Many types of plankton move from the chilly, dark depths of the ocean to the surface, only to drift back down into the shadows in an ongoing cycle. However, it has been a puzzle to understand how single-celled phytoplankton, which typically lack structures for swimming, undertake this journey. A recent study published on October 17 in the journal Current Biology reveals insights about a species of glowing phytoplankton, known as Pyrocystis noctiluca, which expands to six times its original size—just a few hundred microns. This significant swelling enables the phytoplankton to ascend up to 200 meters toward the ocean’s surface to harness sunlight before sinking back down, illustrating a unique approach to long-distance travel in the ocean.
Phytoplankton tend to be 5%-10% denser than seawater, which means they need to find a way to counteract gravity to stay at the surface and perform photosynthesis. “We focused on organisms that seem to lack appendages for swimming,” explains Manu Prakash, a marine biologist and bioengineer at Stanford University, who is the senior author of the study. “Our findings indicate that these P. noctiluca cells behave like tiny submarines, expertly adjusting their density to choose their position within the water column.”
While aboard a research vessel near Hawaii, Prakash and postdoctoral fellow Adam Larson, a leading author of the study, encountered a bloom of P. noctiluca and observed two distinctly different sizes of plankton in their nets. “It took some time to understand why until we recorded videos revealing the cells dramatically inflating,” Larson recounts. “This inflation can occur quite abruptly, so if you step away from the microscope for just 10 minutes, you might miss it.”
To investigate the implications of this rapid expansion, the research team used an innovative device they referred to as a “gravity machine.” “This machine allows us to observe single cells in detail within a virtually endless water environment,” Prakash explains. “It’s akin to a Ferris wheel for small animals, but for single cells. The apparatus is the size of a dinner plate and rotates, tricking the cell into perceiving that it is ascending or descending in its own frame of reference.” By modifying the water pressure and density inside the gravity machine, the researchers could simulate conditions found in ocean depths. They discovered that inflated cells had a lower density compared to the surrounding seawater, empowering them to rise toward the simulated surface, overcoming gravitational forces.
Further examinations revealed that this expansion occurs naturally as part of the plankton’s cell cycle. When a single-celled phytoplankton divides into two, an internal structure known as a vacuole—akin to a flexible tank for water—absorbs fresh water, causing the new cells to greatly increase in size. The resulting daughter cells, now buoyant with lighter freshwater, ascend. “We’ve realized this is a smart strategy for ‘slingshotting’ in the ocean during cell division,” Prakash remarks. “During typical growth, the phytoplankton synthesizes numerous proteins and biomass until they grow too heavy and sink. Then, they divide in deeper waters and use inflation to regain their original size.”
The entire cell cycle spans roughly seven days, aligning with the plankton’s pursuit of light and nutrients. “This cycle illustrates how it may have evolved,” states Prakash. “For the first time, we have solid evidence suggesting that the cell cycle, a fundamental mechanism for managing cell function and division, might be influenced by ecological factors.”
By applying a theoretical framework to their findings, the research team identified the ecological parameters that limit and drive this evolutionary process. “Every cell encounters a gravitational pull downward, and unless they or their descendants resist, they will ultimately sink to the ocean’s floor,” says postdoctoral researcher Rahul Chajwa, another primary author of the study, also at Stanford University. Now, utilizing the insights gained from their gravity machine and their ecological and physiological studies, the team has established a mathematical framework that could potentially apply to all plankton in the ocean.
Moving forward, Prakash’s lab aims to explore additional mysteries surrounding various plankton species that may employ new biochemical strategies to regulate their density and movement through the water column. “We currently have around 600 species documented in our Behavioral Atlas, and we are systematically examining a range of mechanisms. Our research is revealing that there are multiple strategies evolving for this purpose. It’s exciting to know we have a long list of organisms to investigate since millions of species inhabit the ocean—this is just the beginning,” he concludes.
Hongquan Li, a graduate student in Prakash’s lab, is also a contributor to this research.
