File1: VERDI_samples_parameters.xlsx
- Physico-chemical variables obtained from CTD profile
- Inorganic nutrients concentrations
- Chlorophyll-a concentrations
- Organic carbon and nitrogen concentrations
- Phytoplankton abundances
- Detections of chytrids
File 2: VERDI_asv_table.tbl: ASV abundances from natural samples and incubations
File 3: VERDI_tax_table.tbl: Taxonomic assignments of ASVs
File 4: VERDI_asv_seqs.fa: Sequences of ASVs
File 5: VERDI_incubations_images.zip
- Compilation of images taken during incubations with diatoms, The presence of phytoplankton parasites along the water column was explored at the Long Term Ecological Station MareChiara (LTER-MC) in the Gulf of Naples (Mediterranean Sea) in October 2019. Microscopy analyses showed diatoms dominating the phytoplankton community in the upper layers (0-20 m). Metabarcoding data from the water column showed the presence of Chytridiomycota predominantly in the upper layers coinciding with the vertical distribution of diatoms. Laboratory incubations of natural samples enriched with different diatom cultures confirmed parasitic interactions of some of those chytrids – including members of Kappamyces – with diatom taxa. The temporal dynamics of diatoms and chytrids was also explored in a three-year metabarcoding time-series (2011-2013) from surface waters of the study area and in sediment samples. Chytrids were recurrently present at low relative abundances, and some taxa found to infect diatoms in the incubation experiments were also identified in the ASV time-series. However, co-occurrence analyses did not show any clear or recurrent pairing patterns for chytrid and diatom taxa along the three years. The chytrid community in the sediments showed a clearly different species composition compared to the recorded in the water column samples, with higher diversity and relative abundance. The combination of observations, incubations and metabarcoding confirmed that parasites are a common component of marine protist communities at LTER-MC. Host-parasite interactions must be determined and quantified to understand their role and the impact they have on phytoplankton dynamics, - European Union’s Horizon 2020 research and innovation programme under grant agreement No 730984, ASSEMBLE Plus project.
- Spanish MICINN Project SMART (PID2020-112978GB-I00)
- The research program LTER-MC is funded by the Stazione Zoologica Anton Dohrn, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), - Physico-chemical variables obtained from CTD profile - Inorganic nutrients concentrations - Chlorophyll-a concentrations - Organic carbon and nitrogen concentrations - Phytoplankton abundances - Detections of chytrids - Metabarcoding ASV abundances from natural samples and incubations - Metabarcoding Taxonomic assignments of ASVs - Metabarcoding Sequences of ASVs - Compilation of images taken during incubations with diatoms, Peer reviewed

Special issue Marine Benthic Harmful Algal Blooms.-- 11 pages, 5 figures, supplementary materials https://doi.org/10.1016/j.hal.2022.102352.-- Data availability: Data will be made available on request, A new chytrid genus and species was isolated and cultured from samples obtained in the Baltic Sea during a dinoflagellate bloom event. This species is characterized by having a spherical sporangium without papillae and zoospores of 2–3 µm in diameter that are released through 3 discharge pores. Molecular phylogeny based on ribosomal operon showed its sister position to the Dinomyces cluster in Rhizophydiales. Zoospores lack fenestrated cisternae but contain a paracrystalline inclusion, found in a Rhizophydiales representative for the first time. Additionally, the kinetid features are uncommon for Rhizophydiales and only observed in Dinomyces representatives so far. These morphological features and its phylogenetic relationships justify the description of the new genus and speciesParadinomyces triforaminorum gen. nov. sp. nov. belonging to the family Dinomycetaceae. The chytrid was detected during a high-biomass bloom of the dinoflagellate Kryptoperidinium foliaceum. Laboratory experiments suggest this species is highly specific and demonstrate the impact it can have on HAB development. The chytrid co-occurred with three other parasites belonging to Chytridiomycota (Fungi) and Perkinsea (Alveolata), highlighting that parasitic interactions are common during HABs in brackish and marine systems, and these multiple parasites compete for similar hosts, AR and EG were funded by the Spanish MICINN Project SMART PID2020-112978GB-I00. This work was partly supported by the Russian Science Foundation grant no. 21–74–20089 (writing, microscopy analysis), the infrastructure of SAK's laboratory was supported by ZIN RAS program 1021051402849–1, and the C7 strain was incorporated in CCPP ZIN RAS collection supported by a grant 075–15–2021–1069 of the Ministry of Science and Higher Education of the Russian Federation. AK was supported by the Academy of Finland, grant 310,449. MK was supported by JSPS KAKENHI 19H05667. EA was supported by The Royal Society (UK) under the Newton International Fellowship (grant number NF170346, call 2017), With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Peer reviewed

