Editorial: The Responses of Marine Microorganisms, Communities and Ecofunctions to Environmental Gradients

摘要

From estuaries to marginal seas and open oceans, from tropical warm pools to subtropical gyres and polar cryospheres, from sunlit surface water to twilight zone and pitch-black abyssopelagic water, from water columns to sediments and deep subseafloor biospheres, marine ecosystems experience diverse environmental gradients (Karl, 2007 ; Dang and Jiao, 2014 ). In addition to these large-scale gradients, small-scale, and micro-scale gradients of various physicochemical factors are common in the ocean; in particular, in marginal seas and coastal environments (Kappler et al., 2005 ; Stocker, 2012 ). The diverse gradients of physicochemical parameters, nutrients, and chemicals serving as electron donors and acceptors contribute to the creation of habitat heterogeneity and novel locales along a gradient may create unique niches for any given microorganism. Whether at the surface of a marine snow particle or alga, at the edges of an oxygen minimum zone (OMZ), in marginal sea methane-seep sediments, or on a chimney wall of a deep-sea hydrothermal vent, these interfaces provide hotspot habitats with sharp physicochemical gradients that may host diverse yet unknown microorganisms that facilitate yet unknown biogeochemical processes (Hügler and Sievert, 2011 ; Wright et al., 2012 ; Dang and Lovell, 2016 ). With the progress of marine molecular microbial ecology and “omics” techniques, certain environmental keystone microorganisms have been discovered at some of these interfaces: such as the anaerobic methane-oxidizing (ANME) archaea in methane-rich sediments (Valentine and Reeburgh, 2000 ), cable bacteria that facilitate electrogenic sedimentary sulfide oxidation (Nielsen and Risgaard-Petersen, 2015 ), neutrophilic zetaproteobacterial iron-oxidizing bacteria (FeOB) in deep-sea hydrothermal microbial mats and at abyssal basaltic glass-seawater and coastal metal-seawater interfaces (Emerson et al., 2010 ; Dang et al., 2011 ; Henri et al., 2016 ), anaerobic ammonium-oxidizing (anammox) bacteria and SUP05 sulfur-oxidizing bacteria (SOXB) in coastal and oceanic OMZs (Dick et al., 2013 ; Oshiki et al., 2016 ), and sulfur-oxidizing and/or hydrogen-oxidizing Campylobacteria in the proposed new phylum Campylobacterota (formerly known as Epsilonproteobacteria ; Waite et al., 2018 ) at seawater, hydrothermal vent, and subseafloor redox interfaces (Campbell et al., 2006 ; Grote et al., 2012 ; Dick et al., 2013 ; Han and Perner, 2015 ; McNichol et al., 2018 ). Even the ubiquitous marine ammonia-oxidizing Thaumarchaeota , discovered only a decade ago (K?nneke et al., 2005 ), can now be divided into two distinct ecological groups according to the vertical physicochemical profile of marine water, the “shallow clade” and the “deep clade” (Hatzenpichler, 2012 ). The ongoing discovery of unique ecophysiological functions of marine Bacteria and Archaea will contribute to a conceptual rewriting of biogeochemical pathways in the marine C, N, S, and Fe cycles. The characterization of how the abundance and spatial 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 microbial ecology and biogeochemistry of the oceans. This rationale defines the aim and scope of this Research Topic. The contributions of environmental gradients to the diversity of marine microorganisms and their metabolic potentials may play important roles in maintaining the stability and functions of the estuarine, coastal and marginal sea ecosystems, which have been experiencing a multitude of anthropogenic perturbations (Dang and Jiao, 2014 ; Damashek and Francis, 2018 ). The responses of the affected microbial communities to human-induced environmental impacts are currently still difficult to predict and the understanding of microbial processes and mechanisms at the community level is the key for predictive modeling, which also requires the collection of large empirical data sets (Haruta et al., 2013 ; Hanemaaijer et al., 2015 ; Burd et al., 2016 ). Greater understanding of microbial responses to natural and anthropogenic environmental gradients may also help us understand the responses of marine ecosystems to global climate change and other large-scale environmental perturbations such as ocean acidification and spatial and temporal ocean deoxygenation. The authors of this Research Topic contributed a total of 21 publications covering a wide variety of subjects spanning from microbial metabolic dynamics to biogeochemical cycling of C, N, S, and Fe in micro-, small-, and geographic-scale marine gradients. This Editorial aims to highlight some of the main findings reported in these publications and we would like to take this opportunity to thank all participating editors and reviewers for making this Research Topic a success. In order to cover the broad subjects of the articles published in this Research Topi