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Predator-prey interactions play important roles in the cycling of marine organic matter. Here we show that a Gram-negative bacterium isolated from marine sediments (Pseudoalteromonas sp. strain CF6-2) can kill Gram-positive bacteria of diverse peptidoglycan (PG) chemotypes by secreting the metalloprotease pseudoalterin. Secretion of the enzyme requires a Type II secretion system. Pseudoalterin binds to the glycan strands of Gram positive bacterial PG and degrades the PG peptide chains, leading to cell death. The released nutrients, including PG-derived D-amino acids, can then be utilized by strain CF6-2 for growth. Pseudoalterin synthesis is induced by PG degradation products such as glycine and glycine-rich oligopeptides. Genes encoding putative pseudoalterin-like proteins are found in many other marine bacteria. This study reveals a new microbial interaction in the ocean., Predator-prey interactions play important roles in the cycling of marine organic matter. Here the authors show that a Gram-negative bacterium isolated from marine sediments can kill and feed on Gram-positive bacteria by secreting a peptidoglycan-degrading enzyme.

Chromosome replication is an essential process for cell division. The mode of chromosome replication has important impacts on the structure of the chromosome and replication speed., As typical bacterial replicons, circular chromosomes replicate bidirectionally and circular plasmids replicate either bidirectionally or unidirectionally. Whereas the finding of chromids (plasmid-derived chromosomes) in multiple bacterial lineages provides circumstantial evidence that chromosomes likely evolved from plasmids, all experimentally assayed chromids were shown to use bidirectional replication. Here, we employed a model system, the marine bacterial genus Pseudoalteromonas, members of which consistently carry a chromosome and a chromid. We provide experimental and bioinformatic evidence that while chromids in a few strains replicate bidirectionally, most replicate unidirectionally. This is the first experimental demonstration of the unidirectional replication mode in bacterial chromids. Phylogenomic and comparative genomic analyses showed that the bidirectional replication evolved only once from a unidirectional ancestor and that this transition was associated with insertions of exogenous DNA and relocation of the replication terminus region (ter2) from near the origin site (ori2) to a position roughly opposite it. This process enables a plasmid-derived chromosome to increase its size and expand the bacterium’s metabolic versatility while keeping its replication synchronized with that of the main chromosome. A major implication of our study is that the uni- and bidirectionally replicating chromids may represent two stages on the evolutionary trajectory from unidirectionally replicating plasmids to bidirectionally replicating chromosomes in bacteria. Further bioinformatic analyses predicted unidirectionally replicating chromids in several unrelated bacterial phyla, suggesting that evolution from unidirectionally to bidirectionally replicating replicons occurred multiple times in bacteria.

Sea ice is one of the most frigid environments for marine microbes. In contrast to other ocean ecosystems, microbes in permanent sea ice are space confined and subject to many extreme conditions, which change on a seasonal basis. How these microbial communities are regulated to survive the extreme sea ice environment is largely unknown. Here, we show that filamentous phages regulate the host bacterial community to improve survival of the host in permanent Arctic sea ice. We isolated a filamentous phage, f327, from an Arctic sea ice Pseudoalteromonas strain, and we demonstrated that this type of phage is widely distributed in Arctic sea ice. Growth experiments and transcriptome analysis indicated that this phage decreases the host growth rate, cell density and tolerance to NaCl and H2O2, but enhances its motility and chemotaxis. Our results suggest that the presence of the filamentous phage may be beneficial for survival of the host community in sea ice in winter, which is characterized by polar night, nutrient deficiency and high salinity, and that the filamentous phage may help avoid over blooming of the host in sea ice in summer, which is characterized by polar day, rich nutrient availability, intense radiation and high concentration of H2O2. Thus, while they cannot kill the host cells by lysing them, filamentous phages confer properties advantageous to host survival in the Arctic sea ice environment. Our study provides a foremost insight into the ecological role of filamentous phages in the Arctic sea ice ecosystem.

