Ι-Carrageenan Catabolism is Initiated by Key Sulfatases in the Marine Bacterium Pseudoalteromonas Haloplanktis LL1

Marine bacteria contribute substantially to cycle macroalgae polysaccharides in marine environments. Carrageenans are the primary cell wall polysaccharides of red macroalgae. The carrageenan catabolism mechanism and pathways are still largely unclear. Pseudoalteromonas is a representative bacterial genus that can utilize carrageenan. We previously isolated the strain Pseudoalteromonas haloplanktis LL1 that could grow on iota-carrageenan but produce no iota-carrageenase. Here, through a combination of bioinformatic, biochemical, and genetic analyses, we determined that P. haloplanktis LL1 processed a desulfurization-depolymerization sequential pathway for iota-carrageenan utilization, which was initiated by key sulfatases PhSulf1 and PhSulf2. PhSulf2 acted as an endo/exo-G4S (4-O-sulfation-beta-D-galactopyranose) sulfatase, while PhSulf1 was identified as a novel endo-DA2S sulfatase that could function extracellularly. Because of the unique activity of PhSulf1 toward iota-carrageenan rather than oligosaccharides, P. haloplanktis LL1 was considered to have a distinct iota-carrageenan catabolic pathway compared to other known iota-carrageenan-degrading bacteria, which mainly employ multifunctional G4S sulfatases and exo-DA2S (2-O-sulfation-3,6-anhydro-alpha-D-galactopyranose) sulfatase for sulfate removal. Furthermore, we detected widespread occurrence of PhSulf1-encoding gene homologs in the global ocean, indicating the prevalence of such endo-acting DA2S sulfatases as well as the related iota-carrageenan catabolism pathway. This research provides valuable insights into the enzymatic processes involved in carrageenan catabolism within marine ecological systems.