What Are Virioplankton and Why Are They Important?

Aquatic environments, from the open ocean to freshwater lakes, are teeming with microscopic life known as plankton. While many are familiar with phytoplankton and zooplankton, a lesser-known but incredibly abundant group also exists: the virioplankton. These are the viruses of the aquatic world, and their numbers are staggering, often outnumbering bacteria by a significant margin. Although their presence was long suspected, only in recent decades have scientists begun to understand the influence these entities have on planetary ecosystems.

Defining Virioplankton

Virioplankton are the viral component of the plankton community, found in marine and freshwater systems. As obligate intracellular parasites, they can only replicate by infecting a living host cell, distinguishing them from photosynthetic phytoplankton and consumer zooplankton. Their basic structure consists of genetic material, either DNA or RNA, enclosed within a protein shell called a capsid.

A large portion of the virioplankton population is composed of bacteriophages, which are viruses that specifically infect bacteria. The abundance of virioplankton is immense, with estimates suggesting as many as 10^31 viral particles in the world’s oceans, making them the most numerous biological entities in aquatic environments. These viral populations are also dynamic, with their numbers fluctuating based on location, season, and the abundance of their hosts.

The Roles of Virioplankton in Aquatic Ecosystems

Virioplankton influence aquatic ecosystems primarily by controlling the populations of their microbial hosts. By infecting and killing bacteria and other microorganisms, viruses help maintain microbial diversity. This process, sometimes called “killing the winner,” prevents any single host species from becoming overly dominant and contributes to overall ecosystem stability.

Virioplankton are also central to nutrient cycling through a mechanism known as the “viral shunt.” When a virus infects and bursts a host cell (lysis), the cell’s organic contents are released into the water as dissolved and particulate organic matter. This material becomes a food source for other microorganisms, shunting organic matter away from larger organisms and back to the microbial level in biogeochemical cycles.

By infecting phytoplankton, virioplankton can also directly impact primary productivity. A widespread viral infection can terminate a phytoplankton bloom, altering the flow of energy through the ecosystem. The lysis of these photosynthetic organisms releases organic matter and nutrients that can stimulate the growth of other microbial communities.

Virioplankton-Host Dynamics

The interaction between a virioplankton particle and its host is a specific process that drives microbial evolution. The process begins when a virion attaches to a specific receptor on the surface of a host cell. Following attachment, the virus injects its genetic material into the host’s cytoplasm, leaving the capsid outside. Once inside, the viral genetic material commandeers the host cell’s machinery to replicate itself, leading to one of two main outcomes: the lytic or lysogenic cycle.

In the lytic cycle, the host cell is forced to produce thousands of new virions, culminating in the rupture (lysis) of the cell. This releases the newly formed viruses to infect neighboring cells and is a major source of microbial death in aquatic systems.

In the lysogenic cycle, the viral genome integrates itself into the host’s genome, where it is called a prophage. The prophage is replicated along with the host’s own DNA each time the cell divides, without causing immediate harm. Environmental cues, such as stress on the host, can trigger the prophage to exit the host genome and enter the lytic cycle.

Investigating the Invisible: How Scientists Study Virioplankton

Studying virioplankton presents challenges due to their extremely small size and immense genetic diversity. To overcome these issues, scientists rely on powerful magnification, enumeration, and molecular techniques.

• Transmission electron microscopy (TEM) allows for the detailed observation of viral morphology, revealing the shape and size of different virioplankton, which can offer clues about their identity.
• Epifluorescence microscopy involves staining the viral DNA with a fluorescent dye, causing the particles to glow brightly under a specialized microscope for enumeration.
• Flow cytometry is a more advanced and rapid technique that counts and differentiates viral particles as they pass one by one through a laser beam.
• Metagenomics, or viromics, involves sequencing all genetic material directly from an environmental sample to explore viral diversity and genetic potential without lab cultivation.

These techniques collectively provide a window into the world of these invisible yet influential members of aquatic ecosystems.