Bacteria play critical roles in marine nutrient cycles by incorporating and redistributing dissolved organic matter (DOM) and inorganic nutrients in the ocean. TonB-dependent transporter (TBDT) proteins allow Gram-negative bacteria to take up scarce resources from nutrient-limiting environments as well as siderophores, heme, vitamin B12, and recently identified carbohydrates. Thus, the characterization of TBDT distribution and functions is essential to better understand the contribution TBDT to DOM assimilation and its consequences on nutrient cycling in the environment. This study presents the distribution of encoded known and putative TBDT proteins in the genomes of microorganisms and from the Global Ocean Survey data. Using a Lek clustering algorithm and substrate specificities, the TBDT sequences were mainly classified into the following three groups: (1) DOM transporters; (2) Siderophores/Vitamins transporters; and (3) Heme/Hemophores/Iron(heme)-binding protein transporters. Diverse TBDTs were found in the genomes of oligotroph Citromicrobium bathyomarinum JL354 and Citromicrobium sp JLT1363 and were highly expressed in the stationary phase of bacterial growth. The results show that the Gammaproteobacteria and the Cytophaga-Flavobacterium-Bacteroides (CFB) group bacteria accounted for the majority of the TBDT gene pool in marine surface waters. The results of this study confirm the ecological importance of TBDTs in DOM assimilation for bacteria in marine environments owing to a wide range of substrate utilization potential in the ubiquitous Gammaproteobacteria and CFB group bacteria.

Little is known about the functional capability of microbial communities in shallow-sea hydrothermal systems (water depth of <200 m). This study analyzed two high-throughput pyrosequencing metagenomic datasets from the vent and the surface water in the shallow-sea hydrothermal system offshore NE Taiwan. This system exhibited distinct geochemical parameters. Metagenomic data revealed that the vent and the surface water were predominated by Epsilonproteobacteria (Nautiliales-like organisms) and Gammaproteobacteria (Thiomicrospira-like organisms), respectively. A significant difference in microbial carbon fixation and sulfur metabolism was found between the vent and the surface water. The chemoautotrophic microorganisms in the vent and in the surface water might possess the reverse tricarboxylic acid cycle and the Calvin−Bassham−Benson cycle for carbon fixation in response to carbon dioxide highly enriched in the environment, which is possibly fueled by geochemical energy with sulfur and hydrogen. Comparative analyses of metagenomes showed that the shallow-sea metagenomes contained some genes similar to those present in other extreme environments. This study may serve as a basis for deeply understanding the genetic network and functional capability of the microbial members of shallow-sea hydrothermal systems.

Background: Phytoplankton blooms are frequent events in coastal areas and increase the production of organic matter that initially shapes the growth of opportunistic heterotrophic bacteria. However, it is unclear how these opportunists are involved in the transformation of dissolved organic matter (DOM) when blooms occur and the subsequent impacts on biogeochemical cycles. Results: We used a combination of genomic, proteomic, and metabolomic approaches to study bacterial diversity, genome traits, and metabolic responses to assess the source and lability of DOM in a spring coastal bloom of Akashiwo sanguinea. We identified molecules that significantly increased during bloom development, predominantly belonging to amino acids, dipeptides, lipids, nucleotides, and nucleosides. The opportunistic members of the bacterial genera Polaribacter, Lentibacter, and Litoricola represented a significant proportion of the free-living and particle-associated bacterial assemblages during the stationary phase of the bloom. Polaribacter marinivivus, Lentibacter algarum, and Litoricola marina were isolated and their genomes exhibited streamlining characterized by small genome size and low GC content and non-coding densities, as well as a smaller number of transporters and peptidases compared to closely related species. However, the core proteomes identified house-keeping functions, such as various substrate transporters, peptidases, motility, chemotaxis, and antioxidants, in response to bloom-derived DOM. We observed a unique metabolic signature for the three species in the utilization of multiple dissolved organic nitrogen compounds. The metabolomic data showed that amino acids and dipeptides (such as isoleucine and proline) were preferentially taken up by P. marinivivus and L. algarum, whereas nucleotides and nucleosides (such as adenosine and purine) were preferentially selected by L. marina. Conclusions: The results suggest that the enriched DOM in stationary phase of phytoplankton bloom is a result of ammonium depletion. This environment drives genomic streamlining of opportunistic bacteria to exploit their preferred nitrogen-containing compounds and maintain nutrient cycling.

