Do host-associated microbes show a contrarian latitudinal diversity gradient? Insights from Mytilus californianus, an intertidal foundation host

Aim The latitudinal diversity gradient (LDG) is one of the most pervasive large-scale trends in biodiversity. However, whether microbial taxa also conform to this pattern remains poorly known. This study uses the gill and shell-surface microbiomes of the marine bivalve Mytilus californianus to test whether the diversity of host-associated microbial communities declines with increasing latitude, as predicted by the LDG. Location Northeastern Pacific coast, ranging from Sitka, AK to La Jolla, CA, USA (spanning 24.12 degrees of latitude). Taxon Bacteria and Archaea associated with M. californianus gills and shell-surfaces. Methods We amplified and sequenced the 16S rRNA gene from M. californianus gill and shell-surface samples. We used linear and quadratic regressions, accounting for spatial autocorrelation when needed, of average alpha and gamma diversities of the whole microbiome, as well as individual microbial clades, to determine whether these taxa conformed to the traditional LDG. We also used permutational multivariate analysis of variance to determine the level of compositional differentiation among sampling sites. Results Gill and shell-surface microbiota show differing latitudinal diversity patterns, with both micro-environments exhibiting high levels of compositional differentiation along latitude. Individual microbial clades within each micro-environment also show different latitudinal trends, most likely due to differing ecologies and life histories. Main conclusions Mytilus californianus-associated microbial taxa show a flat, hump-shaped or contrarian LDG, suggesting that a steep decline in diversity with increasing latitude may not be a universal pattern across different domains of life.