New & Noteworthy

Yeast on the Brink

July 2, 2012

How scientists are using baker’s yeast to discover the warning signs of impending financial, climate, and species collapse.

Yeast might help us recognize when we are on the edge of a cliff.

Tipping points are all the rage these days. They are discussed with regard to global warming, financial collapses, ecosystems and lots of other situations too.

A tipping point is a point from which something can’t return to what was before. In other words, it is the point at which a new equilibrium is reached.

One of the more interesting tipping points occurs when a population of organisms becomes so low that it may collapse and not be able to recover. This can happen because the beasts are all so interrelated that a disease can wipe them all out.  Or they become so few in number that potential mates have trouble finding one another. Many other reasons can bring a population to this point.

Theory makes a number of predictions about how populations at the tipping point will behave.  Dai and coworkers decided to create a model system using S. cerevisiae to study what populations at the tipping point actually look like experimentally. And to perhaps find easy to study signs that a population is veering close to one of these tipping points.

Their experiments ended up faithfully reproducing a population in the lab that was at a tipping point. This is a big deal in and of itself.  But while they were able to identify signs that a population was at a tipping point, none would be very easy to spot in a wild population. 

Their model system involved using dilutions of yeast grown in sucrose. Since sucrose is hydrolyzed by yeast outside of the cell, a sucrose molecule hydrolyzed by one yeast cell can be used by another. This cooperative effect means that yeast grow better in sucrose at higher cell densities than they do at lower ones. This mimics the effects of low population density in other systems.

The researchers then did a set of simple dilution experiments with this system. They diluted a starting population of yeast by varying amounts into replicate samples and determined how each sample did with subsequent dilutions over time. They found that they reached their tipping point in their system at dilutions of between 500 and 1600. At these dilutions, some replicates survived while others went extinct. 

They confirmed they were at a tipping point by shocking their cultures with high salt. If a population is near a tipping point, it is less able to survive environmental shocks compared to a more robust system. The researchers found that those samples near the tipping point were indeed more vulnerable to salt shock. 

Taken together, these two findings suggested that the researchers had successfully engineered a model system for tipping points. They were now ready to study their population at or near its tipping point to look for any tell tale warning signs.

They found that their model system agreed with a lot of the theory. As a population neared the tipping point it tended to fluctuate more, and to take longer to reach a new stable population. Unfortunately, neither of these is an obvious sign of an impending tipping point. Both effects require lots of observations over a long time period to see.

Given the consequences of going past a tipping point (sea level rise, coral bleaching, the Great Recession, species extinction), recognizing when we are getting close to one is of paramount importance. Perhaps research like this will help us see the warning signs before it is too late to pull back from the brink.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics