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Actinobacteria give soil its earthy smell. They also are the white, powdery "mold" looking substance that forms over the interior surfaces of a hot, new compost pile, especially a pile of wood chips. They can be either aerobic or anaerobic; there is dispute online over which they prefer.

There is dispute as to whether they are dangerous to plants, when they have colonized a pile. I've found extension papers claiming that they are harmless, while other folks claim that they will wipe out plants. Some claim they will wipe out everything but brassicas; some say they destroy symbiotic fungi.

There seems to be a consensus that, like many soil organisms, they are dangerous to breath in.

Some say they are good for the soil, others say they are bad for it.

Elaine Ingham seems to dislike them, but she tends to take many controversial positions.

What do you all think?
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The reason for all the confusion is that there are so many species of this phylum and that most people don't understand that fact.
What results is misinformation being put out as if it were truthful, factual information, because those talking about this phylum are not Microbiologist.

For our needs, what you need to know is that there are "good" species and there are "bad" species that are found in gardens.
Usually the bad ones are anaerobic, all of them, in sufficient numbers can do bad things to our soil.
This is a situation of "All Things In Moderation" What we want is balanced soil microbiology.
Just as a nutrient imbalance is not good for our plants, neither is an imbalance in bacteria or fungi.

Now to give you some of the science (just a taste, the rest you can go to NCBI and read for yourself, should you so desire).

"Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation.
They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds.
Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture.
Actinobacteria play diverse roles in their associations with various higher organisms,
since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma),
soil inhabitants (e.g., Micromonospora and Streptomyces species),
plant commensals (e.g., Frankia spp.), and gastrointestinal commensals (Bifidobacterium spp.).
Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities."

Mostly these organisms serve beneficial roles both in the soil and to the plants that grow in the soil.
Usually their pathogenic forays are along the lines of defenders of the plant roots.
In the role of purveyors of mineral nutrients you are looking at the mycelial forms.  
But when they have an explosive bloom, they can out compete other, necessary organisms and create a soil that is out of balance at the microbe level, this is when they can become deadly to both your soil and your plants.

This is part of a paper from the NCBI , Geonomics of Actinobacteria

"Genomics of Actinobacteria: Tracing the Evolutionary History of an Ancient Phylum†

Actinobacteria constitute one of the largest phyla among Bacteria and represent gram-positive bacteria with a high G+C content in their DNA. This bacterial group includes microorganisms exhibiting a wide spectrum of morphologies, from coccoid to fragmenting hyphal forms, as well as possessing highly variable physiological and metabolic properties. Furthermore, Actinobacteria members have adopted different lifestyles, and can be pathogens (e.g., Corynebacterium, Mycobacterium, Nocardia, Tropheryma, and Propionibacterium), soil inhabitants (Streptomyces), plant commensals (Leifsonia), or gastrointestinal commensals (Bifidobacterium). The divergence of Actinobacteria from other bacteria is ancient, making it impossible to identify the phylogenetically closest bacterial group to Actinobacteria. Genome sequence analysis has revolutionized every aspect of bacterial biology by enhancing the understanding of the genetics, physiology, and evolutionary development of bacteria. Various actinobacterial genomes have been sequenced, revealing a wide genomic heterogeneity probably as a reflection of their biodiversity. This review provides an account of the recent explosion of actinobacterial genomics data and an attempt to place this in a biological and evolutionary context.

In terms of number and variety of identified species, the phylum Actinobacteria represents one of the largest taxonomic units among the 18 major lineages currently recognized within the domain Bacteria (406), including 5 subclasses and 14 suborders (404). It comprises gram-positive bacteria with a high G+C content in their DNA, ranging from 51% in some corynebacteria to more than 70% in Streptomyces and Frankia. An exception to this is the genome of the obligate pathogen Tropheryma whipplei, with less than 50% G+C.

Actinobacteria exhibit a wide variety of morphologies, from coccoid (Micrococcus) or rod-coccoid (e.g., Arthrobacter) to fragmenting hyphal forms (e.g., Nocardia spp.) or permanent and highly differentiated branched mycelium (e.g., Streptomyces spp.) (15). They also exhibit diverse physiological and metabolic properties, such as the production of extracellular enzymes and the formation of a wide variety of secondary metabolites (389). Notably, many such secondary metabolites are potent antibiotics (255), a trait that has turned Streptomyces species into the primary antibiotic-producing organisms exploited by the pharmaceutical industry (29). Furthermore, various different lifestyles are encountered among Actinobacteria, and the phylum includes pathogens (e.g., Mycobacterium spp., Nocardia spp., Tropheryma spp., Corynebacterium spp., and Propionibacterium spp.), soil inhabitants (Streptomyces spp.), plant commensals (Leifsonia spp.), nitrogen-fixing symbionts (Frankia), and gastrointestinal tract (GIT) inhabitants (Bifidobacterium spp.). Unusual developmental features are displayed by many actinobacterial genera, such as formation of sporulating aerial mycelium in Streptomyces species or the persistent nonreplicating state exhibited by certain mycobacteria. Actinobacteria are widely distributed in both terrestrial and aquatic (including marine) ecosystems, especially in soil, where they play a crucial role in the recycling of refractory biomaterials by decomposition and humus formation (152, 403). Furthermore, many bifidobacteria are used as active ingredients in a variety of so-called functional foods due to their perceived health-promoting or probiotic properties, such as protection against pathogens mediated through the process of competitive exclusion, bile salt hydrolase activity, immune modulation, and the ability to adhere to mucus or the intestinal epithelium (273, 329, 407).

The actinobacterial genomes sequenced so far belong to organisms relevant to human and veterinary medicine, biotechnology, and ecology, and the observed genomic heterogeneity is assumed to be a reflection of their biodiversity. This review will give an account of the recent explosion of actinobacterial genomics data and will place this in a biological and evolutionary context.

Taxonomy of Actinobacteria

Actinobacteria include many organisms that exhibit, or have a tendency towards, mycelial growth. 16S rRNA gene sequencing has led to the recognition of 39 families and 130 genera, which also include high-G+C gram-positive bacteria with simpler morphology, such as bifidobacteria and micrococci (Fig. ​(Fig.1)1) (119). The deepest branch separates bifidobacteria from all other known families. The divergence of actinobacteria from other bacteria is so ancient that it is not possible to identify the phylogenetically closest bacterial group to Actinobacteria with confidence (119).

Phylogenetic tree of Actinobacteria based on 1,500 nucleotides of 16S rRNA. Scale bar, 5 nucleotides. Families containing members subjected to complete genome sequencing at the time of this writing are depicted in bold. Orders are indicated.

Actinobacteria have a unique molecular synapomorphy, i.e., a shared derived character: a homologous insertion of about 100 nucleotides between helices 54 and 55 of the 23S rRNA gene (375)."

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