Red Algae
If your lake takes on the rich, deep color of red wine in autumn, don’t immediately run to the store for cheese and crackers. What you are witnessing is probably a cyanobacteria (a.k.a. blue green algae) bloom. Planktothrix rubescens (formerly known as Oscillatoria rubescens) is a species of cyanobacteria that can have a very distinctive red color. Additionally, the cells can attach end-to-end and may form filaments of several millimeters in length, making them easily visible to the naked eye. During winter this cyanobacteria may even seep through fissures in the ice, creating colorful “flowers” across the lake’s surface.
Planktothrix rubescens bloom in a Missouri lake.
For much of the year P. rubescens resides near the metalimnion, that transition area between the warm epilimnion and the colder hypolimnion (see Lake Stratification). P. rubescens blooms are frequently reported in fall when lake temperatures cool and the water column starts mixing deeper. This change in lake thermal structure doesn’t necessarily promote the bloom of P. rubescens, but rather moves the bloom from the metalimnion to the surface. The colorful cyanobacteria become apparent once this metalimnion has mixed with the cooling surface layer of the lake.
Like many cyanobacteria, P. rubescens is capable of regulating its buoyancy, effectively moving itself up and down within the water column. When high in the water column they receive enough light to photosynthesize. During photosynthesis, they produce dense carbohydrates that eventually weigh the cells down, causing them to sink. Once in deeper water (sometimes the hypolimnion) where there isn’t enough light to photosynthesize, P. rubescens will respire, using the carbohydrates for energy, thus lightening their load. A by-product of their respiration is the expansion of their “gas vacuoles”. Eventually, with enough respiration, the gas vacuoles increase buoyancy enough to lift the algal cell back up in the water column.
DENSITY OF P. RUBESCENS DURING STRATIFICATION
This graph shows three variables (temperature, light, and P. rubescens density) as they relate to lake depth. The top and bottom of the graph represent the surface and bottom of the lake, respectively.
The topmost arrow is at the Secchi depth, 12 feet. About 20% of the sun’s light penetrates to this depth. The temperature is the same as at the surface (about 74°F), and there is no “red algae” present.
At 36 feet (the second arrow), there is no sunlight, the temperature is much cooler (about 40°F) and the density of P. rubescens is the highest. This depth marks the bottom of the metalimnion, where there is likely access to a supply of dissolved phosphorus from the nutrient-rich hypolimnion.
P. rubescens has adapted to life in the summer metalimnion by tolerating cool temperatures and low light. The cool, dark and deep water of the hypolimnion tends to have more nutrients, thanks to settling particulates and the release of nutrients from the sediment (see Phosphorus From Within). While in or near the hypolimnion, P. rubescens takes advantage of the higher nutrient levels by stocking up. Thanks to its tolerance of low-light conditions, P. rubescens has an advantage over the algae that are restricted to the epilimnion where nutrient concentrations are lower and competition is higher. Even during turnover, when the water column is mixed, this cyanobacteria competes well under low-nutrient conditions. As a result, its presence in lakes with low nutrient concentrations can be prolonged, highlighting the contrast between the clear water state before turnover and the sudden, startling appearance of the red bloom.
P. rubescens is capable of producing two kinds of toxins; a neurotoxin (which affects the nervous system) and a hepatotoxin (which affects the liver). However, P. rubescens does not produce these toxins at all times. It is not well understood why P rubescens or and other cyanobacteria sometimes produce toxins.
To be safe, avoid swimming during any bloom, whether red, or green.
P. rubescens can create red "flower" patterns by seeping through fissures in ice-covered lakes.
About Cyanobacteria, or blue green algae
Blue green algae (formally known as cyanobacteria) are not technically algae at all, but rather bacteria that contain chlorophyll and have the ability to photosynthesize. Like all bacteria, cyanobacteria are prokaryotes and have no nucleus. Green algae are eukaryotes, with a nucleus and other organelles within their cell membranes. In photosynthesizing eukaryotes, the chlorophyll resides within organelles called chloroplasts.
Chloroplasts are interesting because they have their own DNA and replicate independently from the rest of the cell. Some believe that chloroplasts (and mitochondria) originated as cyanobacteria. Long ago, one cell enveloped another and rather than consuming it, assimilated it. The two cells mutually benefited in a process known as endosymbiosis, setting the stage for the myriad of life forms that would follow.
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