15 pages, 7 figures, 1 table, supporting Information https://doi.org/10.1111/1462-2920.16182.-- Data Availability Statement: DNA sequences data from cultures have been submitted to the GenBank databases under accession number ON081632-ON081642 (SSU rDNA) and ON113503-ON113504 (LSU rDNA Dinomyces arenysensis). Raw data used in the study are available in NCBI SRA PRJNA630546, The interactions of parasitic fungi with their phytoplankton hosts in the marine environment are mostly unknown. In this study, we evaluated the diversity of Chytridiomycota in phytoplankton communities dominated by dinoflagellates at several coastal locations in the NW Mediterranean Sea and demonstrated the most prominent interactions of these parasites with their hosts. The protist community in seawater differed from that in sediment, with the latter characterized by a greater heterogeneity of putative hosts, such as dinoflagellates and diatoms, as well as a chytrid community more diverse in its composition and with a higher relative abundance. Chytrids accounted for 77 amplicon sequence variants, of which 70 were found exclusively among different blooming host species. The relative abundance of chytrids was highest in samples dominated by the dinoflagellate genera Ostreopsis and Alexandrium, clearly indicating the presence of specific chytrid communities. The establishment of parasitoid-host co-cultures of chytrids and dinoflagellates allowed the morphological identification and molecular characterization of three species of Chytridiomycota, including Dinomyces arenysensis, as one of the most abundant environmental sequences, and the discovery of two other species not yet described, This study was funded by the Spanish MICINN Project SMART PID2020-112978GB-I00, the MINECO Project COPAS CTM2017-86121-R. The authors acknowledge the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). Alan Denis Fernández-Valero is funded by the MICIU grant ‘Ayudas para contratos predoctorales para la formación de doctores 2018 (PRE2018-084893)’, Peer reviewed

7 pages, 5 figures, supplementary materials https://doi.org/10.1016/j.hal.2023.102389.-- Data availability: Data will be made available on request, Extracellular Vesicles (EVs) are likely an important strategy of transport and communication in marine microbial community. Their isolation and characterization from axenic culture of microbial eukaryotes represents a technological challenge not fully solved. Here, for the first time, we isolated EVs from a near-axenic culture of the toxic dinoflagellate Alexandrium minutum. Pictures of the isolated vesicles were done with Cryo TEM (Cryogenic Transmission Electron Microscopy). Based on their morphotype the EVs were clustered in five major groups (rounded, rounded electron-dense, lumen electron-dense, double and irregular) and each EV was measured resulting in an average size of 0.36 µm of diameter. Taking in account that in prokaryotes it has been demonstrated that EVs play an important role in the mechanism of toxicity, this descriptive work aims to be the first step to study the possible role of EVs in the toxicity of dinoflagellates, We thank the Spanish MICINN Project SMART (PID2020-112978GB-I00), and the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019- 000928-S), Peer reviewed

16 pages, 7 figures, 1 table, supporting information https://doi.org/10.1111/maec.12726.-- Data Availability Statement: Data generated in this study including environmental parameters, phytoplankton counts, metabarcoding final datasets and results, images and videos generated from incubation experiments are deposited in the public repository DIGITAL.CSIC, accessible by the DOI 10.20350/digitalCSIC/14744, The presence of phytoplankton parasites along the water column was explored at the Long-Term Ecological Station MareChiara (LTER-MC) in the Gulf of Naples (Mediterranean Sea) in October 2019. Microscopy analyses showed diatoms dominating the phytoplankton community in the upper layers (0–20 m). Metabarcoding data from the water column showed the presence of Chytridiomycota predominantly in the upper layers coinciding with the vertical distribution of diatoms. Laboratory incubations of natural samples enriched with different diatom cultures confirmed parasitic interactions of some of those chytrids—including members of Kappamyces—with diatom taxa. The temporal dynamics of diatoms and chytrids was also explored in a 3-year metabarcoding time-series (2011–2013) from surface waters of the study area and in sediment samples. Chytrids were recurrently present at low relative abundances, and some taxa found to infect diatoms in the incubation experiments were also identified in the ASV time-series. However, co-occurrence analyses did not show any clear or recurrent pairing patterns for chytrid and diatom taxa along the 3 years. The chytrid community in the sediments showed a clearly different species composition compared with that recorded in the water column samples, with higher diversity and relative abundance. The combination of observations, incubations, and metabarcoding confirmed that parasites are a common component of marine protist communities at LTER-MC. Host–parasite interactions must be determined and quantified to understand their role and the impact they have on phytoplankton dynamics, The research leading to these results received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement No 730984, ASSEMBLE Plus Project. AR and EG were funded by the Spanish MICINN Project SMART (PID2020-112978GB-I00), and thank the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019–000928-S). The research program LTER-MC is funded by the Stazione Zoologica Anton Dohrn, Peer reviewed

11 pages, 4 figures, 1 table, supplementary information https://doi.org/10.1038/s43705-023-00304-x.-- Data Availability: The datasets presented in this study are available in the NCBI Sequence Read Archive under Bioproject numbers PRJNA891539, PRJNA891506, and are included in the supplementary datasets of this published article, This study aims to investigate the temporal dynamics of the epiphytic protist community on macroalgae, during the summer months, with a specific focus on fungi, and the interactions between zoosporic chytrid parasites and the proliferation of the dinoflagellates. We employed a combination of environmental sequencing techniques, incubation of natural samples, isolation of target organisms and laboratory experiments. Metabarcoding sequencing revealed changes in the dominant members of the epiphytic fungal community. Initially, fungi comprised < 1% of the protist community, mostly accounted for by Basidiomycota and Ascomycota, but with the emergence of Chytridiomycota during the mature phase of the biofilm, the fungal contribution increased to almost 30%. Chytridiomycota became dominant in parallel with an increase in the relative abundance of dinoflagellates in the community. Microscopy observations showed a general presence of chytrids following the peak proliferation of the dinoflagellate Ostreopsis sp., with the parasite, D. arenysensis as the dominant chytrid. The maximum infection prevalence was 2% indicating host-parasite coexistence. To further understand the in-situ prevalence of chytrids, we characterised the dynamics of the host abundance and prevalence of chytrids through co-culture. These laboratory experiments revealed intraspecific variability of D. arenysensis in its interaction with Ostreopsis, exhibiting a range from stable coexistence to the near-extinction of the host population. Moreover, while chytrids preferentially parasitized dinoflagellate cells, one of the strains examined displayed the ability to utilize pollen as a resource to maintain its viability, thus illustrating a facultative parasitic lifestyle. Our findings not only enrich our understanding of the diversity, ecology, and progression of epiphytic microalgal and fungal communities on Mediterranean coastal macroalgae, but they also shed light on the presence of zoosporic parasites in less-explored benthic habitats, This study was funded by the Spanish Ministry of Science, Innovation, and Universities (MICINN) Project SMART PID2020-112978GB-I00, with institutional support from the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S). [...] ADF-V is funded by the MICIU grant “Ayudas para contratos predoctorales para la formación de doctores 2018 (PRE2018-084893)”, Peer reviewed

