Prochlorococcus Biofilm Formation: How Ocean Microbes Shift Lifestyles

Why is the ocean carbon cycle dependent on photosynthetic microbe research?

The ocean's most abundant microbe, Prochlorococcus, has a hidden ability to switch between living as a solitary drifter and forming sticky communities known as biofilms. This organism is the smallest and most abundant photosynthetic microbe in the ocean.

It plays a significant role in the environment, as it is responsible for roughly 10 percent of all carbon removed from the atmosphere by land and oceans each year. Researchers in the Chisholm lab at MIT discovered this dual-lifestyle capability, which allows the microbe to move from planktonic drifting to attached living.

How does Prochlorococcus gene expression drive lifestyle shifts?

The shift between these lifestyles is not the result of genetic mutations. Genomic sequencing confirms that genetically identical cells can switch between being attached or free-floating based on changes in gene expression.

Maya Anjur-Dietrich explains that while these cells share the same genome, they change which parts of their instructions are read out at specific points. This gene expression mechanism allows the same strain to adapt its behavior without altering its underlying genetic code.

Where are Prochlorococcus biofilm communities found in the ocean?

This biofilm phenotype is a universal trait among the strains researchers have studied. After testing diverse ecotypes from every major branch of the Prochlorococcus family tree, the team found that every strain possessed the ability to form these attached communities.

These communities are present throughout the entire ocean, existing from the sunlit surface down to depths of 1,000 meters. Data from the Tara Oceans Project and seawater samples at the Station ALOHA research site in Hawaii indicate that Prochlorococcus accounts for about 12 percent of the microbial community found on particles.

How does biofilm formation change ocean carbon sequestration estimates?

The ability of Prochlorococcus to attach to particles changes how we calculate the amount of carbon moving from the atmosphere to the deep sea. MIT researchers found that the formation of these biofilms likely plays a larger role in carbon sequestration than scientists previously realized. This discovery suggests that current estimates regarding the volume of carbon the ocean transports to the deep sea require a revision.

It is estimated by researchers that this attachment mechanism has the potential to move roughly half a billion metric tons of carbon to the ocean's depths each year. To reach this number, the team conducted a follow-up study that determined the proportion of the Prochlorococcus population associated with particles across various ocean basins. They combined their own findings with archived genetic information to calculate this figure.

Anjur-Dietrich notes that this figure is a very rough, first-order estimate. The calculation relies on several variables, including surface population levels, the rate at which the microbes attach to particles, and the amount of carbon consumed by other organisms as the particles descend.

What are the biological trade-offs for attached Prochlorococcus cells?

Attached cells experience distinct physiological differences compared to their free-floating counterparts. They grow more slowly and produce more chlorophyll than cells that drift alone. While only about 6 percent of cells in a typical culture stick to tube walls, researchers can create populations where roughly 85 percent of cells are attached by repeatedly selecting for that trait over many generations.

This lifestyle choice changes how the cells interact with their environment. In comparison to free-floating cells, those that are attached discharge almost double the amount of organic carbon into the water around them. Maya Anjur-Dietrich is currently investigating if these attached cells provide an easier food source for neighboring microbes on particles, potentially acting as a form of cake that slows the breakdown of sinking particles.

Who led the research on Prochlorococcus biofilm formation?

The study, titled "Biofilm formation and dynamics in the marine cyanobacterium Prochlorococcus," was authored by MIT Department of Civil and Environmental Engineering researchers Katelyn G. Jones, James I. Mullet, Nhi N. Vo, Kurt G. Castro, and Sierra M. Parker. This work builds on research conducted by former postdocs Nikolai Radzinski and Gio Capovilla.

Senior author Chisholm notes that these findings confirm long-held suspicions regarding the organism. According to lead author Maya Anjur-Dietrich, we are now able to view Prochlorococcus as living in two distinct yet linked environments.

The research was supported by the Burroughs Wellcome Fund and grants from the Simons Foundation. Be aware that this information is based on a preprint or press release and has not yet been peer-reviewed.