The microbial ecology of Protea repens (Proteaceae) infructescences and the surrounding environment

Abstract

The infructescences of serotinous Protea trees is an unusual and unique fungal niche, well known for its association with specific fungi and their symbiotic mites. However, except for a single study on Streptomyces species, little is known about any other microbial groups present in this habitat. In this study, we explored the complete microbial diversity associated with the infructescences of Protea repens. Diverse bacterial communities inhabit the infructescences of P. repens. Differences were noted between bacterial communities in infructescences in areas where the plants were regrowing after it was destroyed by fire. Depending on the age of vegetation after fire, some bacterial taxa were present in both the litter and infructescences. Actinomycete bacteria were previously reported from P. repens and P. neriifolia infructescences. In this study, we found two different Streptomyces species consistently present in newly-formed and mature infructescences. In a fire-affected site, actinomycetes rarely colonized infructescences, whereas in an unburnt site, the majority of infructescences were colonized by actinomycetes. However, within three months after infructescences had formed, a much larger proportion of infructescences in the burnt site were colonized by these bacteria. Sporothrix splendens was the most common fungal OTU in the burnt site, but there was increased competition with an OTU identified as a Clavulina sp. in the unburnt site. Although Knoxdaviesia proteae was present in all infructescences examined, it did not have a high relative abundance as was expected. This may be due to biases in PCR, library preparation and sequencing, or it may be that the fungus may simply have been consumed by arthropods as was previously suggested. An attempt was made to identify bacteria present in close proximity to fungal structures in P. repens infructescences. The bacterial communities in P. repens infructescences seemed to be randomly assembled and no consistent bacterial associates of the fungi could be identified. However, it was clear that the presence of the fungi resulted in higher bacterial species richness in the immediate surroundings of the fungi. The fungal communities present in P. repens infructescences were similar in composition to bacterial communities of decomposing deadwood and plant litter suggesting that it provides a similar niche as deadwood- and litter, and is possibly a reservoir for decomposer and soil associated fungi in these fire-prone ecosystems.