12 pages, 5 figures, 2 tables, supplementary data https://doi.org/10.1016/j.ejop.2024.126053.-- Data availability statement: Molecular sequences generated in this study are publicly available in GenBank. Phylogenetic alignments analysed in the present study are available from the corresponding author upon request, We identified two new parasite species of Chytridiomycota isolated during blooms of the dinoflagellate Alexandrium minutum in the coastal Mediterranean Sea. Light and electron microscopy together with molecular characterization of the nuclear 18S, ITS, and 28S rDNA regions led to their identification as two new species, Dinomyces gilberthii and Paradinomyces evelyniae, both belonging to the family Dinomycetaceae, order Rhizophydiales. Dinomyces gilberthii differs from the previously described D. arenysensis by the presence of discharge papillae and the development of a drop-shaped sporangium. Paradinomyces evelyniae differs from the previously described P. triforaminorum by the prominent lipid globule present in early sporangia and by the pointed end producing a rhizoid. The two chytrids differed in their geographical distribution. Dinomyces gilberthii was detected in several Mediterranean habitats, including harbours and beaches, and was particularly prevalent during summer dinoflagellate blooms. Its widespread occurrence in coastal ecosystems suggested a high level of adaptability to this environment. Paradinomyces evelyniae had a more restricted distribution in the coastal-marine environment, occurring in harbour sediments and only occasionally in the water column during winter and early spring. Paradinomyces evelyniae has previously been detected in the Baltic Sea, suggesting that its distribution encompasses contrasting coastal environments, although its presence is rare, This study was funded by the Spanish MICINN Project SMART PID2020-112978GB-I00, with institutional support from the “Severo Ochoa Centre of Excellence” (CEX2019-000928-S). A. D. Fernández-Valero was funded by the MICIU grant “Ayudas para contratos predoctorales para la formación de doctores 2018” (PRE2018-084893). This work was partly supported by the Russian Science Foundation, grant no. 21–74–20089 (light and electron microscopy analysis, manuscript writing). SAK thanks Saint-Petersburg State University for a research grant (project ID Pure 115624290) and ZIN RAS program 122031100260-0, Peer reviewed

Memoria de tesis doctoral presentada por Alan Denis Fernández Valero para obtener el título de Doctor en Ciéncies Marines por la Universitat de Barcelona (UB), realizada bajo la dirección de la Dra. Esther Garcés Pieres y del Dr. Albert Reñé Vicente del Institut de Ciències del Mar (ICM-CSIC).-- 165 pages, [EN] Current knowledge of marine fungi is limited, marked by gaps in the understanding of their diversity, spatio-temporal patterns, and ecological roles within marine ecosystems. The underexplored nature of specific fungal groups, like chytrids occurring during phytoplankton blooms, and the lack of research in most marine regions, such as the Mediterranean Sea, emphasise the need for this thesis.
Consequently, the main objective of this doctoral thesis is to investigate the composition of the fungal community in the Mediterranean coastal ecosystem, with a focus on the basal fungal group Chytridiomycota (chytrids). The study sets for specific objectives: i) to evaluate the spatial distribution of Chytridiomycota diversity in various coastal areas during dinoflagellate proliferations; ii) to evaluate the temporal dynamics of the fungal community in epiphytic communities during benthic dinoflagellate proliferations and to investigate their interaction with hosts; and iii) to identify and describe new parasitic Chytridiomycota species associated with
their host proliferations.
The chytrid community in coastal environments is particularly dynamic and diverse, with a marked specificity towards their preferred hosts. These communities are more diverse and abundant in sediments and on macroalgae surfaces than in the water column. The relative abundance of chytrids increases in the water column during dinoflagellate proliferations, suggesting their opportunistic parasitic behaviour. In the laboratory, strains of four chytrid species, including the most abundant species detected by metabarcoding sequencing, have been cultured and identified by co-culture techniques with dinoflagellates. Three strains correspond to as yet undescribed species and the cultures have allowed the morphological and molecular characterisation of two of them, Dinomyces gilberthii and Paradinomyces evelyniae, which have been newly described in this thesis. In terms of geographic distribution of species, Dinomyces gilberthii is widely distributed in coastal Mediterranean habitats particularly during summer dinoflagellate blooms. Conversely, species like Paradinomyces evelyniae
exhibit a more limited distribution in these habitats. Investigations into the temporal dynamics of the fungal community on poorly explored niches such as biofilms in macroalgae, revealed an alternation in the abundance of fungal groups, significantly in uenced by the epiphytic dinoflagellates dynamics. Dominant chytrid species in these communities, such as Dinomyces arenysensis, exhibit adaptability in their survival strategies, including trophic versatility.
The findings demonstrate that chytrids play a significant role in the structuring of planktonic and benthic microbial communities in the coastal ecosystems, [ES] El conocimiento actual sobre los hongos marinos es limitado, marcado por la falta de conocimiento de su diversidad, patrones espacio-temporales y funciones ecológicas en los ecosistemas marinos. El desconocimiento sobre algunos grupos fúngicos específicos, como los quítridos presentes en proliferaciones fitoplanctónicas, y la falta de investigación en algunas regiones marinas, como el Mar Mediterráneo, enfatizan la necesidad de esta tesis. [...], Alan Denis Fernández Valero was supported by a predoctoral fellowship from the Spanish government (PRE2018-084893). The research was funded by projects COPAS (CTM2017-86121-R) and SMART (PID2020-112978GB-I00) from the Spanish government. The Severo Ochoa Centre of Excellence accreditation (CEX2019-000928-S) from AEI 10.13039/501100011033 is acknowledged, Peer reviewed

