Islands Within Islands: Bacterial Phylogenetic Structure and Consortia in Hawaiian Lava Caves and Fumaroles

Lava caves, tubes, and fumaroles in Hawai‘i present a range of volcanic,
oligotrophic environments from different lava flows and host unexpectedly
high levels of bacterial diversity. These features provide an opportunity
to study the ecological drivers that structure bacterial community
diversity and assemblies in volcanic ecosystems and compare the older,
more stable environments of lava tubes, to the more variable and extreme
conditions of younger, geothermally active caves and fumaroles. Using 16S
rRNA amplicon-based sequencing methods, we investigated the phylogenetic
distinctness and diversity and identified microbial interactions and
consortia through co-occurrence networks in 70 samples from lava tubes,
geothermal lava caves, and fumaroles on the island of Hawai‘i. Our data
illustrate that lava caves and geothermal sites harbor unique microbial
communities, with very little overlap between caves or sites. We also
found that older lava tubes (500–800 yrs old) hosted greater phylogenetic
diversity (Faith's PD) than sites that were either geothermally
active or younger (<400 yrs old). Geothermally active sites had
a greater number of interactions and complexity than lava tubes. Average
phylogenetic distinctness, a measure of the phylogenetic relatedness of a
community, was higher than would be expected if communities were
structured at random. This suggests that bacterial communities of Hawaiian
volcanic environments are phylogenetically over-dispersed and that
competitive exclusion is the main driver in structuring these communities.
This was supported by network analyses that found that taxa (Class level)
co-occurred with more distantly related organisms than close relatives,
particularly in geothermal sites. Network “hubs” (taxa of potentially
higher ecological importance) were not the most abundant taxa in either
geothermal sites or lava tubes and were identified as unknown families or
genera of the phyla, Chloroflexi and Acidobacteria. These results
highlight the need for further study on the ecological role of microbes in
caves through targeted culturing methods, metagenomics, and long-read
sequence technologies.