Biofilm is a pervasive coating found on most surfaces that are in frequent contact with water. Within the film is a highly organized and resilient structure able to host a wide variety of microorganisms, including plant root pathogens like Pythium.
Bacteria in a biofilm exude a matrix consisting of polysaccharides (chains of sugars), proteins, lipids, DNA and other molecules. This matrix secures the biofilm to the surface and provides a structure within which microbes can reproduce. The matrix also lends protection against adverse environmental conditions (high temperature or dryness, for example) and against organisms that feed on bacteria, such as protozoa.
There are four stages of biofilm formation:
- Free Swimming: Free-floating bacteria attach to a surface by exuding what’s known as ‘extracellular polymeric substance,’ a strand-like structure of polysaccharides.
- Attachment: Over time, the adhered bacteria and their exudates created a matrix layer, firmly binding the microbes to the surface and to each other.
- Growth: The microbes in the biofilm proliferate and the biofilm becomes thicker and harder to remove.
- Dispersal: At some point, pieces of biofilm generally detach and will adhere to new areas of a given surface, where it will continue to proliferate.
Due to its structure and substance, biofilm is hard to remove and becomes thicker over time.
The challenges of biofilm in irrigation systems
Biofilm can cause a variety of issues in irrigation systems, whether in greenhouses or for field crops that use source water stored in a pond or reservoir.
Biofilm can clog irrigation drip emitters. A clog prevents water from reaching plant roots, leading to their death and subsequent crop losses. Biofilms also clog filters, resulting in more required maintenance and cleaning.
In addition, biofilm is a refuge for root cell pathogens such as Pythium. A permanent pathogen population within an irrigation system can access every plant served by the system, endangering plant health, yields and profit margins.
Pythium is among the most serious root diseases of greenhouse crops. Pythium species are a large group of fungal-like pathogens that are difficult for even experienced growers to manage, and prevention is key. Phytophthora, related to Pythium, is less common but generally more pathogenic. It most often causes root and crown rot but can also cause foliar blight. Rhizoctonia is also a common cause of root disease and stem canker, and Thielviopsis root and stem rot is another fungal disease concern.
Because greenhouse root diseases are hard to control, especially in the warmer temperatures resulting from global warming trends, their prevention is critical.
Growers therefore need to use a water disinfection method that provides both biofilm removal and ongoing prevention of biofilm buildup.
water disinfection & biofilm control
Physical disinfection methods such as UV radiation kills microbes by destroying their DNA. The UV ‘dose’ that’s delivered depends on several factors and pre-filtration to less than 25 microns is required. Reverse osmosis is another physical water treatment method. Neither of these water disinfection methods have an effect on biofilm formation.
Oxidation is a water disinfection method that kills microbes through cell lysis. It can be achieved through repeated applications of chemicals such as chlorine, chlorine dioxide, copper sulfate or sodium hypochlorite. Passing ozone gas produced on-site through irrigation water provides oxidation, as does adding hydrogen peroxide or peroxyacetic acid. However, none of these methods have been shown to effectively achieve biofilm removal and/or ongoing prevention of biofilm buildup.
Nanobubbles disinfect irrigation water, destroy existing biofilm and prevent biofilm formation. The use of nanobubble technology is a chemical-free and cost-effective oxidation method.
Today, Moleaer nanobubble generators are found at over 500 irrigation sites around the world, treating over 400 million gallons of irrigation water each day. ROI is swift, with 40% of Moleaer horticulture clients reaching ROI within 12 months. Growers using Moleaer’s nanobubble generators see healthier plants and roots resulting in improved yields and experience better irrigation system hygiene allowing for less downtime for operations.
biofilm abrasion with Nanobubbles
Nanobubbles measure 70-120 nanometers in diameter, roughly 2500 times smaller than a grain of salt. At a typical concentration of hundreds of millions of nanobubbles per milliliter and possessing neutral buoyancy, they hover uniformly within a liquid, constantly providing strong, natural oxidative effects. Nanobubbles move randomly and continuously through all the parts of a water system, abrading existing biofilm and preventing further biofilm formation.
Through their constant movement and oxidizing action, nanobubbles penetrate and physically scour biofilm deposits within water basins and irrigation pipes while also preventing biofilm re-formation.
Each nanobubble has a typical ‘lifespan’ of several months. Upon eventual collapse, reactive oxygen species (ROS) such as hydroxyl radicals are produced, which lyse pathogens and other microbes free-floating in the irrigation water or within biofilm.
This 2020 study in Environmental International specifically focuses on the ability of nanobubbles to remove biofilm in irrigation pipes. Researchers found that “nanobubbles reduced microbial biomass and changed community composition in the biofilm” due to “the localized high temperature, pressure, and hydroxyl radicals with strong oxidative activity during the explosion of nanobubbles.”
During the trial, biofilm biomass was reduced at various time intervals by 31.3% - 52.1%. “Nanobubbles induced the significant shifts in biofilm community and reduction of its biodiversity through direct biocidal effect and indirect water quality influence,” stated the scientists. “Nanobubbles were harmful to mutualistic relationships among different microbial species, leading to the formation of simple and small microbial networks, with the shrinking of positive inter-links by 49.1% − 77.1%.”
Other researchers have concluded that “the study of ROS generation caused by nanobubbles is of great importance for its application in both physiological activity promotion aspect and sterilization aspect.”
In addition to removing biofilm through direct physical abrasion and hyper-localized action upon collapse (ROS production, temperature and pressure changes), nanobubbles prevent the formation of biofilm in two ways:
Their constant, random movement and extremely high densities oxidizes (lyses) free-floating microbes before they can adhere to irrigation pipe surfaces and begin to form biofilm.
Secondly, nanobubbles are negatively charged and are attracted to the positive surfaces of irrigation pipes. This attraction causes them to completely cover pipe surfaces, preventing any existing free-floating microbes from attaching and forming biofilm.