15 pages, 5 figures, 1 table, supporting information https://doi.org/10.1111/1758-2229.13299.-- Data Availability: The data that support the findings of this study are openly available in the NCBI repository at https://www.ncbi.nlm.nih.gov/bioproject/PRJEB38773. All code is available on GitHub: https://github.com/adriaaulaICM/bbmo_timeseries_fraction, Marine microbial communities differ genetically, metabolically, and ecologically according to their lifestyle, and they may respond differently to environmental changes. In this study, we investigated the seasonal dynamics of bacterial assemblies in the free-living (FL) and particle-associated (PA) fractions across a span of 6 years in the Blanes Bay Microbial Observatory in the Northwestern Mediterranean. Both lifestyles showed marked seasonality. The trends in alpha diversity were similar, with lower values in spring–summer than in autumn-winter. Samples from both fractions were grouped seasonally and the percentage of community variability explained by the measured environmental variables was comparable (32% in FL and 31% in PA). Canonical analyses showed that biotic interactions were determinants of bacterioplankton dynamics and that their relevance varies depending on lifestyles. Time-decay curves confirmed a high degree of predictability in both fractions. Yet, ‘seasonal’ Amplicon Sequence Variants (ASVs) (as defined by Lomb Scargle time series analysis) in the PA communities represented 46% of the total relative abundance while these accounted for 30% in the FL fraction. These results demonstrate that bacteria inhabiting both fractions exhibit marked seasonality, highlighting the importance of accounting for both lifestyles to fully comprehend the dynamics of marine prokaryotic communities, This work was supported by grants MICOLOR (PID2021-125469NB-C31) to JMG, SMART (PID2020-112978GB-I00) to EG, DIVAS (PID2019-108457RB-I00) to RM, and ECLIPSE (PID2019-110128RB-I00) to IF, all funded by the Agencia Estatal de Investigación from the Spanish Ministry of Science and Innovation. Authors from the ICM also had the institutional support of the Severo Ochoa Centre of Excellence (CEX2019-000928-S). We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative, Peer reviewed

21 pages, 8 figures, supporting information https://doi.org/10.1111/mec.17666.-- Data Availability Statement: Raw metagenomic reads were submitted to the NCBI Sequence Read Archive under Bioproject numbers PRJNA891539 (Fernández-Valero et al. 2023).-- This is the peer reviewed version of the following article: Elisabet Alacid, Albert Reñé, Natàlia Timoneda, Esther Garcés, Molecular Ecology: e17666 (2025), which has been published in final form athttps://doi.org/10.1111/mec.17666. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibite, Marine macroalgae surfaces create a nutrient-rich environment that promotes the formation of epiphyte biofilms. Biofilms are complex systems that facilitate ecological interactions within the community, yet parasitism remains largely unexplored. This study describes the diversity and temporal dynamics of the microeukaryotic community in the biofilm of Mediterranean macroalgae during summer, focusing on parasitic groups. Protist diversity was assessed using metabarcoding sequencing of the V4 region of the 18S rDNA gene using primers biased against metazoans. The macroalgal biofilm exhibited dynamic shifts in the microeukaryotic community structure associated to three phases of biofilm formation. Each phase was characterised by the dominance of specific eukaryotic and parasitic groups with clear successions between them. Our study revealed a high diversity of parasitic protists from different lineages in the macroalgal biofilm. These parasites can infect a wide variety of hosts, including the basibiont, species within the biofilm (micro- and macrocolonizers), nearby marine hosts and terrestrial organisms. The highest diversity and abundance of parasites were found in the mature phase of the biofilm, where the complexity and stability of the system seem to favour parasitism. The parasite assemblage was dominated by Apicomplexa, with many corresponding to unknown diversity, demonstrating that biofilms are a hotspot of unknown parasitic interactions. These parasites could potentially affect the dynamics of these communities and facilitate ecological interactions between the biofilm and surrounding organisms, suggesting that parasitism play a key, but still unexplored role, in shaping complex marine biofilms network, This study was funded by the Spanish Ministry of Science, Innovation, and Universities (MICINN) Project SMART PID2020-112978GB-I00, with institutional support from the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). It also contributes to the state-of-the-art objectives of the Horizon Europe GES4SEAS project HORIZON-CL6-2021-BIODIV-01-04. [...] E. Alacid was supported by the Beatriu de Pinós Postdoctoral Program (grant number BP2020-00174) of the Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya, Peer reviewed

