I received an e-mail message earlier this week with images showing colonies of a cyanobacterium, Nostoc commune, growing in openings created by turfgrass succumbing to some other issue. The sender asked me to identify the "black stuff" they believed was killing their lawn.
Of course, the cyanobacterium did not kill the turfgrass. Something else was killing the lawn and the cyanobacterium was just an innocent bystander taking advantage of an opening; literally.
Cyanobacteria are single-celled organisms with great aspirations. They can exist in multicellular states with chains of cells stuck together within a sheath to form a filament. This allows the cells to communicate and share nutrients over a large area which is how many Nostoc species form large mats.
Nostoc commune and others in this genus of unusual organisms are often misidentified because of their changing appearance. During periods of wet weather, Nostoc commune may look like an agglomeration of rubbery yellowish-green to bluish-black material is "bubbling up" from the soil.
The otherworldly appearance of hydrated Nostoc commune is responsible for several common names. It was once believed the alien-looking masses originated from the dust of shooting stars (a.k.a. meteors) which accounts for the common names of star-jelly, star-shot, and star-slime. Other common names such as "witch's-butter" are self-explanatory.
During periods of dry weather, the odd-looking masses collapse and turn black; the "black stuff" in the picture of struggling turfgrass that was sent to me. In fact, this is the most common appearance of Nostoc spp. that does not enjoy irrigation.
Drying occurs quickly; however, this causes no harm to the cyanobacteria. It's actually a survival strategy supported by a range of polymers that keep the overall structure intact.
The bottom line is that Nostoc commune and other mat-forming members of this genus are not toxic; they cause no harm to plants or animals including harm to the health of concerned Ohio landowners. However, the mucoid hydrated mates can become slippery when wet, so tread carefully.
When an Alga is Not an Alga
Cyanobacteria have long been mislabeled as "blue-green algae." That's because they have slightly bluish green photosynthetic pigments in their cytoplasm. Since the bacteria act like plants by producing their own "food" (polysaccharides) and generating oxygen, cyanobacteria were once lumped in with the algae.
However, algae, as well as other plants (and animals), are "Eukaryotes" meaning their nuclei are gift-wrapped in a membrane. Cyanobacteria lack membrane-bound nuclei; a condition that makes them a "Prokaryote."
Cyanobacteria have a number of interesting tricks up their prokaryotic sleeves. They have specialized cells called heterocysts that can grab nitrogen out of the atmosphere in a process called "nitrogen fixation" to convert the nitrogen into molecular forms that make the element available for the bacteria as well as plants.
For this reason, some cyanobacteria may be found growing on the surfaces of plants, including certain algae, which once added to the confusion with learning the true identity of these unusual bacteria. Some, such as Nostoc commune, also have specialized pigments in their cells that absorb UV light to protect against UV radiation. This allows this bacterium to survive intense solar radiation.
In fact, Nostoc spp. have a worldwide distribution and may be found thriving under some of the most extreme environmental conditions. Some biologists consider them to be a type of "extremophile" which is important to remember when you're trying to eliminate them!
A Shortened Long History
There are several interesting evolutionary connections between cyanobacteria and the rise of plants. Biologists generally agree that chloroplasts in plants have their ancestral origins with cyanobacteria through an evolutionary process called "symbiogenesis." Without cyanobacteria, higher plants as we know them would not exist. In fact, neither would we.
Earth has been around for about 4.55 billion years. Some believe slightly longer, but what's few billion more or less among friends. There was no oxygen in Earth's early atmosphere; it was "anaerobic." That didn't mean life didn't evolve, but it was life that lived without oxygen. Some of those anaerobic microbes still exist, but not where they're exposed to the oxidizing effects of oxygen.
The geologic record in the form of bands of rust in rocks shows that something remarkable happened around 2.45 billion years ago. It's referred to as the Great Oxygenation Event (GOE) and marks the beginning of a rapid change in Earth's atmosphere from anaerobic to aerobic.
The rise of oxygen contributed to the decline of free-floating iron in the early oceans. The iron oxidized; it rusted. The iron oxide (= rust) dropped to the ocean floor and was compressed into rocks. Eventually, the red-banded rocks were exposed through tectonic forces to provide an "Aha Moment" among geologists.
GOE occurred at about the same time that cyanobacteria appeared on the scene so it was once believed that these bacteria were entirely responsible for the rise of oxygen (literally). However, recent research has shown that other microorganisms were already producing some oxygen before the cyanobacteria. Regardless, it's still generally accepted that these remarkable bacterial oxygen pumps played a significant role in pushing atmospheric oxygen towards the current 21% that we enjoy today. So, you should thank cyanobacteria with your next breath.
Managing an Ancient
Although Nostoc commune is a remarkable organism that causes no harm and has even been investigated for use in land reclamation sites, it and other cyanobacteria in this genus can grow in large mats that present a slippery safety concern in nurseries and greenhouse walk-ways. Blackened, dehydrated colonies can create an unsightly mess on the bottoms of plant containers. The same is true with colonies growing in gravel driveways.
Still, it's almost impossible to eliminate an organism that has survived numerous mass extinctions from rampaging volcanoes to a dinosaur-killing meteor strike. However, an article titled, A Biological Introduction to Cyanobacterial Mats that was published in the March 2016 Issue of Digger Magazine (Oregon Association of Nurseries) provides some helpful insights.
Nostoc spp. are terrestrial organisms that can survive long periods looking like blackened, dried-out mucus. However, they must have periodical infusions of water to thrive. So, moisture management through improved drainage is essential. This includes addressing both shallow as well as deep drainage issues.
For example, dried Nostoc mats can clog the drainage holes in ground cover and weed control fabrics. Using a coarse bristled push broom to remove dried Nostoc can open the drainage holes; however, nothing will be gained if the underlying soil does not drain rapidly.
Although Nostoc can generate its own food and grab nitrogen from the atmosphere, it must acquire other nutrients from its environment. In fact, phosphorous is considered the most limiting nutrient in Nostoc development. This has also been observed with the aquatic cyanobacteria that periodically foul lake waters; but don’t call the problem "algal blooms!" So, limiting phosphorous runoff can reduce the development of Nostoc mats.
Algaecides such as sodium carbonate peroxyhydrate and copper sulfate products provide limited to no control of Nostoc. Herbicides such as glyphosate (e.g. Roundup) not only provide no control, but the release of phosphorus from dead plants can actually support Nostoc growth.
Herbicidal soap products based on ammoniated soap of fatty acids or potassium salts of fatty acids can provide limited efficacy. However, trials conducted by Oregon State University (the Other OSU!) showed that the most effective herbicidal product is Scythe (pelargonic acid). Not only did Scythe kill Nostoc but it also prevented regrowth for several weeks and this product is labeled for treatment of algae, moss, or liverworts in container nurseries. Of course, hydrated Nostoc must be targeted, dried mats are not susceptible. Also, this contact herbicide can damage or kill preferred plants so avoid direct contact as well as drift.