diversity of environments where this symbiotic reliance has evolved.
The Global Importance of Bacterivory
The widespread consumption of bacteria is a major driver of global ecological processes, extending far beyond the individual animal’s nutrition. Bacterivory plays a fundamental role in nutrient cycling by remineralizing organic matter trapped within bacterial biomass. When grazers consume bacteria, they excrete excess nutrients, such as nitrogen and phosphorus, back into the environment. These dissolved nutrients become readily available for primary producers like algae and plants.
This process also facilitates the transfer of energy from the microscopic scale up to higher trophic levels, known as the “microbial loop.” Bacteria consume dissolved organic carbon, packaging this energy into their cell bodies, which tiny grazers then consume. These grazers are subsequently eaten by larger invertebrates and fish. By consuming bacteria, animals regulate bacterial populations, controlling their abundance and community composition in both water and soil.