20 pages, 5 figures, 1 table.-- Data Availability Statement: All datasets generated can be accessed in GitHub, https://github.com/ParAqua-COST/ParAquaSeq_Repository, including sequence files, metadata, explanatory documents, and scripts used to access the information. All datasets follow FAIR principles, and they are stored at Zenodo under the DOI: https://doi.org/10.5281/zenodo.14851382 (Van den Wyngaert et al. 2024), Amplicon sequencing tools such as metabarcoding are commonly used for thorough characterisation of microbial diversity in natural samples. They mostly rely on the amplification of conserved universal markers, mainly ribosomal genes, allowing the taxonomic assignment of barcodes. However, linking taxonomic classification with functional traits is not straightforward and requires knowledge of each taxonomic group to confidently assign taxa to a given functional trait. Zoosporic parasites are highly diverse and yet understudied, with many undescribed species and host associations. However, they can have important impacts on host populations in natural ecosystems (e.g., controlling harmful algal blooms), as well as on industrial-scale algae production, e.g. aquaculture, causing their collapse or economic losses. Here, we present ParAquaSeq, a curated database of available molecular ribosomal sequences belonging to zoosporic parasites infecting aquatic vascular plants, macroalgae and photosynthetic microorganisms, i.e. microalgae and cyanobacteria. These sequences are aligned with ancillary data and other information currently available, including details on their hosts, occurrence, culture availability and associated bibliography. The database includes 1131 curated sequences from marine, freshwater and industrial or artificial environments, and belonging to 13 different taxonomic groups, including Chytridiomycota, Oomycota, Phytomyxea, and Syndiniophyceae. The curated database will allow a comprehensive analysis of zoosporic parasites in molecular datasets to answer questions related to their occurrence and distribution in natural communities. Especially through meta-analysis, the database serves as a valuable tool for developing effective mitigation and sustainable management strategies in the algae biomass industry, but it will also help to identify knowledge gaps for future research, A.R. acknowledges the institutional support of the grant ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) funded by AEI 10.13039/501100011033. S.V.W was supported by the Research Council of Finland (Decision Numbers 340659 and 346387). I.T. and M.S. were supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grant no. 451-03-66/2024-03/200178). A.T. has been supported by the project “LifeWatchPLUS” of the “Programma Operativo Nazionale (PON) Ricerca e Innovazione 2014-2020 - Azione II, potenziamento di Infrastrutture di Ricerca” of the Italian Ministry of University and Research. B.S. was supported by the Icelandic Research Fund in the frame of the project “Per- and poly-fluoroalkyl substances (PFASs) meet Arctic and sub-Arctic marine diatoms and their associated symbionts” (RANNIS ID: 239548-051). H.-P.G. received funding by the German Science Foundation (DFG) through the projects MostFun (GR1540/51-1), FUNACTION (GR1540/47-1), and Polar Parasites (GR1540/33-1 and GR1540/48-1). G.B.S was supported by the National Laboratory for Water Science and Water Security program (RRF-2.3.1-21-2022-00008) and NKFIH KKP 144068. S.N. received funding by the Austrian Science Fund (FWF) Y0801-B16. L.G. received fundings by the Ministry of Universities and Research (Italy) through the Biodiversa+ secretariat, co-funded by the European Commission, projects FUNACTION (GA no. 101052342) and MostFun, and with the funding organisations Ministry of Universities and Research (Italy), and the European Commission—NextGenerationEU, Project “Strengthening the MIRRI Italian Research Infrastructure for Sustainable Bioscience and Bioeconomy”, code n. IR0000005. D.S.S. received funding through the projects GloMEc (AXA Research Fund), FishME (ANR-21-BIRE-0002-01), and OneAquaHealth (EC-Project 101086521). N.T. was funded by the Spanish MICINN Project SMART PID2020-112978GB-I00, Peer reviewed

