Rickettsiales are obligate intracellular bacteria originally found in metazoans, but more recently recognized as widespread endosymbionts of various protists. One genus was detected also in several green algae, but reports on rickettsialean endosymbionts in other algal groups are lacking. Here we show that several distantly related eustigmatophytes (coccoid algae belonging to Ochrophyta, Stramenopiles) are infected by Candidatus Phycorickettsia gen. nov., a new member of the family Rickettsiaceae. The genome sequence of Ca. Phycorickettsia trachydisci sp. nov., an endosymbiont of Trachydiscus minutus CCALA 838, revealed genomic features (size, GC content, number of genes) typical for other Rickettsiales, but some unusual aspects of the gene content were noted. Specifically, Phycorickettsia lacks genes for several components of the respiration chain, haem biosynthesis pathway, or c-di-GMP-based signalling. On the other hand, it uniquely harbours a six-gene operon of enigmatic function that we recently reported from plastid genomes of two distantly related eustigmatophytes and from various non-rickettsialean bacteria. Strikingly, the eustigmatophyte operon is closely related to the one from Phycorickettsia, suggesting a gene transfer event between the endosymbiont and host lineages in early eustigmatophyte evolution. We hypothesize an important role of the operon in the physiology of Phycorickettsia infection and a long-term eustigmatophyte-Phycorickettsia coexistence.

Centre for Phycology and Biorefinery Research Centre of Competence Institute of Botany CAS Dukelská 135 Třeboň CZ 379 82 Czech Republic

Department of Life Sciences Coimbra Collection of Algae University of Coimbra Coimbra 3000 456 Portugal

Faculty of Science Department of Biology and Ecology Life Science Research Centre University of Ostrava Chittussiho 10 Ostrava 710 00 Czech Republic

Faculty of Science Department of Biology and Ecology Life Science Research Centre University of Ostrava Chittussiho 10 Ostrava 710 00 Czech Republic Faculty of Science Institute of Environmental Technologies University of Ostrava Chittussiho 10 Ostrava 710 00 Czech Republic

Sackler Institute for Comparative Genomics American Museum of Natural History Central Park West at 79th Street New York NY 10024 USA Division of Invertebrate Zoology American Museum of Natural History Central Park West at 79th Street New York NY 10024 USA

Unité de Recherche en Maladies Infectieuses et Tropicales Emergentes UM63 CNRS7278 IRD198 INSERMU1095 Institut Hospitalo Universitaire Méditerranée Infection Aix Marseille Université Faculté de Médecine 27 boulevard Jean Moulin Marseille cedex 5 13385 France

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