Phylogenetic relatedness drives protist assembly in marine and terrestrial environments

Aim Assembly of protist communities is known to be driven mainly by environmental filtering, but the imprint of phylogenetic relatedness is unknown. In this study, we aimed to test the degree to which co-occurrences and co-exclusions of protists in different phylogenetic relatedness classes deviate from random expectation in two ecosystems, in order to link them to ecological processes. Location Global open oceans and Neotropical rain-forest soils. Major taxa Protists. Time period 2009-2013. Methods Protist metabarcoding data originated from two large-scale studies. Co-occurrence and co-exclusion networks were constructed using a recent method combining a null distribution model with Spearman's rank correlation coefficients among pairs of operational taxonomic units. Phylogenetic relatedness was estimated using either global pairwise sequence distance or phylogenetic distance inferred from best maximum-likelihood trees derived from multiple alignments of operational taxonomic unit representative sequences. The significance of observed patterns relating networks and phylogenies was evaluated by distance classes against two null models, in which either the tips of the phylogenetic trees or the network edges were randomized. Results Closely related protists co-occurred more often than expected by chance in all datasets, but also co-excluded less often than expected by chance in the marine dataset only. Concurrent excesses of co-occurrences and co-exclusions were observed at intermediate phylogenetic distances in the marine dataset. Main conclusions This suggests that environmental filtering and dispersal limitation are the dominant forces driving protist co-occurrences in both environments, whereas a signal of competitive exclusion is detected only in the marine environment. Differences in co-exclusion are potentially linked to the individual environments, in that marine waters are more homogeneous, whereas rain-forest soils contain a myriad of nutrient-rich microenvironments, reducing the strength of mutual exclusion.