Vorticella are microscopic, bell-shaped ciliates that feed primarily on bacteria, including cyanobacteria and other picoplankton. They are indiscriminate suspension feeders, meaning they don’t pick and choose specific prey. Instead, they sweep in whatever bacteria-sized particles the surrounding water delivers, filtering mainly by size rather than type.
What Vorticella Eat
The bulk of Vorticella’s diet consists of free-floating bacteria, typically particles in the range of 1 to 3 micrometers long. That includes common waterborne bacteria, cyanobacteria (photosynthetic microbes sometimes called blue-green algae), and tiny bits of organic debris suspended in the water. In laboratory experiments, Vorticella readily consumed multiple strains of picocyanobacteria regardless of their pigment type or genetic lineage, confirming that these organisms aren’t picky eaters. If a particle is small enough to fit in, Vorticella will eat it.
In wastewater treatment plants, Vorticella has been observed preying directly on E. coli and other suspended bacteria. It also ingests tiny particles of biomass, which helps reduce suspended solids in the water. This makes Vorticella one of the most important protozoa in both natural water purification and engineered sewage treatment systems.
How Vorticella Captures Food
Vorticella lives in two forms: a free-swimming stage and a stalked, stationary stage. Most of its feeding happens while it’s anchored to a surface by its stalk, with its bell-shaped body (called a zooid) open to the water. Two rings of tiny hair-like structures called cilia surround its mouth opening. The inner ring of cilia beats in coordinated waves, generating a miniature vortex that pulls water toward the organism at speeds of roughly 0.4 to 2.5 millimeters per second. The outer ring acts as a filter, sorting particles by size so that bacteria-sized food gets directed inward while larger debris is deflected away.
This feeding vortex is remarkably efficient for such a small organism. Think of it like a tiny whirlpool funnel that continuously draws in fresh water, strips out the bacteria, and ejects the filtered water. The process runs nonstop as long as the cilia are beating.
How Much a Single Vorticella Can Eat
A single small Vorticella can consume nearly 3,000 bacteria per hour, one of the highest grazing rates recorded among sessile (attached) ciliates. That rate depends heavily on how many bacteria are available in the surrounding water. When bacterial concentrations are high, Vorticella eats more; when concentrations drop, intake slows accordingly.
Temperature also plays a significant role. One species, Vorticella natans, showed roughly three times higher ingestion rates at 10 to 20°C compared to 5°C when food was abundant. At moderate food levels, though, temperature had less impact, suggesting that food availability is the stronger driver of feeding speed. Below about 10°C, feeding slows considerably regardless of how many bacteria are present.
How Vorticella Digests Its Food
Once a bacterium is swept into Vorticella’s mouth opening, it gets enclosed in a small bubble of fluid inside the cell called a food vacuole. This vacuole acts like a temporary stomach. When it first forms, the fluid inside is close to neutral (pH 6.6 to 7.0), and any live prey trapped inside is still moving. Within five to ten minutes, the vacuole acidifies sharply to a pH of about 4.3, which is roughly as acidic as stomach acid in humans. This acid environment kills the prey and breaks it down.
As digestion progresses and nutrients are absorbed through the vacuole wall into the rest of the cell, the pH gradually climbs back up. Old vacuoles containing leftover indigestible material settle around pH 5.4 to 5.6 before being expelled from the cell. The entire digestion cycle in ciliates like Vorticella takes only a few minutes, far faster than digestion in multicellular organisms.
Why Vorticella’s Feeding Matters
Vorticella’s appetite for bacteria has real consequences for water quality. In activated sludge systems used at wastewater treatment plants, Vorticella is often the dominant sessile ciliate, sometimes accounting for nearly 100% of attached ciliates in the sludge. By consuming suspended bacteria and fine particulate matter, these organisms help clarify the water and reduce pathogen loads. Studies on aerobic granular sludge found that protozoan predation, including by Vorticella, was the primary pathway for removing E. coli, contributing an additional 0.5 to 2.5 log reduction beyond what non-predatory biological and chemical processes could achieve. In practical terms, that means predation by Vorticella and similar ciliates can remove 90% to over 99% of certain bacteria from treated water.
In natural freshwater environments, Vorticella plays a similar role. By grazing on bacteria and picoplankton in lakes, ponds, and streams, these ciliates help regulate microbial populations and recycle nutrients through the food web. Their feeding keeps bacterial blooms in check and converts bacterial biomass into a form that larger organisms, like small crustaceans, can consume when they eat Vorticella in turn.