Biotic and abiotic surfaces in marine waters are rapidly colonized by microorganisms. Surface colonization and subsequent biofilm formation and development provide numerous advantages to these organisms and support critical ecological and biogeochemical functions in the changing marine environment. Microbial surface association also contributes to deleterious effects such as biofouling, biocorrosion, and the persistence and transmission of harmful or pathogenic microorganisms and their genetic determinants. The processes and mechanisms of colonization as well as key players among the surface-associated microbiota have been studied for several decades. Accumulating evidence indicates that specific cell-surface, cell-cell, and interpopulation interactions shape the composition, structure, spatiotemporal dynamics, and functions of surface-associated microbial communities. Several key microbial processes and mechanisms, including (1) surface, population, and community sensing and signaling, (2) intraspecies and interspecies communication and interaction, and (3) the regulatory balance between cooperation and competition, have been identified as critical for the microbial surface association lifestyle.

In this review, recent progress in the study of marine microbial surface colonization and biofilm development is synthesized and discussed. Major gaps in our knowledge remain. We pose questions for targeted investigation of surface-specific community- level microbial features, answers to which would advance our understanding of surface-associated microbial community ecology and the biogeochemical functions of these communities at levels from molecular mechanistic details through systems biological integration.

The key points presented in this review include:

1) Marine particle- and surface-associated microorganisms usually possess unique species composition, ecological processes and biogeochemical functions, which are generally different from their free-living counterparts. Particle- and surface-associated microbiota play important roles in nutrient regeneration, bioactive element (such as C, N, P, S and Fe) cycling, environmental pollutant biodegradation, and the flows of matter, energy and genetic information in marine food webs.

2) The formation and development of surface-associated marine microbial communities generally follow successional patterns, in which marine Roseobacter clade bacteria are usually the early-stage pioneer colonizers, particularly in coastal waters. Complex interactions such as cooperation and competition are prevalent among surface-associated microorganisms, laying the foundation for the establishment of compositional diversity and functional processes of the marine surface-associated microbiota.

3) Surface-associated microbiota influence, via microbial respiration, the biodegradation of marine particulate organic matter (POM), and thus the remineralization depth of sinking POM, the carbon sequestration efficiency of the biological pump, and the ocean's modulation capacity of atmospheric CO₂ concentration and climate change. Furthermore, surface-associated microbiota prompt the regeneration of nutrients through POM biodegradation, thus enhance photosynthetic CO₂ fixation in the euphotic zone and chemolithoautotrophic CO₂ fixation in the twilight and aphotic zones (Fig. 1). Therefore, surface-associated microorganisms play important roles in the modulation of global climate change and the ocean's environmental change.