Growth and development of iron-oxidizing bacterial films in an iron-rich drainage system in central CT, USA

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Roger C. Ort and Roger Laushman, Biology, Oberlin College, Oberlin, OH

Background/Question/Methods
Leptothrix discophora is one of many aerobic iron-oxidizing bacteria that grow in metal-encrusted organic sheaths. Despite being a widespread genus in nature, the underlying mechanisms behind Leptothrix biofilm formation and appearance are poorly understood and nearly absent from the literature. Iron-oxidizing bacterial films vary in appearance, but the mechanics of thin-film interference can likely explain many of the observed patterns and color varieties. We attempted a multi-faceted observational approach to studying these biofilm-forming iron-bacteria in nature. Using digital photographs, time-lapse videos, field samples, and frequent field observations, we observed biofilm development, distribution, and appearance in situ. We developed a simple sampling technique to facilitate the preservation of unaltered, intact 2”x3” biofilm samples on microscope slides, producing 63 samples featuring characteristic patterns, textures, and colors for analysis. Collecting films on glass slides allows viewing both sides of the film, as opposed to being restricted to viewing only the open-air side in the field. With the understanding that thin-film interference may be responsible for biofilm color and that film thickness increases with age, we used frequent field observations to make conclusions about relative film growth rates and patterns of development.
Results/Conclusions
We observed film lifespans ranging from less than 24 hours to greater than a week. Film age correlated with physical characteristics e.g., color, pattern, and texture. As films aged, they were observed going through a color sequence in exact alignment with the Michel-Lévy interference color chart. Films infrequently achieved the latter stage “third order” colors due to disturbance and buildup of particulate iron, which also contributed to texture. Older films often featured wrinkled areas where the film lifted from the water surface. Continuous field monitoring revealed that growth rates varied among microsites, indicating that bacteria films had "preferred" microenvironments. Films located in deeper, running water often were smaller, shorter-lived, and grew more slowly when compared to films that formed in shallow water or even saturated soils. Comparing both sides of films showed that color, transparency, and granularity varied with habitat and age. Further research is needed to quantify biofilm growth and characteristics, but our preliminary observations might provide valuable insights into the natural conditions under which biofilms may form, as well as provide a method for approximating film age upon observation of color alone. Understanding iron bacteria biofilm development may be useful for monitoring stream pollution.