Marine macroalgae affect abundance and community richness of bacterioplankton in close proximity1

Many macroalgae employ chemical means to suppress epibiosis by micro‐ and macroorganisms. Previous studies have focused on the effects of tissue extracts of entire algae on a few bacterial isolates, thereby missing not only ecologically relevant bacteria but also natural delivery mechanisms of algal antimicrobial agents. In this study, we investigated the potential antimicrobial effects of waterborne macroalgal metabolites utilizing a culture‐independent approach to compare the bacterial community richness in seawater in the presence and absence of macroalgae. The methodology comprised the collection of planktonic bacteria in algal culture water on membrane filters followed by filter PCR and denaturant gradient gel electrophoresis (DGGE) of 16S rRNA gene sequences of harvested bacteria with universal primers. Similarity analysis distinguished two groups of macroalgae under investigation, one of which showed >55% difference, and the other <50% difference in bacterial community composition in comparison to natural seawater. The bacterial abundance in algal culture water of different algae was reduced between 20% and 50%. Further experiments demonstrated that the observed effects were caused by waterborne algal compounds. However, some bacterial types were exclusively eliminated in the presence of algae, indicating causative modes of action other than direct exposure of bacteria to waterborne compounds, such as surface‐mediated antimicrobial effects.     

Allelochemicals produced by Caribbean macroalgae and cyanobacteria have species-specific effects on reef coral microorganisms

Coral populations have precipitously declined on Caribbean reefs while algal abundance has increased, leading to enhanced competitive damage to corals, which likely is mediated by the potent allelochemicals produced by both macroalgae and benthic cyanobacteria. Allelochemicals may affect the composition and abundance of coral-associated microorganisms that control host responses and adaptations to environmental change, including susceptibility to bacterial diseases. Here, we demonstrate that extracts of six Caribbean macroalgae and two benthic cyanobacteria have both inhibitory and stimulatory effects on bacterial taxa cultured from the surfaces of Caribbean corals, macroalgae, and corals exposed to macroalgal extracts. The growth of 54 bacterial isolates was monitored in the presence of lipophilic and hydrophilic crude extracts derived from Caribbean macroalgae and cyanobacteria using 96-well plate bioassays. All 54 bacterial cultures were identified by ribotyping. Lipophilic extracts from two species of Dictyota brown algae inhibited >50% of the reef coral bacteria assayed, and hydrophilic compounds from Dictyota menstrualis particularly inhibited Vibrio bacteria, a genus associated with several coral diseases. In contrast, both lipo- and hydrophilic extracts from 2 species of Lyngbya cyanobacteria strongly stimulated bacterial growth. The brown alga Lobophora variegata produced hydrophilic compounds with broad-spectrum antibacterial effects, which inhibited 93% of the bacterial cultures. Furthermore, bacteria cultured from different locations (corals vs. macroalgae vs. coral surfaces exposed to macroalgal extracts) responded differently to algal extracts. These results reveal that extracts from macroalgae and cyanobacteria have species-specific effects on the composition of coral-microbial assemblages, which in turn may increase coral host susceptibility to disease and result in coral mortality.     

Antimicrobial activity of polyphenolic compounds from Spirulina against food-borne bacterial pathogens

Food-borne drug-resistant bacteria have adverse impacts on both food manufacturers and consumers. Disillusionment with the efficacy of current preservatives and antibiotics for controlling food-borne pathogens, especially drug-resistant bacteria, has led to a search for safer alternatives from natural sources. Spirulina have been recognized as a food supplement, natural colorant, and enriched source of bioactive secondary metabolites. The main objectives of this study were to isolate polyphenolic compounds from Spirulina and analyze their antibacterial potential against drug-resistant food-borne bacterial pathogens. We found that fraction B of methanol extract contained a high quantity of polyphenols exhibiting broad spectrum antimicrobial effects against drug-resistant food-borne bacterial pathogens. Potential secondary metabolites, such as benzophenone, dihydro-methyl-phenylacridine, carbanilic acid, dinitrobenzoate, propanediamine, isoquinoline, piperidin, oxazolidin, and pyrrolidine, were identified by gas chromatography and mass spectrophotometry (GCMS). These metabolites are active against both gram-positive and gram-negative pathogens. Our work suggests that phenolic compounds from Spirulina provide a natural and sustainable source of food preservatives for future use.     

