*4.1. Bacterial Strains, Plasmids, and Growth Conditions*

All bacterial strains and plasmids used in this study are listed in Table S5. *P. rubra* S4059 was isolated during the Galathea 3 expedition [36] and cultured in marine broth (MB, BD Difco 2216, Le Pont de Claix, France) or APY medium [37] at 25 ◦C, 200 rpm. *P. rubra* S4059 carrying a chromosomal-integrated suicide plasmid was cultured in MB containing 30 μg/mL chloramphenicol (Sigma, C0378, St. Louis, MO, USA) at 25 ◦C, 200 rpm. All chitin used in this study are α-chitin. *P. rubra* S4059 mutants and wild type were cultured in a marine minimal medium (MMM) [38] supplemented with four different carbon sources (0.2% mannose, 0.2% crystalline chitin, 0.2% colloidal chitin, or 0.2% NAG) and growth, biofilm formation, and secondary metabolome determined. To compare chitin degradation of mutants and wild type S4059, they were grown on plates containing 2% Sea Salt (Sigma, S9883, St. Louis, MO, USA), 1.5% agar, and 0.2% chitin (crystalline or colloidal). Colloidal chitin was prepared as described in a previous study [39].

All *Escherichia coli* strains were cultured in Luria Bertani (LB) Broth (BD Difco 244520, Le Pont de Claix, France) at 37 ◦C, 200 rpm. *E. coli* GB *dir*-*pir*116 [40] was used for constructing suicide plasmid. *E. coli* WM3064 was used as the donor strain in intergeneric conjugation experiments. *E. coli* WM3064 is a *dapA* mutant that requires exogenously supplied diaminopimelic acid (DAP, Sigma, D1377, St. Louis, MO, USA) with a final concentration of 0.3 μM for growth [41]. Plasmid pDM4 was used as the backbone of suicide vectors [42]. *E. coli* strains harboring pDM4, or its derivatives were grown in LB Broth with 10 μg/mL chloramphenicol or in LB agar with 15 μg/mL chloramphenicol.

## *4.2. Whole Genome Sequencing and Assembly of P. rubra S4059*

Genomic DNA of *P. rubra* S4059 was extracted using the Genomic DNA buffer set (QIAGEN, 19060, Hilden, Germany) following the supplier's instructions. The closed genome of *P. rubra* S4059 was obtained by minION sequencing using the EXP-FLP002 flow cell priming kit, SQK-RAD004 rapid sequencing kit, FLO-MINSP6 flow cell, and the associated protocol (version RSE\_9046\_V1\_revB-17Nov2017 and RSE\_9046\_V1\_revB-14 AUG2019). lllumina reads, obtained from a previous study [7], were combined with the minION reads for hybrid assembly using the Unicycler package [43]. Before assembly, the minION reads were filtered using the Filtlong package, only keeping the top 500,000,000 bp. The genome was annotated using Prokka [44]. The genome is available at the National Center for Biotechnology Information (NCBI) under the accession number CP045429 and CP045430.