18 pages, 7 figures, 1 table, supplementary data https://doi.org/10.1093/femsec/fiaf081, Zoosporic parasites significantly influence aquatic ecosystems by infecting various phytoplankton taxa, but their interactions in brackish ecosystems remain largely unexplored. This study explores microbial communities and parasitic interactions with summer phytoplankton communities at six brackish coastal sites in the northern Baltic Sea. We hypothesized that small-scale spatial heterogeneity in environmental conditions would lead to distinct assemblages of microbial communities and phytoplankton–parasite interactions. By combining DNA metabarcoding, single-cell sequencing, and microscopy, we provide the first community-level qualitative and quantitative assessment of zoosporic parasites infecting summer phytoplankton in the Baltic Sea. Microbial communities varied significantly across sites, with salinity as primary driver of eukaryotic diversity. Chytrid fungi were the dominant parasites, infecting green algae, diatoms, and filamentous cyanobacteria, with infection rates up to 5.8% of phytoplankton biomass. Sequences from brackish chytrids clustered with those from freshwater environments, reflecting polyphyletic patterns linked to host taxa. Phytoplankton–parasite interactions were influenced by host abundance and site-specific conditions with correlation analysis suggesting broader host ranges and potential generalist behavior in some chytrid species. Additionally, an unidentified oomycete infected up to 85% of the toxic bloom-forming cyanobacterium Nodularia spp. This study highlights the ecological relevance of zoosporic parasites in the Baltic Sea and emphasizes the need for further research into their role in phytoplankton bloom dynamics, This work was supported by funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 730984, Assemble Plus project. This work was further supported by the German Research Foundation (projects WY175/1-1 to S.V.d.W. and WU890/2-1 to C.W.); the Research Council of Finland (340659 and 346387 to S.V.d.W.); the Beatriu de Pinós postdoctoral program of the Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya (grant number BP2020-00174 to E.A.); the Spanish Ministry of Science, Innovation, and Universities (MICINN) (Project SMART PID2020-112978GB-I00 to E.G. and A.R.) with institutional support from the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S), Peer reviewed

This study describes the diversity and temporal dynamics of the microeukaryotic community in the biofilm of Mediterranean macroalgae during summer, focusing on parasitic groups, This study was funded by the Spanish Ministry of Science, Innovation, and Universities (MICINN) Project SMART PID2020-112978GB-I00, with institutional support from the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). It also contributes to the state-of-the-art objectives of the Horizon Europe GES4SEAS project HORIZON-CL6-2021-BIODIV-01-04. High-performance computing analyses were run at the Marine Bioinformatics Service (MARBITS) of the Institut de Ciències del Mar (ICM-CSIC) in Barcelona. E. Alacid was supported by the Beatriu de Pinós Postdoctoral Program (grant number BP2020-00174) of the Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya, Peer reviewed

