Supplemental Figure Legends:

Fig. S1 Diatom abundance and nutrient (silicate, nitrogen, phosphate) concentrations from March 1st to May 30th 2016 at the Helgoland sampling site.

Fig. S2: Bubble plots showing normalized read abundance of members of the Bacteroidetes (a), (b), and Rhodobacteriaceae (minimum read abundance 0.005%) for each substrate incubation and the unamended incubation at t0, 3d, and 6d for Hel_1 to Hel_4. NMDS plots (right) based on Bray-Curtis dissimilarity reveal separation in Bacteroidetes, Gammaproteobacteria, and Rhodobacteriaceae composition across bloom phases with time. ANOSIM (R:Vegan) indicates significant difference between dissimilarity across bloom phases.

Fig. S3: Shifts in microbial communities (1% minimum abundance) during incubation with FLA-PS. Percentage change in relative read abundance of bacterial genera (and phyla) are shown; the bar chart compares t0 abundances of each to 3d and 6d abundances and shows increase or decrease in each genus. ARA = arabinogalactan, CA = carrageenan, CH = chondroitin sulfate, LA = laminarin, XY = xylan, NA = no addition control. (a) Hel_1; note there are no data for CA at 6d (b) Hel_2; Note there are no data for xylan because there are no data from the t0 sample; (c) Hel_3; there are no data for the NA at 6d. Note also that there are no chondroitin data because this substrate was not used at the Hel_3 sample point. (d) Hel_4.

Fig. S4: Relative abundance (% of DAPI stained cells) of Alteromonas (black; ALT1413) and Bacteroidetes (gray; CF319a) during incubations Hel_1 to Hel_4 enumerated by FISH.

Fig. S5: Percentage of total DAPI-stainable cells at t0 timepoint (~ 15 minutes after substrate addition) showing staining by laminarin, xylan, chondroitin sulfate, and arabinogalactan for Hel_1 to Hel_4. Note that no chondroitin incubation was carried out for Hel_3. These data are the same as the t0 timepoints in Fig. 6a, replotted here for clarity.

3 per. Mov. Avg.(Diatoms) Diatoms SiO4 NOx PO4 3.5 x 106 30

3 x 106 25 ) -1 L 2.5 x 106

(cells 20

2 x 106

15 µM

abundance 1.5 x 106

10 Diatom 1 x 106

5 x 105 5

0 0 1.3 8.3 5.4 3.5 15.3 22.3 29.3 12.4 19.4 26.4 10.5 17.5 24.5 Date a) Bacteroidetes HEL_1 HEL_2 HEL_3 HEL_4 Substrate AR CA CH LA XY NA AR CA CH LA XY NA AR CA LA XY NA AR CA CH LA XY NA Genus Time(days) 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 Rhodothermaceae unc. NS11.12 marine group NS9 marine group ● ANOSIM R = 0.76,p- value = 0.001 Winogradskyella 0.5 Stress= 0.12 Wenyingzhuangia ● ● ● Flavobacteriaceae unc. ●●● ● ● ● ●● ●● HEL_2 Ulvibacter ● ● ●HEL_3 Tenacibaculum ●● ● ● ● ●●● Pseudofulvibacter ●●●● ● Polaribacter 4 ● ● Polaribacter 3 HEL_4 Polaribacter 2 0.0 ● ● ●●●● Polaribacter NMDS2 ●●● ● NS5 marine group ● NS4 marine group ● ● ● NS3a marine group ●● ● ● Hel-1 NS2b marine group ● ●●●●● ●● Hel-2 Formosa ● ●HEL_1 Flavicella Hel-3 Dokdonia ● Hel-4 Aurantivirga −0.5 NA Aquibacter 0.0 0.5 1.0 Algibacter Polaribacter huanghezhanensis NMDS1 Cryomorphaceae unc. Fluviicola Crocinitomix Marinoscillum Saprospiraceae unc. Lewinella Aureispira Relative abundance (%) 0.05 0.5 1 b) Gammaproteobacteria HEL_1 HEL_2 HEL_3 HEL_4 Substrate AR CA CH LA XY NA AR CA CH LA XY NA AR CA LA XY NA AR CA CH LA XY NA Genus Time(days) 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6

