Bacteria are an incredibly diverse and abundant group of microorganisms that underpin many of Earth’s ecosystems. Far from being only harmful, bacteria serve as a foundational food source for countless animals across various environments. Their vast numbers and rapid reproduction rates make them a readily available and energy-rich component at the base of numerous food webs. Many animals have evolved specialized mechanisms to consume these microscopic organisms, transferring energy through ecological systems.
Microscopic Grazers and Predators
The smallest animals directly consume bacteria, playing a significant role in microbial food webs. Single-celled organisms, known as protozoa, are prominent grazers of bacteria. Amoebas, for instance, ingest bacteria by extending temporary projections called pseudopods to encircle and engulf their prey through a process called phagocytosis. Ciliates, another group of protozoa, use hair-like structures called cilia to sweep bacteria and other food particles into a mouth-like opening, forming food vacuoles for digestion. Both amoebas and ciliates are important in soil and aquatic ecosystems, where they regulate bacterial populations and contribute to nutrient cycling.
Very small multicellular creatures also feed on bacteria. Rotifers, tiny aquatic invertebrates, primarily consume organic detritus, dead bacteria, algae, and protozoans. Nematodes, or roundworms, are also significant consumers of bacteria in soil and sediment ecosystems, with some species capable of eating up to 5,000 bacteria per minute. Even tardigrades, often called water bears, include bacteria in their varied diet. These microscopic consumers form a crucial link in the “microbial loop,” returning dissolved organic carbon to higher trophic levels by incorporating it into their biomass.
Aquatic Filter Feeders
A wide array of aquatic animals, ranging from invertebrates to large vertebrates, obtain nutrition by filtering bacteria directly from the water. Sponges, for example, draw water through their porous bodies, trapping bacteria and other microorganisms. Bivalve mollusks like clams, mussels, and oysters are highly efficient filter feeders, consuming vast amounts of phytoplankton, bacteria, and other particles from the water. An individual mussel or oyster can filter several liters of water per hour, contributing significantly to water clarity and ecosystem health.
Some fish species also exhibit filter-feeding behaviors that include bacteria in their diet. The American paddlefish, for instance, filters plankton from the water, which can include bacterial aggregates. Baleen whales, while primarily known for consuming krill, also indirectly benefit from the bacterial food web as krill themselves feed on bacteria. The ecological role of these filter feeders extends beyond their own sustenance, as they play a part in cleaning water bodies and transferring energy from the microbial level up the food chain. They help regulate nutrient cycles and can even enhance the activity of beneficial bacteria that convert nitrogen gas, permanently removing it from the water.
Internal Symbiotic Relationships
Many animals rely on bacteria that live inside their bodies for essential nutrition, forming intricate symbiotic relationships. Ruminant animals, such as cows, sheep, and goats, possess a specialized stomach chamber called the rumen, which harbors a dense community of bacteria and other microorganisms. These ruminal microbes are uniquely capable of breaking down cellulose, a complex carbohydrate found in plant cell walls that the animal’s own digestive enzymes cannot process. The bacteria ferment the cellulose, releasing nutrients that the ruminant can then absorb, enabling these animals to thrive on a diet of grass and other fibrous plant materials.
Termites also depend on internal bacteria, alongside protozoa, to digest wood. These insects consume wood, but it is their gut symbionts that possess the necessary enzymes to break down cellulose and other wood components into usable nutrients for the termite. Without these microbial partners, termites would be unable to extract energy from their woody diet.
In deep-sea environments, where sunlight does not penetrate, some organisms form remarkable symbioses with chemosynthetic bacteria. Tube worms, certain bivalves, and yeti crabs living near hydrothermal vents and cold seeps host bacteria that convert inorganic compounds like hydrogen sulfide or methane into organic compounds. These chemosynthetic bacteria provide their animal hosts with a primary food source, forming the base of these unique ecosystems. This internal reliance on bacteria highlights their pervasive influence on animal nutrition, extending beyond direct consumption to encompass metabolic partnerships that unlock otherwise inaccessible food sources.