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A Good Diatom Gone Astray

2010 April 23

This seems to happen more and more these days. A good organism, doing its job in the world, eking out life in its established niche. Suddenly, something goes wrong – conditions change or the organism is transported to a new location. Sometimes death or extinction result, but other times rampant growth occurs, resulting in a perfectly good organism becoming a nuisance. Such is the case with Didymosphenia.

Didymosphenia, or Didymo for short, is a beautiful and rather large diatom. Diatoms are a type of algae that grow on all sorts of substrates in a stream; they produce oxygen during photosynthesis, thereby sustaining aquatic life. Many diatoms have specific habitat requirements and have been used as indicators of water quality over the years. Finding Didymo in a water sample has traditionally meant that the lake or river was oligotrophic: very cold and clean, with very little available nutrients. Finding Didymo today might signal alarm.

Didymo occurs naturally across the northern hemisphere, typically in high altitude streams, but has recently started to flourish in a number of atypical environments. Scientists are monitoring stretches of rivers in Colorado, Idaho, Montana, Wyoming, Oregon, North Dakota, Vancouver Island, parts of Europe, and most recently, New Zealand. Established Didymo populations in some watersheds have completely covered rocks and plants, often for 10-20 km or more, effectively taking over an entire riverbed.

To understand how a microscopic alga can consume a river bottom, consider how Didymosphenia grows. Diatoms are constructed like tiny glass boxes; when they reproduce vegetatively, the frustules (or the lid and bottom of the box) divide. Each frustule produces a new half, resulting in two new ‘boxes’ or complete diatoms. Each time Didymo divides, it also exudes a mucilaginous stalk, which can be hundreds of times the length of an individual frustule. In places where Didymo colonies grow rampantly, the appearance and texture becomes quite gross. “The stuff is really slimy, at least on the top layer,” says Rex Lowe, diatom specialist from Bowling Green State University. “But the stalks are rather tough and cottony when pulled off a rock. When you step on it, the top is slippery but it squishes down, similar to stepping on an outdoor carpet.” Because the stalks are non-photosynthetic, they can take on a whitish appearance. “The stalks make up most of the density of the colony. They dry out almost like paper – in fact, where Didymo colonies have dried out (post high water mark), they look like toilet paper or cardboard along the stream.” The stalks are resistant to decomposition, so there is no odor to the algal mat, but because of the resemblance to toilet paper, it can appear that a river has sewage discharge problem.

Dr. Lowe is currently involved with BioSecurity New Zealand, where Didymo, or ‘rock snot’ as it is called down-under, has launched a significant invasion. Didymo first appeared in 2004 and has since then rapidly colonized ten rivers. How did it get there? More than likely it arrived by trout anglers. “There is just a steady torrent of trout fishermen going over to New Zealand,” says Dr. Lowe. “If they’re from western states and they don’t clean their boots, they are going to be carrying microbes.” New Zealand scientists have taken an extremely aggressive approach to minimizing the spread of the diatom: initially they closed many rivers, actually posting personnel to prevent physical access. Presently, they are considering ways to chemically control the invasion, using chlorine or copper sulfate. “I would think that the most ‘hopeful’ thing is to let it run its course, and then become controlled naturally by a virus or bacteria.” Dr. Lowe compares the New Zealand invasion to a similar event that occurred on Vancouver Island. There, Didymo populations exploded for about 10 years. Since then they have dropped back naturally, not unlike other boom and bust cycles that typify invaders.

But what everyone really wants to know is what is causing Didymo to suddenly bloom? A number of theories have emerged, including increased ultraviolet light and global warming. Dr. Lowe, however, feels that the sudden spread is due to a genetic variant. “On Vancouver Island, the diatom was reported in the late 1800’s; it didn’t explode until the mid-1990’s and their streams have not undergone any dramatic changes. In New Zealand, there wasn’t much diatom growth at all and now suddenly Didymo is there and doing really well.” In other words, Didymosphenia seems to have mutated just enough to radically widen its once narrow environmental window.

Here in the states, Sarah Spaulding of the Environmental Protection Agency is closely monitoring the spread of Didymo. Currently, the diatom seems to be confined to the west, with recent reports confirmed in North Dakota, Oregon, and Wyoming. According to Dr. Spaulding, “There has been little scientific research on the fallout of this invasion, in terms of the insect life and fisheries.” The stalks seem to present most of the problem. As the colonies become dense, fine sediments are trapped in them, excluding typical grazers (i.e., mayflies, caddisflies, etc.). Chironomids seem to like the mats, and data indicate increased populations in streambeds overcome with Didymo. No one has looked at long-term effects on the fisheries however. “The Federation of Fly Fishers has shown great interest in the spread of Didymo,” says Dr. Spaulding. “In fact, they are cosponsoring a symposium with the EPA and the American Fisheries Society.” The symposium will address the science of the diatom, its growth and spread, and its overall impact on the entire watershed.

Didymo has yet to be reported in the Midwest, but an ounce of prevention beats the cure. Fly fishers travel from one stream to another, often tossing wet waders and boots into a plastic bag or storage container. Diatoms and other microorganisms are capable of surviving quite a while, as long as they have some moisture. Since it only takes one live cell to start a colony, the potential exists to transport Didymo to new streams. According to Spaulding, anglers can prevent the spread by cleaning and treating boots and waders. Scrubbing in hot water and then soaking for a few minutes in a bleach solution (1/2 cup in a 2 gallon bucket) will kill the diatom as well as other organisms (think zebra mussels for starters). It is important to not allow the cells to wash down the drain, as then they are directed right into the watershed. Alternatively, boots and waders can be completely dried for at least 48 hours. Any area that can harbor trapped moisture, however, can also harbor the diatom, so chemical treatment is preferred.

For information on the symposium, contact Spaulding.sarah@epa.gov or visit www.fisheries.org/AFSmontana or www.epa.gov/Region8/water. To report suspected Didymo colonies, collect material in a clean, stoppered vial. Send materials to Sarah Spaulding with exact location of collection site (refer to her email for additional instructions).

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