This study addresses the pervasive challenge of biofouling in seawater desalination systems, which compromises membrane performance and longevity, by introducing a multifunctional PDA-SP-cTA coating. This bio-inspired coating effectively mitigates biofouling in seawater reverse osmosis systems without requiring biocide. The coating is applied to both hydrophilic polyamide membranes and hydrophobic polypropylene feed spacers through in-situ and ex-situ polymer deposition methods, involving a single-step process with polydopamine and sodium-periodate, followed by surface tailoring with citric acid-blended tannic acid. Extensive surface characterization, primarily conducted on polypropylene feed spacers, confirms coating deposition. Antibiofouling properties are evaluated through long-term biofouling tests simulating industrial conditions. The findings demonstrate that the ex-situ applied coating significantly reduces relative feed channel pressure drop increase due to biofilm growth by 75 % and lowers biomass accumulation (88 % total cell counts, 70 % adenosine-triphosphate, 91 % carbohydrates, and 69 % proteins). The coating inhibits the colonization of biofouling-causing bacterial genus Alteromonas, drastically decreases active bacterial gene copy numbers, and alters microbial composition, leading to reduced biofilm viability and loosely attached biofilms that could enhance cleaning efficiency. This comprehensive study encompasses the entire process from the strategic selection and systematic characterization of the coating to extensive biofouling tests and stability assessments offering a holistic solution to combat biofouling without biocides. With demonstrated durability and stability across various pH conditions over time, this coating could be a widely applicable and scalable solution for biofouling mitigation in diverse industrial contexts.