, All authors approved the final manuscript, CMMG supervised the experiments; BC, MMP and CMMG wrote the manuscript, with contributions of all other authors, vol.1004, pp.1005-1006

R. Chandra, H. M. Iqbal, G. Vishal, H. Lee, and S. Nagra, Algal biorefinery: A sustainable approach to valorize algal-based biomass towards multiple product recovery, Bioresource Technol, 2019.

K. Sudhakar, R. Mamat, M. Samykano, W. H. Azmi, W. F. Ishak et al., An overview of marine macroalgae as bioresource, Renew Sust Energ Rev, vol.91, pp.165-179, 2018.

J. M. Cock, L. Sterck, P. Rouze, D. Scornet, A. E. Allen et al.,

C. Peters, P. Pommier, J. Potin, H. Poulain, B. Quesneville et al., The Ectocarpus genome and the independent evolution of multicellularity in brown algae, Nature, pp.617-621, 2010.
URL : https://hal.archives-ouvertes.fr/cea-00906990

J. Collen, B. Porcel, W. Carre, S. G. Ball, C. Chaparro et al., Genome structure and metabolic features in the red seaweed Chondrus crispus shed light on evolution of the Archaeplastida, vol.110, pp.5247-5252, 2013.
URL : https://hal.archives-ouvertes.fr/hal-01073830

N. Ye, X. Zhang, M. Miao, X. Fan, Y. Zheng et al.,

P. Shi, D. Ji, Z. Li, C. Guan, Z. Shao et al., Saccharina genomes provide novel insight into kelp biology, vol.6, p.6986, 2015.

E. K. Hwang, F. L. Liu, K. H. Lee, D. S. Ha, and C. S. Park, Comparison of the cultivation performance between Korean (Sugwawon No. 301) and Chinese strains (Huangguan No. 1) of kelp Saccharina japonica in an aquaculture farm in Korea, Algae, vol.33, pp.101-108, 2018.

R. Loureiro, C. M. Gachon, and C. Rebours, Seaweed cultivation: potential and challenges of crop domestication at an unprecedented pace, New Phytol, pp.489-492, 2015.

N. Robinson, P. Winberg, and L. Kirkendale, Genetic improvement of macroalgae: status to date and needs for the future, Journal of Applied Phycology, vol.25, pp.703-716, 2013.

M. Valero, M. Guillemin, C. Destombe, B. Jacquemin, C. M. Gachon et al., Perspectives on domestication research for sustainable seaweed aquaculture, Perspectives in Phycology, pp.33-46, 2017.

J. Zhang, T. Liu, R. Feng, C. Liu, and S. Chi, Genetic map construction and Quantitative Trait Locus (QTL) detection of six economic traits using an F2 population of the hybrid from Saccharina longissima and Saccharina japonica, PLoS One, vol.10, p.128588, 2015.

J. L. Collier, Flow cytometry and the single cell in phycology, Journal of Phycology, vol.36, pp.628-644, 2000.

L. Lefebvre-legendre, Y. Choquet, R. Kuras, S. Loubery, D. Douchi et al., A nucleus-encoded chloroplast protein regulated by iron availability governs expression of the photosystem I subunit PsaA in Chlamydomonas reinhardtii, Plant Physiol, vol.167, pp.1527-1657, 2015.
URL : https://hal.archives-ouvertes.fr/hal-02323591

M. J. Veldhuis and G. W. Kraay, Phytoplankton in the subtropical Atlantic Ocean: towards a better assessment of biomass and composition, vol.51, pp.507-530, 1357.
URL : https://hal.archives-ouvertes.fr/hal-01663377

H. Berberoglu and L. Pilon, Experimental measurements of the radiation characteristics of Anabaena variabilis ATCC 29413-U and Rhodobacter sphaeroides ATCC 49419, Int J Hydrogen Energ, vol.32, pp.4772-4785, 2007.

A. Joubert, B. Calmes, R. Berruyer, M. Pihet, J. P. Bouchara et al., Laser nephelometry applied in an automated microplate system to study filamentous fungus growth, pp.399-404, 2010.

S. Pochon, P. Simoneau, S. Pigne, S. Balidas, N. Bataille-simoneau et al., Dehydrin-like proteins in the necrotrophic fungus Alternaria brassicicola have a role in plant pathogenesis and stress response, Plos One, vol.8, 2013.
URL : https://hal.archives-ouvertes.fr/hal-01209939

V. Redou, M. Navarri, L. Meslet-cladiere, G. Barbier, and G. Burgaud, Species richness and adaptation of marine fungi from deep-subseafloor sediments, Appl Environ Microb, vol.81, pp.3571-3583, 2015.

C. Butterwick, S. Heaney, and J. Talling, A comparison of eight methods for estimating the biomass and growth of planktonic algae, British Phycological Journal, vol.17, pp.69-79, 1982.

C. M. Gachon, M. Strittmatter, D. G. Müller, J. Kleinteich, and F. C. Küpper, Detection of differential host susceptibility to the marine oomycete pathogen Eurychasma dicksonii by real-time PCR: not all algae are equal, Appl Environ Microb, vol.75, pp.322-328, 2009.

C. M. Gachon, M. Strittmatter, Y. Badis, K. I. Fletcher, P. Van-west et al., Pathogens of brown algae: culture studies of Anisolpidium ectocarpii and A. rosenvingei reveal that the Anisolpidiales are uniflagellated oomycetes, European Journal of Phycology, vol.52, pp.133-148, 2017.

