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A marine symbiosis has been recently discovered between prymnesiophyte species and the unicellular diazotrophic cyanobacterium UCYN-A. At least two different UCYN-A phylotypes exist, the clade UCYN-A1 in symbiosis with an uncultured small prymnesiophyte and the clade UCYN-A2 in symbiosis with the larger Braarudosphaera bigelowii. We targeted the prymnesiophyte-UCYN-A1 symbiosis by double CARD-FISH (catalyzed reporter deposition-fluorescence in situ hybridization) and analyzed its abundance in surface samples from the MALASPINA circumnavigation expedition. Our use of a specific probe for the prymnesiophyte partner allowed us to verify that this algal species virtually always carried the UCYN-A symbiont, indicating that the association was also obligate for the host. The prymnesiophyte-UCYN-A1 symbiosis was detected in all ocean basins, displaying a patchy distribution with abundances (up to 500 cells ml − 1 ) that could vary orders of magnitude. Additional vertical profiles taken at the NE Atlantic showed that this symbiosis occupied the upper water column and disappeared towards the Deep Chlorophyll Maximum, where the biomass of the prymnesiophyte assemblage peaked. Moreover, sequences of both prymnesiophyte partners were searched within a large 18S rDNA metabarcoding data set from the Tara-Oceans expedition around the world. This sequence-based analysis supported the patchy distribution of the UCYN-A1 host observed by CARD-FISH and highlighted an unexpected homogeneous distribution (at low relative abundance) of B. bigelowii in the open ocean. Our results demonstrate that partners are always in symbiosis in nature and show contrasted ecological patterns of the two related lineages.

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Photosynthetic species of the dinoflagellate genus Dinophysis retain cryptophyte plastids from the Teleaulax/Plagioselmis/Geminigera group via their ciliate prey Mesodinium rubrum, but other cryptophyte and algal sources have occasionally been found. Identifying the specific prey of ciliates fed upon by mixotrophic Dinophysis species is a requisite to improve predictive capabilities of their bloom formation. Here we examined the origin of Dinophysis plastids from Galician waters and their transfer in cross-feeding experiments in the laboratory. Plastid 23S rDNA sequences were obtained from 60 Dinophysis specimens from the Galician Rías Baixas and shelf waters. Most sequences in Dinophysis cells were identical to Teleaulax amphioxeia. Galician shelf samples also yielded T. amphioxeia-type sequences, although one of these was closer to a freshwater cryptophyte, and a few others were related with other taxa (diatoms, red algae and proteobacteria). Mesodinium cf. major, an alternative prey to M. rubrum, was identified. Crossfeeding tests in the laboratory showed that T. amphioxeia, T. minuta, T. gracilis, and Plagioselmis prolonga sustained growth of M. rubrum. D. acuminata cultivated on a M. rubrum-T. amphioxeia system was transferred to M. rubrum fed upon T. minuta, T. gracilis and P. prolonga. After > 2 mo of acclimation, T. amphioxeia plastid 23S rDNA and psbA gene sequences from D. acuminata were replaced by those of secondary cryptophytes. Here we confirm 2 cryptophytes, T. minuta and P. prolonga, as suitable prey for M. rubrum. Nevertheless, field and laboratory results show that, at least for D. acuminata, T. amphioxeia represents the main source of plastids. KEY WORDS: DinophysisrDNA • psbA gene Resale or republication not permitted without written consent of the publisher *D. caudata P2-21/11/11 *D. acuminata ST28-07/03/13 D. acuminata P2-13/06/11 *D. acuminata ST28-07/03/13 *D. caudata P2-21/11/11 *D. acuminata ST28-07/03/13 D. acuminata P2-13/06/11 *D. acuminata P2-28/05/12 *D. acuminata ST28-07/03/13 *D. acuminata P2-10/10/11 D. acuminata P2-11/07/11 *D. acuminata ST28-07/03/13 *D. caudata P2-21/11/11 *D. acuminata ST28-07/03/13 D. acuminata P2-08/08/11 *D. acuminata ST28-07/03/13 *D. forƟi ST8-07/03/13 *D. forƟi ST8-07/03/13 *D. acuta P2-21/11/11 D. acuminata P2-08/08/11 *D. caudata P2-21/11/11 *D. forƟi ST8-07/03/13 D. acuminata P2-11/07/11 *D. acuminata P2-10/10/11 D. acuminata P2-08/08/11 D. acuminata P2-28/05/12 *D. forƟi ST8-07/03/13 *D. acuminata+T.amphioxeia *D. acuminata+T.amphioxeia *D. caudata P2-21/11/11 *D. acuminata+T.amphioxeia *D. acuminata+T.amphioxeia *D. caudata P2-21/11/11 *D. acuminata+T.gracilis D. sacculus+P.prolonga D. caudata P2-26/09/11 D. caudata P2-28/06/10 *D. caudata P2-21/11/11 D. acuminata P2-16/03/11 *D. acuminata+T.amphioxeia D. caudata P2-28/06/10 D. tripos P2-28/06/10 D. tripos P2-28/06/10 Teleaulax amphioxeia AND-AO710 Mesodinium rubrum AND-AO711 P.nannoplancƟca JX470950.1 Plagioselmis prolonga CR10EHU *D. acuta ST8-07/03/13 *D. acuminata+P.prolonga *D. acuminata+P.pr...

Marine cyanobacteria of the genera Synechococcus and Prochlorococcus are the most abundant photosynthetic organisms on earth, spanning vast regions of the oceans and contributing significantly to global primary production. Their viruses (cyanophages) greatly influence cyanobacterial ecology and evolution. Although many cyanophage genomes have been sequenced, insight into the functional role of cyanophage genes is limited by the lack of a cyanophage genetic engineering system. Here, we describe a simple, generalizable method for genetic engineering of cyanophages from multiple families, that we named REEP for REcombination, Enrichment and PCR screening. This method enables direct investigation of key cyanophage genes, and its simplicity makes it adaptable to other ecologically relevant host-virus systems. T7-like cyanophages often carry integrase genes and attachment sites, yet exhibit lytic infection dynamics. Here, using REEP, we investigated their ability to integrate and maintain a lysogenic life cycle. We found that these cyanophages integrate into the host genome and that the integrase and attachment site are required for integration. However, stable lysogens did not form. The frequency of integration was found to be low in both lab cultures and the oceans. These findings suggest that T7-like cyanophage integration is transient and is not part of a classical lysogenic cycle.

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