Mystery of the smelly purple lake

Ah! The sun is shining. The day is warm, and a friendly breeze sweeps by just enough to tease at your hat. On days like these, there’s nothing like a nice spring or summer afternoon by the lake. But wait. Something else came in on the breeze, something that doesn’t go with good times or fresh air and summer swims. It smells like rotten eggs. And the lake ripples purple across the way, looking like Kool-Aid gone bad.

This is Little Gaynor Lake, a small, naturally occurring basin called a "prairie pothole." The lake is one of two potholes in Boulder County. As ice melts and water warms, the lake periodically turns purple and begins to emit an unsavory odor, an odor that so offended the neighbors that Boulder County Open Space asked CIRES researchers Prof. William Lewis and Dr. James McCutchan to figure out what was going on with this watering hole. The answer might not be welcome news for the surrounding residents, but could bode well for biofuel research.

The source of the dilemma appears to be a natural and rare phenomenon. “As far as


we know, there’s no overland source of pollution because there’s no tributary to this lake,” said Dr. Lewis. “So even if there were no people around, the lake would probably still be doing this.”

The Science

Unpleasant as it may be, the ecology at work in Little Gaynor Lake could have positive implications for biofuel production.

A lake with only a small amount of groundwater flowing through it has become a sulfur-rich environment. With that distinguishing character comes an unusual ecology. Organisms specially adapted to exploit oxygen-poor waters have colonized the lake. An alga, Anabaenopsis elenkenii, produces organic matter that uses up oxygen near the lake bottom and leads to the formation of sulfide. One bacterium, Chromatium, uses sulfide for photosynthesis and is responsible for the lake’s occasional purple color. When wind churns the water, the smelly sulfide rises

to the top, and, coupled with light, fuels photosynthesis by the purple-colored bacteria.

That the unusual lake is natural raises uncertainty about what, if anything, Boulder County should do to fix it. The upside is that the organisms responsible for the smelly, purple lake could offer clues to a new form of biofuel production. The alga is hardy, continuing to grow and release energy even in environments deprived of light and oxygen. That makes it a good candidate for biofuel.

The fact that the alga is so abundant is another plus. The theoretical cap on chlorophyll concentrations, an indicator for algae densities, for a lake of this size is about 300 mg/L. Little Gaynor Lake showed concentrations upwards of 2,000 mg/L. "When we saw that we said, ‘Wow!’ We haven’t seen concentrations that high before," said McCutchan.

Need another reason to like this odorous environment? The alga’s affinity for salty waters could aid biofuel production in places where agriculture is otherwise off-limits. The End