Flavobacterium arcticum SM1502T, isolated from the surface seawater of King's Fjord, Svalbard, Arctic, is an aerobic, non-flagellated, rod-shaped and yellow-pigmented bacterium belonging to the family Flavobacteriaceae. It is the first isolated Arctic Flavobacterium bacterium. To date, no genomes of Flavobacterium species isolated from the Arctic region have been sequenced. Here, we reported the complete genomic compositions and metabolic features of F. arcticum SM1502T. The genome consists of 2,970,356 bp with an average GC content of 35.0%. A total of 2652 protein-coding sequences and 50 RNA genes were identified in the genome. SM1502T had lower protein pIs with higher ratios of acidic residues to basic residues compared to the relative bacteria and contains genes related to iron transport and organic osmoprotectant uptake, which could maintain cellular osmotic balance. These related genes and protein pI characteristics indicated that SM1502T has high salt tolerance and could accommodate the changing Arctic salty environments.

Bacterial extracellular proteases are important for bacterial nutrition and marine sedimentary organic nitrogen degradation. However, only a few proteases from marine sedimentary bacteria have been characterized. Some subtilases have a protease-associated (PA) domain inserted in the catalytic domain. Although structural analysis and deletion mutation suggests that the PA domain in subtilases is involved in substrate binding, direct evidence to support this function is still absent. Here, a protease, P57, secreted by Photobacterium sp. A5-7 isolated from marine sediment was characterized. P57 could hydrolyze casein, gelatin and collagen. It showed the highest activity at 40°C and pH 8.0. P57 is a new subtilase, with 63% sequence identity to the closest characterized protease. Mature P57 contains a catalytic domain and an inserted PA domain. The recombinant PA domain from P57 was shown to have collagen-binding ability, and Phe349 and Tyr432 were revealed to be key residues for collagen binding in the PA domain. This study first shows direct evidence that the PA domain of a subtilase can bind substrate, which provides a better understanding of the function of the PA domain of subtilases and bacterial extracellular proteases from marine sediment.

Bacterial extracellular proteases are important for bacterial nutrition and marine sedimentary organic nitrogen degradation. However, only a few proteases from marine sedimentary bacteria have been characterized. Some subtilases have a protease-associated (PA) domain inserted in the catalytic domain. Although structural analysis and deletion mutation suggests that the PA domain in subtilases is involved in substrate binding, direct evidence to support this function is still absent. Here, a protease, P57, secreted by Photobacterium sp. A5-7 isolated from marine sediment was characterized. P57 could hydrolyze casein, gelatin and collagen. It showed the highest activity at 40°C and pH 8.0. P57 is a new subtilase, with 63% sequence identity to the closest characterized protease. Mature P57 contains a catalytic domain and an inserted PA domain. The recombinant PA domain from P57 was shown to have collagen-binding ability, and Phe349 and Tyr432 were revealed to be key residues for collagen binding in the PA domain. This study first shows direct evidence that the PA domain of a subtilase can bind substrate, which provides a better understanding of the function of the PA domain of subtilases and bacterial extracellular proteases from marine sediment.

Although several serine collagenolytic proteases from bacteria were reported, none has been used to prepare bioactive collagen peptides. MCP-01 is the most abundant extracellular protease of deep-sea Pseudoalteromonas sp. SM9913 and is a serine collagenolytic protease with high efficiency on fish collagen hydrolysis. Here, we set up a pilot scale process to ferment SM9913 for extracellular protease production. With SM9913 extracellular protease as a tool, a process to prepare collagen oligopeptide-rich hydrolysate from codfish skin was set up, which was further scaled up to pilot (100 L) and plant (2000 L) levels with yields >66%. The hydrolysates from laboratory-, pilot- and plant-scales had quite similar quality, containing ~95% peptides with molecular weights lower than 3000 Da and approximately 60% lower than 1000 Da, in which collagen oilgopeptides account for approximately 95%. Bioactivity analyses showed that the hydrolysate had moisture-retention ability, antioxidant activity, and promoting effect on cell viability of human dermal fibroblasts. Safety evaluation showed that the hydrolysate was nontoxic and nonirritating to skin. Therefore, SM9913 extracellular protease is a good enzyme to prepare bioactive oligopeptides from fish skin. The results also suggest that the collagen oligopeptides-rich hydrolysate may have potentials in biomedical, functional food, pharmaceutical and cosmetic industries.