Late Summer Red Tide: Myrionecta

Written September 17, 2012, published late September 2012

In late summer to early fall each year, water in the lower Columbia River, from Tongue Point to the entrance, turns purple to blood red. This so-called red tide is not toxic, and has been happening here for many decades. It’s caused by the rapid growth of a tiny ciliated protozoan (a single-celled animal with rows of tiny movable hairs, called cilia), Myrionecta rubra, which lives in brackish to salt water. It was formerly called Mesodinium rubrum. 

Aerial photo of Chinook Basin on Baker's Bay, during late summer when Myrionecta is blooming. Myrionecta patches are very dark; while waters with low levels of this protozoan are green. Photo by Kathleen Sayce

Myrionecta rubra looks like two balls stuck together, one slightly smaller than the other. It has two rows of cilia where the balls join, which move in rhythm like flexible, very fast beating galley oars. The whole animal is so small that in a water sample without magnification, all you can see is a red blur in the water. Under a light microscope, Myrionecta cells live only a few minutes before they overheat and die, rupturing the cell wall and spewing the intercellular contents out into the water. When healthy, they zip around as though jet propelled, bouncing off the edges of the slide, and rocketing from one side to another, fast, agile, tumbling, and changing directions with ease. 

Myrionecta cells look red because inside each cell, which is 50 µm long and 20 µm wide, are even tinier red algae, each one a few microns in diameter. Green plants have green plastids called chloroplasts, which were once free-living green photosynthetic bacteria. Myrionecta’s red plastids are red algae that have learned how to live inside cells; they can also live on their own as well. This relationship is often called a symbiosis, because the algae give the cells in which they live sugars for food, being photosynthetic, and gain a protective cell wall to live within. 

Myrionecta is around most of the year in brackish to salty water. In the years when I collected water samples to look at plankton species, I saw them in samples from the Columbia River, the Columbia plume offshore, in the ocean surf, in Willapa Bay, and also on rivers, including the Palix and Willapa Rivers. During August to October, they become very abundant. A few cells in the water don’t change the color, but billions of cells turn the water blood red. 

Sailboat ont the Columbia River near Desdemona Sands off Astoria, sailing through a dense red patch of Myrionecta. Aerial photo by Kathleen Sayce.

This color change is easily seen from the Astoria-Megler Bridge on Highway 101. The darkest colors can be seen as streaks and swirls, especially from the spans over the north and south channels, or from an airplane. The red color appears in August, first as a purplish tinge to otherwise blue waters, strengthens in September, and persists until fall storms begin, usually sometime in October. During this period, the number of Myrionecta cells in the water is easily in the millions of cells per cubic meter of water. The next time you are sitting in your car on the high span at the south end of the bridge in late summer or early fall in warm dry weather, watch the water and see if Myrionecta rubra is ‘blooming.’ 

Unlike many harmful algal blooms, this dramatic red bloom happens every summer and early fall with no bad side effects.  Myrionecta rubra doesn’t make biotoxins, does not make seafood poisonous, or cause illness or death in fish, birds or humans. It’s been going on for many decades. Not all blooms in local waters are so benign, but this particular one doesn’t seem to be a problem.