Members of phylum Bacteroidetes are distributed across diverse marine niches and Flavobacteria is often the predominant bacterial class decomposing algae-derived polysaccharides. Here, we report the complete genome of Gramella flava JLT2011 (Flavobacteria) isolated from surface water of the southeastern Pacific. A remarkable genomic feature is that the number of glycoside hydrolase (GH) genes in the genome of G. flava JLT2011 is more than two-fold higher than that of other Gramella species. The functional profiles of the GHs suggest extensive variation in Gramella species. Growth experiments revealed that G. flava JLT2011 has the ability to utilize a wide range of polysaccharides for growth such as xylan and homogalacturonan in pectin. Nearly half of all GH genes were located on the multi-gene polysaccharide utilization loci (PUL) or PUL-like systems in G. flava JLT2011. This species was also found to harbor the two xylan PULs and a pectin PUL, respectively. Gene expression data indicated that more GHs and sugar-specific outer-membrane susC-susD systems were found in the presence of xylan than in the presence of pectin, suggesting a different strategy for heteropolymeric xylan and homoglacturonan utilization. Multi-omics data (transcriptomics, proteomics, and metabolomics) indicated that xylan PULs and pectin PUL are respectively involved in the catabolism of their corresponding polysaccharides. This work presents a comparison of polysaccharide decomposition within a genus and expands current knowledge on the diversity and function of PULs in marine Bacteroidetes, thereby deepening our understanding of their ecological role in polysaccharide remineralization in the marine system.

Organic sulfur compounds are not only essential for organismal survival but also indispensable for the sulfur cycle. Over the past few decades, dimethylsulfoniopropionate (DMSP) and dimethyl sulfide (DMS) cycling in the upper ocean have been well characterized from the genetic to the ecosystem level. Recent advances in the study of marine sulfonate transformation have indicated that phytoplankton and microbes play key roles in oceanic sulfur and carbon fluxes. This review provides biochemical details of the major sulfur metabolites, and presents an interlinked reaction network with genetic information on the microbial transformation and mineralization of sulfur compounds. This review also discusses future prospects for the discovery and characterization of novel substrates and enzymes involved in organosulfur cycling, as well as for investigations of deep sea and sedimentary organic sulfur.

Eutrophication-induced water deoxygenation occurs continually in coastal oceans, and alters community structure, metabolic processes, and the energy shunt, resulting in a major threat to the ecological environment. Seasonal deoxygenation events have occurred in the Bohai Sea (China), however, how these affect the functional activity of microorganisms remains unclear. Here, through the use of absolute quantification of 16S rRNA genes amplicon sequencing and metatranscriptomics approaches, we investigated the structure of the microbial community and the patterns of transcriptional activity in deoxygenated seawaters. The dominant phyla were Proteobacteria (average value, 1.4 × 10⁶ copies ml^-1), Cyanobacteria (3.7 × 10⁵ copies ml^-1), Bacteroidetes (2.7 × 10⁵ copies ml^-1), and the ammonia-oxidizing archaea Thaumarchaeota (1.9 × 10⁵ copies ml^-1). Among the various environmental factors, dissolved oxygen, pH and temperature displayed the most significant correlation with microbial community composition and functional activity. Metatranscriptomic data showed high transcriptional activity of Thaumarchaeota in the deoxygenated waters, with a significant increase in the expression of core genes representing ammonia oxidation, ammonia transport, and carbon fixation (3-hydroxypropionic acid/4-hydroxybutyric acid cycle) pathways. The transcripts of Cyanobacteria involved in photosynthesis and carbon fixation (Calvin-Benson-Bassham cycle) significantly decreased in low oxygen waters. Meanwhile, the transcripts for the ribulose bisphosphate carboxylase-encoding gene shifted from being assigned to photoautotrophic to chemoautotrophic organisms in surface and bottom waters, respectively. Moreover, the transcription profile indicated that heterotrophs play a critical role in transforming low-molecular-weight dissolved organic nitrogen. Elevated abundances of transcripts related to microbial antioxidant activity corresponded to an enhanced aerobic metabolism of Thaumarchaeota in the low oxygen seawater. In general, our transcriptional evidences showed a population increase of Thaumarchaeota, especially the coastal ecotype of ammonia oxidizers, in low oxygen aquatic environments, and indicated an enhanced contribution of chemolithoautotrophic carbon fixation to carbon flow.