The ecology and diversity of microbial eukaryotes in geothermal springs

Abstract

Decades of research into the Bacteria and Archaea living in geothermal spring ecosystems have yielded great insight into the diversity of life and organismal adaptations to extreme environmental conditions. Surprisingly, while microbial eukaryotes (protists) are also ubiquitous in many environments, their diversity across geothermal springs has mostly been ignored. We used high-throughput sequencing to illuminate the diversity and structure of microbial eukaryotic communities found in 160 geothermal springs with broad ranges in temperature and pH across the Taupō Volcanic Zone in New Zealand. Protistan communities were moderately predictable in composition and varied most strongly across gradients in pH and temperature. Moreover, this variation mirrored patterns observed for bacterial and archaeal communities across the same spring samples, highlighting that there are similar ecological constraints across the tree of life. While extreme pH values were associated with declining protist diversity, high temperature springs harbored substantial amounts of protist diversity. Although protists are often overlooked in geothermal springs and other extreme environments, our results indicate that such environments can host distinct and diverse protistan communities.

Fig. 1

New Zealand geothermal springs harbor diverse protists. a Phylogenetic tree of the 1088 protist phylotypes recovered across 160 geothermal springs. The tree is a conservative estimate of diversity as it only includes phylotypes present (>10 reads) in a given sample. The outer ring bars are scaled to show the relative abundance of each phylotype. Ring and clade color indicate taxonomic group. For a few taxa, clade membership did not reflect taxonomic assignment and no corrections were made. b Most spring communities are dominated by a few phylotypes and individual protist phylotypes are rarely found in more than a few springs. (Top) The number of phylotypes found in each spring. (Bottom) The number of springs in which each phylotype occurred

Fig. 2

Distribution of individual protist lineages across temperature (°C) and pH for groups identified as sensitive to either environmental variable. The plots are arranged according to the observed preferences for particular conditions. Each circle represents a geothermal spring sample; gray circles indicate a particular taxon was not found in a spring and if found, circle color ranges from yellow to red based on log-relative-abundance. Size of circles corresponds to their proportional abundance

Fig. 3

Phylogenetic distribution of the protist phylotypes sensitive to temperature and pH across springs. Clade markers signify type for each taxon: red is hot, blue is cold, yellow is acidic, and green is alkaline

Fig. 4

The protistan, bacterial, and archaeal taxa that co-occur across geothermal springs. Node size is proportional to abundance of taxa and edge width corresponds to strength of co-occurrence (positive ρ value). Gray nodes represent bacterial and archaeal taxa and white nodes represent protistan taxa. Node outline colors indicate the five modules (groups of co-occurring taxa) identified using the Louvain community detection algorithm [55]. Node fill color designate protist taxonomic groups that contain the phylotypes identified as indicators for a particular environmental condition (shown in Fig. 3). There are 138 nodes (taxa) with 940 connections (edges)