The fourth edition of this bestselling text has been rigorously updated to reflect major new discoveries and concepts since 2019. This new edition reflects important new findings in marine microbiology revealed by development of high-throughput sequencing, meta-omics, and single-cell technologies, and the ability to analyse and interpret large datasets. Furthermore, it presents major new insights into topics such as the physiology of planktonic microbes and their role in ocean processes; the impacts of rising CO2 levels and climate change on the microbial community structure and ocean processes; the evolution of viruses, organelles, and eukaryotic life; interactions of microbes with plastic pollution; and emerging diseases of marine life and their control. All chapters have been significantly expanded, with full colour diagrams, illustrations, and boxes to aid students’ interest and understanding. Novel pedagogy is designed to encourage students to explore current high-profile research topics. This is the only textbook addressing such a broad range of topics in the specific area of marine microbiology, now a core topic within broader marine science degrees.