The cyanobacteria are the largest and most diverse group of photosynthetic bacteria. Due to the similarity of the photosynthetic processes and the pigments found in cyanobacteria and the chloroplasts of plants, it is generally believed that cyanobacteria are the evolutionary precursors to the chloroplasts. These bacteria perform oxygenic photosynthesis, which results in the evolution of oxygen. Cyanobacteria are very tolerant of environmental extremes and are present in almost all aquatic and terrestrial environments.
Onondaga Lake located in upstate New York near Syracuse is contaminated with mercury and other hazardous materials in the sediment and water from past manufacturing operations. Industrial activity around Onondaga Lake dates back to the late 1700¹s and the salt (NaCl) industry. In 1884, the Solvay process for the production of sodium carbonate (Na2CO3) was established (Doerr et al., 1994). The chlor-alkali process, which produced sodium hydroxide (NaOH) and chlorine gas (Cl2), is responsible for the release of mercury into Onondaga Lake. This process used a floating mercury electrode as the cathode and as a solvent for sodium. Theoretically, mercury should not have been released into the environment because the process was designed to recycle and retain its intermediates. However, the Allied Chemical plant released mercury into Onondaga Lake through leakage, vaporization, and when the electrolytic cells were cleaned and removed. It is estimated that from 1946 to 1953, nearly 11 lbs. of mercury entered the lake each day. From 1953 to 1970, the mercury discharge was approximately 22 lbs. per day (Chemistry in Context, 1994). Over the 24 year span in which the Allied Chemical Company operated, it is estimated that 75,000 kg of Hg were released into Onondaga Lake (Effler, 1987).
How do living systems deal with chemicals, such as mercury and other heavy metals, which are inherently toxic to their fundamental biochemical processes? In a great variety of bacteria, the mechanism of resistance to mercury compounds is the transformation of the toxic form (either organic or ionic) to the less toxic elemental form Hg(0). The mercury resistance operon is found in both Gram negative and Gram positive organisms and is widely distributed in both pristine and chemically impacted environments (Summers, 1992; Silver and Misra, 1988). The operon consists of genes coding for operon regulation, mercury transport and mercuric reductase, merA, which reduces Hg(II) to Hg(0).
Our goal was to collect water samples from Onondaga Lake and to isolate metal-resistant organisms from this metal polluted environment. We determined the percentage of mercury resistant organisms at each sampling site. We also isolated cyanobacteria from our water samples. Once the cyanobacterial cultures are isolated from other organisms, we plan to analyze the organization of the mercury resistance operon. Oneida Lake, located northeast of Syracuse, was used as a control lake. This lake has no record of industrial activity and/or metal contamination.
We made three trips to Syracuse on May 29, June 30, and August 3, 1998. Water samples were collected from sites around the edge of each lake, chosen at random but sampled each time, and also up-river and down-river from each lake (Figure 1). Water samples were plated on plate count agar (in order to count total organisms) and on plate count agar plus 50 µM mercury (in order to count mercury resistant organisms). Plates were incubated for 24 hours at 240C and bacterial colonies were counted. Water samples were also plated on media to enrich for fresh-water cyanobacteria. These plates were incubated at 240C under low light intensity for several weeks until cyanobacterial colonies were observed.
Our total plate counts have given us interesting insight into the presence and distribution of mercury resistant organisms in Onondaga and Oneida Lakes. In Onondaga Lake the highest level of mercury resistant organisms (19.7% of the total organisms counted) were found at collection site #1 (Figure 2), directly east of the Allied waste beds. Other collection sites around Onondaga Lake had approximately 2% mercury resistant organisms. Each of the other collection sites (all river sites and Oneida Lake sites) had 1% or fewer mercury resistant organisms (Figure 1).
Eubacterial and cyanobacterial isolates will be analyzed by polymerase chain reaction (PCR). PCR is a very sensitive technique used to analyze the genome of many organisms. We will use primers designed from the known merA sequence on the eubacterial isolates from Onondaga and Oneida Lakes. Restriction digests of merA amplified from non-human primate feces revealed nine different mer loci (Liebert et al., 1997). We are interested in whether our eubacterial isolates fall into these designated groups or extend these observations.
The first complete genome sequence of a cyanobacterium, Synechocystis sp. PCC 6803, reveals a merA homolog. We are interested in the restriction pattern from cyanobacterial merA and whether these also fit into the identified mer loci. Our first attempt to amplify merA from cyanobacterial isolates from Onondaga and Oneida Lakes revealed amplification of a merA homolog from the Onondaga Lake sample, but not in the Oneida sample. More experiments must be done before any conclusions can be drawn.
REFERENCES
Doerr, S.M., S.W. Effler, K.A. Whitehead, M.T. Auer, M.G. Perkins and T.M. Heidtke. 1994. Chloride model for polluted Onondaga Lake. Water Resource. 28:849-861.
Effler, S.W. 1987. The impact of a chlor-alkali plant on Onondaga Lake and adjoining systems. Water, Air, Soil Pollution. 33:85-115.
Liebert, C.A., J. Wireman, T. Smith and A.O. Summers. 1997. Phylogeny of mercury resistance (mer) operons of Gram-negative bacteria isolated from the fecal flora of primates. Applied & Environmental Microbiology. 63:1066-1076.
Silver, S. and T.K. Misra. 1988. Plasmid-mediated heavy metal resistances. Annual Review of Microbiology 42:717-743.
Summers, A.O. 1992. Untwist and shout: a heavy metal-responsive transcriptional regulator. Journal of Bacteriology. 174:3097-3101.
Chemistry in Context: applying chemistry to society. 1994. American Chemical Society. W.M. C. Brown Communications, Inc. 182-199.