The term Black Beard Algae (BBA) refers to algae from the genera Audouinella, which contains many sub-species. Audouinella species occur in both marine and freshwater. These algae appear as short reddish or black turfs of hair. They grow in small bunches and first appear as small furry black dots as small as the tip of a pen. They are often found on the edges of older leaves of plants, or on hardscape and filter outlets. Some species can grow to more than a centimeter in length.
Why does BBA attach especially readily to plant leaf edges?
While they appear still and unmoving on the outside, there is complex biochemistry on going inside the plant as the plant generates carbohydrates, sugars and other metabolites from photosynthesis. Plants are leaky organisms and aquatic plants eject a portion of excess metabolites directly through their leaf surfaces and margins. Algae feed off these waste organics. When plants are under stress, for example, when adapting to a new environment, which requires a significant amount of re-programming of proteins and enzymes, the excretion of waste proteins and metabolites is increased. The plant becomes particularly vulnerable to algae. On the flip side, when plants are healthy, they generate defensive antimicrobial chemicals and enzymes to ward off pathogens and algae.
Stressed/unhealthy plants are thus algae magnets while healthy plants are very resistant towards algae. Older leaves leak more organics, and plants defend older leaves less as they are less valuable. This is why in most tanks, you see algae attaching only onto some plants but not others, and most commonly to older leaves first. Plant stress can be caused by many factors. Recently transplanted plants tend to face some adaptation stress as they adapt to the new environment. Insufficient CO2 or nutrient levels can cause poor growth as well. Frequently moving plants and making large changes to the tank environment are both actions that can cause plant stress as they try to catch up to the environmental changes.
In a tank where there is a lot of old deteriorating old leaves on plants, the environment is especially conducive for algae. In a tank where there is dominant plant mass and the plants are growing robustly with fresh new leaves, algae will find it difficult to survive. Thus, plant husbandry plants a big role in managing algae. Cutting off old leaves to make space for new growth, and replanting healthy tops/discarding old deteriorating bottoms are tasks that should be done regularly to maximize growth of new fresh leaves.
This approach of rejuvenating growth to avoid algae applies to BBA but is a universal concept that can be applied to manage all algae.
Black beard algae
High levels of organic waste & detritus in the planted tank encourages BBA to spawn. If they are found attached to plants, it is most commonly caused by stress induced by fluctuating CO2 levels. In tanks where plants are healthy, but there a high level of organic waste in the tank environment, BBA will be found attached to hardscape instead.
If found on hardscape or driftwood, it is almost always found in fast flow areas in the path of CO2. Often this is due to aquarists obsessed with overly strong flow. It's commonly found on filter outflows, and on CO2 diffusers - which are often placed in the path of high flow. In such tanks, slowly down the flow or redirecting it to have more even flow distribution has a big impact.
Barr report suggests that BBA strives when CO2 is present but at low levels; between 10-15ppm. Non injected tanks (low tech tanks) and tanks with higher levels of CO2 are less vulnerable - to this end, it seems to occur most often in tanks where CO2 injection is used but optimized poorly.
Black brush algae has little connection to nutrient levels or light spectrum. Both tanks with very lean water column fertilization and rich water column fertilization are vulnerable to it. It has very strong correlation to organic waste levels, plant husbandry and CO2 fluctuations.
American flagfish, mollies and SAE (Siamese algae eater) pick at it when there are no tasty alternatives. However, they are seldom adequate solutions.