Sep. 9, 1998 MOSCOW, Idaho - Certain naturally-occurring bacteria living in the floor of Lake Coeur d'Alene may be helping to prevent the release of contaminants such as lead and zinc into lake waters, according to a University of Idaho scientist. These benefits, however, could be offset by other bacteria whose activities favor the release of other contaminants, such as arsenic.
UI biology Professor Frank Rosenzweig and graduate students Jim Harrington and Dave Cummings, both of Moscow, recently published research findings that indicate microorganisms living within or near the floor of the lake are transforming some of the most potentially dangerous heavy metals - such as lead and zinc - from a water-soluble state to a much safer solid state. The viability - and consequently the activity - of many such organisms depends on keeping the sediments free of oxygen.
"This is a fertile area for research because these microbes can actually change elements from one state to another," Rosenzweig said. "My gut feeling right now is the best thing we can do for the lake is leave it alone. To go in and dredge the lake as some have suggested would be, to say the very least, unwise. But leaving it alone may also entail minimizing human impacts that often result from over development."
Rosenzweig's research is funded through the National Science Foundation's EPSCoR project and is being published in refereed scientific journals such as, "Environmental Science and Technology" and "Journal of Environmental Quality." He said that until now the vast majority of research done on northern Idaho's Lake Coeur d'Alene has been chemical in nature. "There really has been no microbiological work done on sediments in the lake," he said.
Rosenzweig, Cummings and Harrington have taken samples from the lake floor at several sites near the mouth of the Coeur d'Alene River near Harrison. That's where previous research has shown the highest level of heavy metals and mine tailings to be. They also took samples from the lake floor where the St. Joe River flows into the lake. It has virtually no heavy metal contamination, and therefore, serves as a control site. The sampling method involves retrieving sediments as intact, 0.5-meter cores encased in PVC tubes.
In previous studies similar samples were shipped to laboratories outside the region. Rosenzweig's group preserved both the temperature and pressure conditions in which the samples - both those at the surface of the floor and up to 60 centimeters into the floor - were found. They rushed the samples back to the Moscow campus to conduct a thorough chemical analysis to pinpoint the location and state of the heavy metals.
The samples were then exposed to harsher and harsher chemical processes to see under what conditions the heavy metals would be stripped from the sediment and returned to a water-soluble state. "We found that irrespective of depth, the majority of metal contaminants were associated with a sulfidic phase. This phase is insoluble under the neutral pH, oxygen-free conditions that prevail in these lake sediments," Rosenzweig said. "Furthermore, lead and zinc are almost always found together in the deepest layers of the sediment."
Significantly, the scientists also found that arsenic and iron - heavy metals presumably deposited at the same time as the lead and zinc - were most prevalent in uppermost sediment layers, often within 4 to 5 inches of the overlying waters. Their location seems to coincide with the point where oxygen disappears.
The observation that metals have different distribution patterns led Rosenzweig and his group to hypothesize that metals are being actively transformed within Lake Coeur d'Alene. These transformations involve the change of metals from one compound to another. Since different compounds have different solubilities in water, they can result in the movement of metals between solid and liquid phases.
"These 'transformations' are both chemical and biological in nature," Rosenzweig said. "But the extent to which microbes control these processes has only recently become apparent. Bacteria not only deplete the sediments of oxygen, they can also respire iron, arsenic and sulfate in much the same way we use oxygen."
Microbes that respire sulfate produce hydrogen sulfide (marsh gas). Sulfide can chemically combine with metals to render them insoluble, and therefore, less likely to enter the food chain. "A metal sulfide that is familiar to everybody is fool's gold or pyrite," said Rosenzweig. "The sparkle at the bottom of our streams testifies to the stability of these compounds in water."
On the other hand, microbes that respire iron or arsenic could contribute to iron and arsenic instability.
"The activities of microbes underlie the peculiar distribution patterns of these elements in contaminated sediments such as Lake Coeur d'Alene," Rosenzweig said. "Whether or not these particular elements could ever be released into overlying waters is an open question. Clearly, we need to better understand the system with respect to the consequences of human disturbance. And disturbance in my book includes not just mechanical things, such as dredging, but increased nutrient levels that could occur if the lake were overdeveloped."
Each spoonful of Lake Coeur d'Alene sediment contains more than a million microbes. Preliminary work suggests that these sediments support tremendous biodiversity. The UI scientist and his graduate students have isolated "entirely new species and genera of bacteria."
"When I started graduate school I never thought I would ever have the fun of describing and naming new organisms," Rosenzweig said. "Moreover, these discoveries may prove to have commercial benefits.
"Depending on the conditions you design - or face - these bacteria could be used for biological mining or for the bioremediation of existing mining sites. After all, in order to survive, these creatures either had to have evolved or come equipped with the ability to tolerate extremely high levels of heavy metals."
Rosenzweig and his group are headed back to the waters of Lake Coeur d'Alene early this month to take more core samples of the lake floor. The group now includes post-doctoral fellow Bruce Wielenga, and doctoral students Tony March, Srivi Ramamoorthy, Allison Niggenmeyer and Matthew LaForce. Among their projects will be microcosm experiments to test how the bacterial community responds to additional sedimentation as well as increases in nutrients like nitrogen or phosphate. The team will then record how these changes affect the location and abundance of heavy metals.
"To me, the science seems to be saying, 'Leave the lake alone' and 'Restrict development,'" Rosenzweig said. "If we do that, the amount of heavy metals in the lake water may remain acceptable. But a definite answer as to how much development the lake can tolerate will depend strictly on more research."
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