M. Strittmatter, T. Guerra, J. Silva, and C. M. Gachon, A new flagellated dispersion stage in Paraphysoderma sedebokerense, a pathogen of Haematococcus pluvialis, J Appl Phycol, vol.28, pp.1553-1558, 2016.

C. Rad-menendez, M. Gerphagnon, A. Garvetto, P. Arce, Y. Badis et al., Rediscovering Zygorhizidium affluens Canter: Molecular taxonomy, infectious cycle, and cryopreservation of a chytrid infecting the bloom-forming diatom Asterionella formosa, European Journal of Phycology, vol.50, pp.148-148, 2015.
URL : https://hal.archives-ouvertes.fr/hal-01981931

. R_core_team, R: A language and environment for statistical computing, 2013.

C. Rousseau, E. Belin, E. Bove, D. Rousseau, F. Fabre et al., High throughput quantitative phenotyping of plant resistance using chlorophyll fluorescence image analysis, Plant Methods, p.9, 2013.
URL : https://hal.archives-ouvertes.fr/inserm-00837088

P. G. Falkowski, Light-shade adaptation in marine phytoplankton, Primary productivity in the sea, pp.99-119, 1980.

A. Lehmuskero, M. S. Chauton, T. Boström, and . Light, A review of cellular-and molecular-scale optical processes, Progress in oceanography, 2018.

M. Chioccioli, B. Hankamer, and I. L. Ross, Flow cytometry pulse width data enables rapid and sensitive estimation of biomass dry weight in the microalgae Chlamydomonas reinhardtii and Chlorella vulgaris, Plos One, vol.9, p.97269, 2014.

S. Van-den-wyngaert, O. Vanholsbeeck, P. Spaak, and B. W. Ibelings, Parasite fitness traits under environmental variation: disentangling the roles of a chytrid's immediate host and external environment, Microbial ecology, vol.68, pp.645-656, 2014.

M. Ermakova, T. Huokko, P. Richaud, L. Bersanini, C. J. Howe et al., Distinguishing the roles of thylakoid respiratory terminal oxidases in the cyanobacterium Synechocystis sp, Plant Physiol, vol.6803, pp.1307-1319, 2016.

A. M. Wood, R. Everroad, and L. Wingard, Measuring growth rates in microalgal cultures, Algal culturing techniques, vol.18, pp.269-288, 2005.

K. Avia, S. M. Coelho, G. J. Montecinos, A. Cormier, F. Lerck et al., High-density genetic map and identification of QTLs for responses to temperature and salinity stresses in the model brown alga Ectocarpus, Scientific reports, vol.7, pp.43241-1358, 2017.
URL : https://hal.archives-ouvertes.fr/hal-01510346

F. L. Liu, Z. R. Shao, H. N. Zhang, J. D. Liu, X. L. Wang et al., QTL Mapping for frond length and width in Laminaria japonica Aresch (Laminarales, Phaeophyta) using AFLP and SSR Markers, Mar Biotechnol, vol.12, pp.386-394, 2010.

M. Bonhomme, O. Andre, Y. Badis, J. Ronfort, C. Burgarella et al., High-density genome-wide association mapping implicates an Fbox encoding gene in Medicago truncatula resistance to Aphanomyces euteiches, New Phytol, vol.201, pp.1328-1342, 2014.
URL : https://hal.archives-ouvertes.fr/hal-01208691

S. Rasconi, M. Jobard, L. Jouve, and T. Sime-ngando, Use of Calcofluor White for detection, identification, and quantification of phytoplanktonic fungal parasites, Appl Environ Microb, vol.75, pp.2545-2553, 2009.
URL : https://hal.archives-ouvertes.fr/hal-00527082

T. A. Klochkova, J. B. Shim, M. S. Hwang, and G. H. Kim, Host-parasite interactions and host species susceptibility of the marine oomycete parasite, Olpidiopsis sp., from Korea that infects red algae, Journal of Applied Phycology, vol.24, pp.135-144, 2012.

M. Valero, S. Richerd, V. Perrot, and C. Destombe, Evolution of alternation of haploid and diploid phases in life-cycles, Trends Ecol Evol, vol.7, pp.25-29, 1992.

J. M. Cock, O. Godfroy, N. Macaisne, A. F. Peters, and S. M. Coelho, Evolution and regulation of complex life cycles: a brown algal perspective, Curr Opin Plant Biol, vol.17, pp.1-6, 2014.
URL : https://hal.archives-ouvertes.fr/hal-01806434

S. M. Coelho, A. F. Peters, B. Charrier, D. Roze, C. Destombe et al., Complex life cycles of multicellular eukaryotes: New approaches based on the use of model organisms, vol.406, pp.152-170, 2007.

C. Destombe, J. Godin, M. Nocher, S. Richerd, and M. Valero, Differences in response between haploid and diploid isomorphic phases of Gracilaria verrucosa (Rhodophyta, Gigartinales) exposed to artificial environmental conditions, Hydrobiologia, vol.261, pp.131-137, 1993.

B. R. Robertson, D. K. Button, and A. L. Koch, Determination of the biomasses of small bacteria at low concentrations in a mixture of species with forward light scatter measurements by flow cytometry, Appl Environ Microb, vol.64, pp.3900-3909, 1998.

H. B. Steen, Flow cytometer for measurement of the light scattering of viral and other submicroscopic particles, Cytom Part A, vol.57, pp.94-99, 2004.