Vibrio UBA4486 ANOSIM R = 0.24,p- value = 0.001 ● SUP05 cluster ● Stress= 0.09 Thiohalorhabdaceae unc. ● Candidatus Tenderia ● Woeseia ● ● ● SAR86 clade 0.5 ● ● Pseudomonas ●● OM182 clade HEL_3 ● Pseudohongiella ● ● ● ● HEL_2 ● Nitrincolaceae unc. ●● ● ● ● Neptunomonas ● ●● ● ● ● Marinobacterium 0.0 ● ● ●

Amphritea NMDS2 ● Marinomonas HEL_4 ● ● ● ● ● Legionellaceae unc. ● ●● ● ● HglApr721 ●●● ● ● ● Spongiibacteraceae unc. ● HEL_1● ● BD1.7.clade ● ● Hel-1 SAR92 clade Hel-2 Porticoccus ●● Hel-3 −0.5 ●● OM60 NOR5. clade ● Hel-4 Luminiphilus NA Halioglobus OM43 clade −0.5 0.0 0.5 RS62 marine group Arenicella NMDS1 Psychromonas Pseudoalteromonas Moritella Thalassotalea Colwellia Paraglaciecola Glaciecola Alteromonas Relative abundance (%) 0.05 0.5 1 c) Rhodobacteraceae HEL_1 HEL_2 HEL_3 HEL_4 Substrate AR CA CH LA XY NA AR CA CH LA XY NA AR CA LA XY NA AR CA CH LA XY NA Time(days) 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 Genus 0.5 ANOSIM R = 0.56,p- value = 0.001 Rhodobacteraceae unc. Stress= 0.09 Tateyamaria ● ●●● ● ● ● Sulfitobacter ● ● ● ● ● ● ● ● Roseobacter clade ●●● ● HEL_3 ● HEL_2● ● ● ● NAC11.7 lineage ● ●● Roseobacter ● ● ● Pseudorhodobacter 0.0 ● ● ● ● ● ● ● ● ●●● HEL_4 Planktotalea ●● ●● ●●

NMDS2 ● ● ● ● Planktomarina ● ● HEL_1 ●● Loktanella ● ●● ● ● ● Hel-1 Lentibacter ● ● Hel-2 Halocynthiibacter Hel-3 Hel-4 −0.5 Ascidiaceihabitans Na

Amylibacter −0.5 0.0 0.5 NMDS1 Albimonas

Relative abundance (%) 0.05 0.5 1

Reintjes et al. Supplementary Figure S2 a) HEL_1 b) HEL_2 c) HEL_3 d) HEL_4

Other

50 SAR86 SAR92 Colwellia Glaciecola 25 Gammaproteobacteria Amylibacter Planktomarina 0 Alphaproteobacteria Ulvibacter Flavicella Bacteroidetes -25 Relative increase and decrease −50

Time (days) 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 Substrate ARA CA CH LA XY NA ARA CA CH LA NA ARA CA LA XY NA ARA CA CH LA XY NA

Reintjes et al. Supplementary Figure S3 Laminarin Bacteroidetes (CF319a) Alteromonas (ALT1413) 60%

40%

DAPI) 20% (% 0% *

Relative abundance 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 HEl_1 HEL_2 HEL_3 HEL_4 Time (days) Xylan

60%

40%

DAPI) 20% (% 0% * Relative abundance 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 HEl_1 HEL_2 HEL_3 HEL_4 Time (days) Chondroitin Sulfate

60%

40% DAPI) 20% (%

Relative abundance 0% * * * * * * 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 HEl_1 HEL_2 HEL_3 HEL_4 Time (days) Arabinogalactan 60%

40%

DAPI) 20% (%

Relative abundance 0% * * 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 HEl_1 HEL_2 HEL_3 HEL_4 Time (days) No addition control 60%

40% DAPI) 20% (%

Relative abundance 0% * 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 0 1 3 6 9 HEl_1 HEL_2 HEL_3 HEL_4 Time (days)

Reintjes et al. Supplementary Figure S4 Laminarin Xylan Chondroitin sulfate Arabinogalactan 25%

20% DAPI) (% 15%

10%

5%

Relative abundance 0% HEL_1 HEL_2 HEL_3 HEL_4 HEL_1 HEL_2 HEL_3 HEL_4 HEL_1 HEL_2 HEL_3 HEL_4 HEL_1 HEL_2 HEL_3 HEL_4

Reintjes et al. Supplementary Figure S5