Marine Snow ― A Hotspot Microenvironment for Microbial Horizontal Gene Transfer and Enhanced Metabolism

Marine snow is the hotspot for microbial biogeochemical processes in the ocean. Compared to their free-living counterparts, marine snow-associated microorganisms (MSAM) usually possess higher diversity, increased abundance, and enhanced activities such as respiration and extracellular enzymatic hydrolysis of biopolymers. MSAM are usually copiotrophic, while many free-living microorganisms, particularly those thriving in the open ocean, are oligotrophic. The sinking of marine snow, along with zooplankton fecal pellets and algal phytodetrital aggregates, contributes to the export and sequestration of particulate organic carbon in the ocean. The enriched abundance and diversity of MSAM may facilitate intraspecies and interspecies horizontal gene transfer due to the close spatial juxtaposition of the microbial cells and to the surface-stimulated gene expression for natural competency. The enhanced genetic exchange induced by living on marine snow may lay a foundation for the microbial (micro)diversity and (micro)evolution, especially in the deep ocean where the abundance of the resident free-living microorganisms is generally low. The enhanced genetic exchange among MSAM may also facilitate the dispersion and acquirement of unique metabolic genes, particular for recalcitrant organic matter in the organically depleted deep ocean waters. Thus, the enhanced horizontal gene transfer frequency in marine snow-associated microbial communities may help to explain the biogeography of deep-sea microorganisms. Furthermore, this community genetic characteristic may influence carbon sequestration in the ocean by modulating both the biological pump and the microbial carbon pump. Under increased anthropogenic impacts, marine snow warrants in depth investigation, especially on its perspective of enhanced microbial horizontal gene transfer and metabolic activities.