A.R. acknowledges the institutional support of the grant ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) funded by AEI 10.13039/501100011033. S.V.W was supported by the Research Council of Finland (Decision Numbers 340659 and 346387). I.T. and M.S. were supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grant no. 451-03-66/2024-03/200178). A.T. has been supported by the project “LifeWatchPLUS” of the “Programma Operativo Nazionale (PON) Ricerca e Innovazione 2014-2020 - Azione II, potenziamento di Infrastrutture di Ricerca” of the Italian Ministry of University and Research. B.S. was supported by the Icelandic Research Fund in the frame of the project “Per- and poly-fluoroalkyl substances (PFASs) meet Arctic and sub-Arctic marine diatoms and their associated symbionts” (RANNIS ID: 239548-051). H.-P.G. received funding by the German Science Foundation (DFG) through the projects MostFun (GR1540/51-1), FUNACTION (GR1540/47-1), and Polar Parasites (GR1540/33-1 and GR1540/48-1). G.B.S was supported by the National Laboratory for Water Science and Water Security program (RRF-2.3.1-21-2022-00008) and NKFIH KKP 144068. S.N. received funding by the Austrian Science Fund (FWF) Y0801-B16. L.G. received fundings by the Ministry of Universities and Research (Italy) through the Biodiversa+ secretariat, co-funded by the European Commission, projects FUNACTION (GA no. 101052342) and MostFun, and with the funding organisations Ministry of Universities and Research (Italy), and the European Commission—NextGenerationEU, Project “Strengthening the MIRRI Italian Research Infrastructure for Sustainable Bioscience and Bioeconomy”, code n. IR0000005. D.S.S. received funding through the projects GloMEc (AXA Research Fund), FishME (ANR-21-BIRE-0002-01), and OneAquaHealth (EC-Project 101086521). N.T. was funded by the Spanish MICINN Project SMART PID2020-112978GB-I00, Peer reviewed

11 pages, 4 figures, 1 table, supplemental material https://doi.org/10.1080/00275514.2025.2577604.-- Data Availability Statement: The new species rDNA sequence (18S-ITS1-5.8S-ITS2-28S) is available in GenBank under the accession number LC877790. All data sets generated can be found in the Supplementary Material (https://doi.org/10.20350/digitalCSIC/18104), including the list of primers used for sequencing and the phylogenetic sequence list data sets with corresponding accession numbers. The alignments created for the phylogenetic analysis of this species are available as a FASTA file on GitHub (https://github.com/nuriapousola/Algophthora-mediterranea.git), This study describes a novel parasitic fungus that infects the benthic and epiphytic dinoflagellate Ostreopsis cf. ovata during a coastal microalgal bloom in the Mediterranean Sea. Microscopic observations revealed a distinctive, irregularly shaped zoosporangium during the mature stages and spherical, posteriorly uniflagellate zoospores. This supports its affiliation within the phylum Chytridiomycota. Concatenated phylogenetic analysis based on 18S, 5.8S, and 28S ribosomal DNA placed the fungus within the order Lobulomycetales, thus establishing it as a distinct lineage separate from previously described species. Additional phylogenetic analyses including environmental DNA sequences revealed a close phylogenetic relationship with previously reported freshwater sequences. This suggests a possible ecological link between marine and freshwater habitats. Cross-infection experiments confirmed the ability of the fungus to infect healthy cells of both dinoflagellate and diatom species, rendering it the first known chytrid with a broad phytoplankton host range. Additionally, it is the first member of this order known to parasitize dinoflagellate species and only the second known to parasitize marine algae. Infection prevalence was higher in dinoflagellates than in diatoms. Furthermore, under laboratory conditions, the chytrid also developed zoosporangia on pollen grains, using them as an alternative nutrient source. Based on these findings, this study describes a new genus and species of zoosporic fungus, Algophthora mediterranea, within the order Lobulomycetales, This work was supported by fundings from Spanish MICINN Project DIVINE [PID2023-150672NB-I00]; and Project SMART [PID2020-112978GB-I00], with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation [CEX2019– 000928- S]. The Japan Science and Technology Agency [JPMJCR24J2]; Japan Society for the Promotion of Science [JSPS KAKENHI 19H05667]; and Japan Society for the Promotion of Science [JSPS KAKENHI 24K18177], Peer reviewed

All data sets generated can be found in the Supplementary Material, including the list of primers used for sequencing and the phylogenetic sequence list data sets with corresponding accession numbers, This work was supported by fundings from Spanish MICINN Project DIVINE [PID2023-150672NB-I00]; and Project SMART [PID2020-112978GB-I00], with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation [CEX2019– 000928- S]. The Japan Science and Technology Agency [JPMJCR24J2]; Japan Society for the Promotion of Science [JSPS KAKENHI 19H05667]; and Japan Society for the Promotion of Science [JSPS KAKENHI 24K18177], Peer reviewed