From estuaries to coastal seas and open oceans, from sunlit surface water to twilight zone and abyssopelagic water, from water columns to sediments and deep biospheres, marine ecosystems constitute diverse environmental gradients. In addition to these large-scale gradients, small-scale and micro-scale gradients of various physicochemical factors are common in marine environments. A unique locale in a gradient may create significant site heterogeneity and unique niches for a wide range of microorganisms. The interfaces at the surface of a marine snow particle or alga, at the edge of an oxygen minimum zone (OMZ), in marginal sea methane-seep sediments, or on a chimney wall of a deep-sea hydrothermal vent constitute hotspot environments with sharp physicochemical gradients that may be home to yet unknown microorganisms and biogeochemical processes. With the progress of marine molecular microbial ecology, several environmental keystone microorganisms have been discovered at some of these interfaces: such as the anaerobic methane-oxidizing (ANME) archaea in methane-rich sediments, cable bacteria that conduct electrogenic sedimentary sulfide oxidation, neutrophilic Fe-oxidizing Zetaproteobacteria in deep-sea microbial mats and at metal-seawater interfaces, anaerobic ammonium-oxidizing (anammox) bacteria and SUP05 sulfur-oxidizing bacteria in coastal and oceanic OMZs, and Epsilonproteobacteria at hydrothermal vents and other redox interfaces. Even the ubiquitous marine ammonia-oxidizing Thaumarchaeota, which were discovered only a decade ago, can be divided into two distinct ecological groups according to the vertical physicochemical profile of marine water, the "shallow clade" and the "deep clade." The ongoing discovery of unique ecophysiological functions of marine Bacteria and Archaea contributes to a conceptual rewriting of biogeochemical pathways in the marine C, N and S cycles.

The characterization of how the abundance and distribution of marine microorganisms, the structure of microbial communities and their provided ecosystem functions respond to the diverse environmental gradients is of fundamental importance to our understanding of the biogeochemistry of the oceans. The contributions of ecological gradients to microbial community stability and function are of particular importance for the estuarine, coastal and marginal sea environments, which have been experiencing a multitude of anthropogenic perturbations. The responses of the affected microbial communities to the human-mediated environmental impacts are difficult to predict and future modeling will require the collection of large empirical data sets. Greater understanding of microbial responses to natural and anthropogenic environmental gradients may also help us to understand the responses of marine ecosystems to global climate change and other large-scale environmental perturbations such as ocean acidification and spatial ocean deoxygenation.

This Research Topic focuses on studies (including original research, methods, perspectives, minireviews and reviews) that investigate and discuss the response of marine microorganisms, communities and their ecological functions to natural and anthropogenic environmental gradients. Contributions that describe ecological characteristics or the development of methods suitable for the study of marine microbial communities that participate in the biogeochemical cycling of C, N and S, in micro-, small-, or geographic-scale gradients are particularly encouraged.

Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.