Hi everyone - I hope your summer went well and you had a great growing/composting season. I had busy summer mainly helping take care of the newest additional to our family, Osten, who is now 5 months old (below). We had a fairly successful backyard garden, with ~50 tomato plants and a little bit of just about everything else. I'm finally finding the time to get some more blog posts written up so sorry for the delay!
I wanted to mention I was recently accepted as a speaker for the the Virginia Association for Biological Farming (VABF) Conference January 19th to 21st 2024. if you are in the area you might want to check it out. I have primarily been speaking to composters and academics, but I am excited to present some of the data from the Vermi-Microbiome Project to farmers explicitly interested in biological solutions for their farms. I really hope some of this data can increase adoption of vermicompost and teas.
I'm sure many of you have heard of the new Worm Farming Conference coming up soon in Valdosta, Georgia. I was hoping they might ask me to speak there as well, but I found out about it pretty late. Nonetheless, they have some great speakers there and it would likely be worth your time to attend.
Beneficial Bacteria of Vermicompost
On to the main topic -- who are these beneficial bacteria found in vermicompost and what are they doing? I've talk a little about some of these types of organisms in previous posts but I wanted to get it all in one place and in more detail.
Actinobacteria are a group of related microorganisms at the Class Level (remember Kingdom -> Phyla -> Class -> Order -> Family -> Genus -> Species). This is large group of organisms, but they all have some unique similarities. In vermicompost, these organisms are usually the most abundant group of known plant growth promoting organisms comprising 20-30% of all bacteria present. They also have many pharmaceutical benefits which we'll discuss briefly as well.
What do they look like?
Actinobacteria actually look a lot like fungus. While most other bacteria look like little single cellular dots under a microscope, Actinos actually form long branching structures called "hyphae" that form the mycelium...just like many fungi. The name of this group of organisms has actually changed a bit and used to be called Actinomycetes. "mycetes" actually means fungus -- reflecting the fact that used to be classified as fungi. While Actinobacteria look similar to fungi under a microscope, their hyphae are smaller than fungi and their cell walls are made of peptidoglycan and not chitin like fungi. In many vermicompost Actinomycetes is the the most abundant class of organisms. I wonder if it's possible this is the origin of some of the "fungal dominant" composts? I'll save this for another blog post.
The figure below shows their life cycle and what they look like at each stage.
What do they do?
First off Actinobacteria, are nearly all aerobic saprophytes, meaning they breath oxygen and are great plant material decomposers. They are abundant in soils but can also live in fresh and salt water. They have a similar life cycle to fungi in that they form mycelium in substrate, an aerial fruiting body and the spores. Spore formation is common in soil organisms as it allows them to "hibernate" during unfavorable conditions (ex:drought) and reemerge when conditions are more favorable.
Actinobacteria are also responsible for the production of "about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds." (Barka et al, 2015)
In an agricultural context Actinobacteria can basically do it all and are one of the most commercialized "biofertilizers." I'm basically just going to summarize this article "Plant Growth Promoting Actinobacteria, the Most Promising Candidates as Bioinoculants?" because it is both recent and comprehensive. If we consider all of the plant growth promoting properties I wrote about recently in this post they can do it all. This includes, nitrogen fixation, phosphorous and iron solubilization, phytohormone production, biocontrol, plant stress reduction, and even bioremediation. On top of this they are one of the best types of organisms at surviving arid conditions. The paper I linked above concludes that "it should play a key role in formulating multi-strain inoculants with synergistic actions for promoting sustainable agriculture."
What do they look like?
Bacilli, like Actinobacteria, are a large class of organisms all of which are aerobic and form endospores. They are rod shaped and some form chain like structures. In the image below you can see the endospores (white circles) while the cells are stained purple. This purple stain is called a Gram stain. Cells that retain the purple stain are called Gram positive (G+) and cells that stain pink are Gram negative (G-). This is just one basic way of differentiating cells under a microscope based on their cell wall structure.
What do they do?
Like Actinobacteria, Bacilli organism are ubiquitous in soils. They also also known to produce a variety of enzymes, ferment foods, and are thought to be a promising probiotic. You might recognize the the Bacillus name from your yogurt such as Bacillus Bulgaricus used to make Belgian yogurt.
From a plant health perspective, Bacilli have many of the beneficial traits including nitrogen fixation, plant growth hormone production, and some disease suppression. Of these functions what they really seem to excel at is auxin production and mineral solubilization (phosphorous mainly). They are able to make phosphate available in the soil by exuding a lot of small carbon molecules that bind to the phosphorous atoms in the soil and free it from the calcium or iron it's bound to. This makes the phosphorous biologically available to both plants and microorganisms.
In vermicompost, we are ideally seeing these organisms in the 10-20% range although they don't seem to be present in all vermicomposts. A paper published in 2017 out of India, looked at a strain of Aneurinibacillus aneurinilyticus (I can't pronounce it either). They found this strain had nearly every plant growth promoting property. When a cell suspension of this organism was added to tomato seedlings a "Remarkable increase was observed in seed germination (27.07%), shoot length (42.33%), root length (52.6%), shoot dry weight (62.01%) and root dry weight (45.7%)" when compared to the uninoculated controls.
Rhizobiales are probably the most well known plant growth promoting organism as they were discovered early in legume (peas beans and peanuts) root nodules. They are most well known for fixing atmospheric nitrogen however there is new research coming out demonstrating that they also associate with other plant types and do more than fix nitrogen. I have seen them in all vermicompost and they usually comprise 5-10% of the community.
What do they look like?
Unlike the previous two types of organisms these are non-spore formers. Some genus of organisms like Hyphomicrobium (right side below) are rod shaped but can form a hyphal filament or stalk. This is actually part of their reproductive cycle. The left side of the figure below is a pictures of a root (green) and the pink dots are attached Rhizobacterium. This pictures is an example of fluorescent microscopy in which colored probes are used to identify specific organism types.
What do they do?
As I said, they are mainly known as nitrogen fixers, however there is growing evidence that not all organisms from this family can fix nitrogen, yet are still found attached to plant roots. This indicates that they are likely providing benefits other than nitrogen fixation.
Some of the most promising organisms from this family I've read about are from the genera Devosia and Mesorhizobium. In addtion to nitrogen fixation, Devosia are organisms also known to produce auxin. Mesorhizobium species have been seen solubilize phosphorous and iron on top of auxin and nitrogen fixation capabilities.
I first want to clarify that these are not the only beneficial organisms associated with vermicompost- only the most abundant groups. Organisms in the clades of Xanthomonadales, Sphingomonadales, Psudomondales, Flavobacteriales, and Burkholderiales also contain species of beneficial organisms and are commonly found in vermicompost but in less abundance. I'll cover some of these groups of organisms when I write a similar post to this one covering organisms found in vermitea.
Secondly, many of the publications this information comes from are quite recent - usually in the last few years before this project started. There is still much research to be to comprehensively understand which organisms are helping plants thrive. I also want to write about fungi and fungi to bacteria ratios so stay tuned for that as well.
Any questions or comments please feel free to reach out or post a comment below!