Table 1. List of algal strains and samples used for 16S metabarcoding experiments.

Name	Sex	Origin	Year	Isolator	ITS accession	Culture medium	Growth in DSWM	Culture medium			Alga
								No. samples	Dsim	No. OTUs	No. samples	Dsim	No. OTUs
371	Sp	Hopkins River, AUS	1995	J West	FR668844	SWM	Yes	1	0.15	43	3	0.13-0.16	74-84
						DSWM	Yes	1	0.18	95	3	0.13-0.18	71-80
Bft 1a	f	Beaufort, USA	1978	DG Müller	FR668733	SWM	No	1	0.34	39	1	0.19	61
Bft 15b	m	Beaufort, USA	1978	DG Müller	FR668734	SWM	No	1	0.14	76	1	0.33	53
Par 10b	f	Port Aransas, USA	1978	DG Müller	FR668791	SWM	No	1	0.50	51	1	-c	19c
PAr 10n_2	f	Port Aransas, USA	1978	DG Müller	FR668792	SWM	Yes	1	0.14	72	1	0.12	48
PAr 27a	m	Port Aransas, USA	1978	DG Müller	FR668795	SWM	Yes	1	0.20	53	1	0.12	54
PAr 1027	sp	Port Aransas, USA	1978	DG Müller	unpublished	SWM	No	1	0.35	69	1	0.27	59
Tam 2b	m	Tampa, USA	1978	DG Müller	FR668803	SWM	Yes	1	0.53	51	1	0.22	67
Tam 12b	m	Tampa, USA	1978	DG Müller	FR668804	SWM	No	1	0.37	56	1	0.50	52
Tam 18b	f	Tampa, USA	1978	DG Müller	FR668805	SWM	Yes	1	0.27	85	1	0.27	33
Tam 19c	m	Tampa, USA	1978	DG Müller	FR668806	SWM	C.d.	1	0.24	59	1	0.20	67
Grh 6c	sp	Grinhigen, SW	1979	DG Müller	FR668907	SWM	C.d.	1	0.17	40	1	0.39	39
Kal 1c	sp	Killvik, SW	1979	DG Müller	FR668909	SWM	Yes	1	0.65	64	1	0.16	59
Bri_1	sp	Pill, UK	2008	AF Peters	unpublished	SWM	Yes	1	0.37	56	1	0.70	68
Bri_3	sp	Pill, UK	2008	AF Peters	unpublished	SWM	Yes	1	0.17	54	1	0.13	60
DEV11_1a	m	Kingsbridge, UK	2011	AF Peters	unpublished	SWM	No	1	0.43	49	1	0.21	60
Ter11_1	sp	Terenez, FR	2011	AF Peters	unpublished	SWM	No	1	0.27	61	1	0.17	60
Ter11_2	sp	Terenez, FR	2011	AF Peters	unpublished	SWM	Yes	1	0.20	68	1	0.09	33
Rsa001	sp	Muizenberg, RSA	2004	D Schroeder	unpublished	SWM	Yes	1	0.31	52	1	0.70	35
Rsa002	Sp	Muizenberg, RSA	2004	D Schroeder	unpublished	SWM	Yes	1	0.13	58	1	-c	31c
Ec32b	m	San Juan de Marcona, Peru	1988	AF Peters	genome	SWM	C.d.	1	0.18	56	1	0.17	59

Abbreviations: DSWM, diluted seawater medium; f, female strain; ITS, internal transcribed spacer; m, male strain; OTU, operational taxonomic unit; sp, heterozygous sporophytes; SWM, seawater medium. The capacity of a strain to grow in DSWM was determined in a separate experiment with three biological replicates. ‘C.d.'=cell death, that is, cultures showed clear bleaching after 5 weeks of exposure.

^aThis strain is likely to represent a species different from the remaining strains, despite high phenotypic similarity.

^bThis strain is the genome-sequenced purely marine strain of E. siliculosus.

^cFor these samples, less than 7891 sequences were obtained and no diversity-index was calculated.