July 15, 2013
Phytoplankton Join ‘Social Mixers’ In Ocean Turbulence
[Watch the video: Phytoplankton Social Mixers]
Lee Rannals for redOrbit.com - Your Universe OnlineThe motility of phytoplankton allows the tiny ocean plants to determine their fate in ocean turbulence, according to scientists at MIT and Oxford University.
Researchers wrote in the journal Nature Communications that the individual vortices that make up ocean turbulence are like social mixers for phytoplankton. This social mixer brings similar cells into close proximity, helping to enhance sexual reproduction and other ecologically desirable activities.
Scientists previously believed that phytoplankton were passive drifters, unable to defy even the weakest currents orto travel of their own accord. However, researchers have now shown that many species can actually swim, and do so to optimize light exposure and to avoid predators. The latest study found that when these micro-plants are caught in an ocean vortex, they form highly concentrated patches at the center of the swirl. This process continues to repeat itself, allowing phytoplankton to move from social mixer to social mixer.
"Based on our intuition of turbulence and turbulent mixing, we expected homogeneity to reign," says Roman Stocker, an associate professor of civil and environmental engineering at MIT who led the study. "Instead, the phytoplankton surprised us by forming highly concentrated clusters of cells - it's turbulent un-mixing. For the phytoplankton, this is a vehicle to effectively find cells of the same species without any sensory information on each other's location or the need to invest in costly means of chemical communication."
These highly concentrated patches can also have a downside since phytoplankton form the base of the ocean food chain and proximity makes them easy prey.
"While patchiness increases the chance of a fatal encounter with a predator, it also increases the chance of finding other phytoplankton cells, which is needed to form resilient cysts that can survive harsh winter conditions," says William Durham, the paper's first author and a lecturer at Oxford University who began working on this study as a doctoral student at MIT.
"This mechanism suggests phytoplankton might tune their motility to have the best of both worlds, minimizing patchiness when there are a lot of predators around while maximizing patchiness when the time is ripe for cyst formation."
The researchers team first performed experiments using phytoplankton in the lab, then extended their observations to a turbulent ocean using high-resolution simulations. They used a transparent box shaped like the letter H with seawater flowing upward through the vertical bars to create two inner-directed vortices within the horizontal bar. When the team added a red-tide-forming species of phytoplankton known as Heterosigma akashiwo to the mix, they formed dense patches at the centers of the swirls.
The team found that patchiness increased more than tenfold when phyotoplankton swam. This research led to the conclusion that the microorganisms might have evolved to develop the ability to actively adjust their swimming speeds to modulate interactions with others of the same species.
"Life is turbulent in the vast expanses of the ocean - and it's fascinating to learn how some of the most important organisms on our planet fare and behave in their daily turbulent lives," Stocker concluded.