Not a Fuel Slick! …

A few years ago I was walking along the nature trail that runs south of Finch Drive towards the Adventure Centre. As I crossed over a small stream bed I noticed an intensely-coloured iridescent sheen that looked very much like a petrochemical slick. When I touched the surface of the standing water, the slick broke up but did not quickly re-form as gasoline might do. It was not slippery to the touch as oil might be, and it had an herbal smell rather than the kind of smell I associate with fossil fuels. Another odd clue was a layer of rusty-brown sludge on the bottom and banks of the stream.

iNaturalist to the rescue! I posted some photos that prompted some interesting comments and suggestions from scientists and citizen scientists in California, Iowa, and Virginia, as well as from Squamish. The conversation developed along these lines:

• “It’s common to find what looks like oil on the surface of still water: this can be associated with rotting vegetation.”
• “Our soil around Squamish is known to be rich in iron.”
• The rusty sludge (referred to as “flox” or “floc”) is “thought to be caused by bacteria that gets its energy by oxidizing dissolved iron.”
• “The oily sheen stuff is distinctly different from the iron flox.  The iron flox emissions (rusty, reddish brown, semi-solid plumes) result from a process in watersheds with higher iron content at or below surface. As water moves through the water table, areas with higher iron concentrations and other ‘right’ geological conditions will produce the flox. The ‘oil slick’ and the flox can occur simultaneously, but if they do, it is for different reasons.”
• “The red flox and the oily sheen are quite possibly produced by two different organisms: we can certainly say that the “oil” is a byproduct of methane formation by anaerobic bacteria (it seems to be a gelatinous precipitate of iron-hydroxide), while the red “floc” is the iron bacteria gatekeeper between the deoxygenated, iron-rich water rising up from the swamp and the atmosphere…or oxygen being contributed by the adjacent plants.”
• “I’ve never seen the oil without some degree of the floc, but I’ve seen the floc without the oil so I’ve just assumed that the bacteria producing the floc also sometimes produce the oil. That may not be correct, however.”
• The final thoughts came from a biogeochemist  who noted that: “These types of oily-looking films have been reported in the literature as forming in association with anoxic and ferrous iron-bearing groundwater. … It may be indicative of a genus called Leptothrix, which are Fe2+ and Mn2+ oxidizing bacteria. This genus is in the phylum Beta Proteobacteria, as are many of the neutrophilic (neutral pH-loving) iron-oxidizing bacteria. …  As with most bacteria, taxonomic assignment is impossible without 16S rRNA sequencing, although many of the “iron bacteria” have characteristic morphologies that can be identified with light microscopy.”

To summarize: we’re unlikely to be able to identify the organism that causes this oily-looking slick, but we’ve had fun wondering.  By the way, that slick is persistent in the area just south of Finch Drive, so if you’re curious, have a look.

Gwen