Eelgrass (Zostera spp.) associated phytomyxids are host-specific congeneric parasites and predominant eukaryotes in the eelgrass rhizosphere on a global scale

Together with increasing environmental and anthropogenic pressures, pathogenic diseases are one of important factors contributing to the ongoing decline of seagrass meadows worldwide; yet the diversity and ecology of the microorganisms acknowledged as seagrass parasites remain critically understudied. Here we investigate phytomyxid parasites (Rhizaria: Endomyxa: Phytomyxea) of three different eelgrass (Zostera spp.) species found in the Northern hemisphere. We present molecular evidence that Plasmodiophora bicaudata, a long-recognized parasite of dwarf eelgrass taxa, is closely related to the novel phytomyxid recently discovered in root hairs of Zostera marina, and together they form a distinct clade within the order Phagomyxida, proposed here as Feldmanniella gen. nov. A full life cycle is systematically described in a phagomyxid representative for the first time, proving its conformity with the generalized phytomyxid life history, despite previous uncertainties. The presence of primary infection stages in nearly all collected eelgrass specimens, and subsequent analysis of amplicon sequences from a global Z. marina dataset, reveal phytomyxids to be ubiquitous and one of the predominant microeukaryotes associated with eelgrass roots on a global scale. Our discoveries challenge the current view of Phytomyxea as rare entities in seagrass meadows and suggest their generally low pathogenicity in natural ecosystems. Originality-Significance StatementThis study addresses a group of microbial parasites critically understudied in the marine environment. It presents complex evidence that Phytomyxea - obligate intracellular biotrophs previously considered to be rare entities in the oceans, are in fact ubiquitous endobionts of seagrasses of the genus Zostera - foundation species and important primary producers in coastal areas worldwide. Our work represents a significant contribution to the fields of aquatic microbiology and seagrass ecology and is seminal to understanding the biology of Phytomyxea outside of terrestrial ecosystems.