Antimicrobial activity observed among cultured marine epiphytic bacteria reflects their potential as a source of new drugs

The surfaces of marine eukaryotes provide a unique habitat for colonizing microorganisms where competition between members of these communities and chemically mediated interactions with their host are thought to influence both microbial diversity and function. For example, it is believed that marine eukaryotes may use their surface-associated bacteria to produce bioactive compounds in defence against competition and to protect the host against further colonization. With the increasing need for novel drug discovery, marine epibiotic bacteria may thus represent a largely underexplored source of new antimicrobial compounds. In the current study, 325 bacterial isolates were obtained from the surfaces of marine algae Delisea pulchra and Ulva australis . Thirty-nine showed to have antimicrobial activity and were identified via 16S rRNA gene sequencing. The majority of those isolates belonged to Alpha- and Gammaproteobacteria . Interestingly, the most commonly isolated bacterial strain, Microbulbifer sp., from the surface of D. pulchra has previously been described as an ecologically significant epibiont of different marine eukaryotes. Other antimicrobial isolates obtained in this study belonged to the phyla Actinobacteria , Firmicutes and Bacteroidetes . Phylogenetically, little overlap was observed among the bacteria obtained from surfaces of D. pulchra and U. australis . The high abundance of cultured isolates that produce antimicrobials suggest that culturing remains a powerful resource for exploring novel bioactives of bacterial origin.     

Extracts of seaweeds as potential inhibitors of quorum sensing and bacterial growth

Macroalgae are an important source of antimicrobial compounds. However, it is unclear if these compounds are produced by the algae themselves, by their associated bacteria, or by both. The main aim of this study was to investigate the potential of macroalgae and their associated microorganisms to inhibit bacterial quorum sensing (QS) and growth. Before extraction, half of the algal specimens were treated with 30% ethanol to remove surface associated bacteria. Canistrocarpus cervicornis extracts were able to inhibit QS of the reporter Chromobacterium violaceum CV017, where extracts with associated bacteria were more efficient than those without bacteria. However, not all algal extracts that inhibited QS of CV017 were able to inhibit bacterial attachment of Pseudomonas aeruginosas PA01, showing specific activity of algal metabolites. Only 58% of the extracts showed antibacterial activity against eight marine fouling and pathogenic bacterial strains tested. Our data suggests that algae and their associated microbiota are important sources of antimicrobial compounds which potentially can be used in future biotechnological applications.     

Planktonic bacteria and fungi are selectively eliminated by exposure to marine macroalgae in close proximity

To test whether macroalgae affect microbial colonizers in close proximity in a phylum-specific fashion, the community richness of planktonic bacteria and fungi was analyzed with selective oligonucleotide probes targeting the Cytophaga/Flavobacterium/Bacteroides (CFB), Alphaproteobacteria and Roseobacter group and the ITS1 region of marine fungi. Naturally occuring planktonic microorganisms were incubated in the presence of macroalgae or in seawater previously conditioned with macroalgal metabolites. The red algae Ceramium rubrum and Mastocarpus stellatus as well as seawater conditioned with these algae reduced the community composition of bacteria to a greater extent than the brown alga Laminaria digitata, indicating that metabolites differed among macroalgae or that the susceptibility of planktonic bacteria towards alga-derived antimicrobials correlated with their phylogenetic affiliation. The most affected phylotypes belonged to the CFB and the Roseobacter clade. The planktonic fungal community was only affected in the presence of macroalgae and not in algal-conditioned water, but with a specificity different from that observed for bacteria. The macroalgae L. digitata and M. stellatus exhibited more pronounced antifungal effects than C. rubrum. This study demonstrates macroalgal defenses against epiphytic microorganisms based on natural delivery mechanisms of allelochemicals utilizing a culture-independent approach, thus minimizing the ecological bias inherent to culture-dependent studies based on few microbial isolates.     

Epibacterial community patterns on marine macroalgae are host-specific but temporally variable

Marine macroalgae are constantly exposed to epibacterial colonizers. The epiphytic bacterial patterns and their temporal and spatial variability on host algae are poorly understood. To investigate the interaction between marine macroalgae and epiphytic bacteria, this study tested if the composition of epibacterial communities on different macroalgae was specific and persisted under varying biotic and abiotic environmental conditions over a 2-year observation time frame. Epibacterial communities on the co-occurring macroalgae Fucus vesiculosus, Gracilaria vermiculophylla and Ulva intestinalis were repeatedly sampled in summer and winter of 2007 and 2008. The epibacterial community composition was analysed by denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene libraries. Epibacterial community profiles did not only differ significantly at each sampling interval among algal species, but also showed consistent seasonal differences on each algal species at a bacterial phylum level. These compositional patterns re-occurred at the same season of two consecutive years. Within replicates of the same algal species, the composition of bacterial phyla was subject to shifts at the bacterial species level, both within the same season but at different years and between different seasons. However, 7-16% of sequences were identified as species specific to the host alga. These findings demonstrate that marine macroalgae harbour species-specific and temporally adapted epiphytic bacterial biofilms on their surfaces. Since several algal host-specific bacteria were highly similar to other bacteria known to either avoid subsequent colonization by eukaryotic larvae or to exhibit potent antibacterial activities, algal host-specific bacterial associations are expected to play an important role for marine macroalgae.     

Screening of antimicrobial activities in red, green and brown macroalgae from Gran Canaria (Canary Islands, Spain)

Extracts from 44 species of seaweed from Gran Canaria (Canary Islands, Spain) were screened for the production of antibacterial and antifungal compounds against a panel of gram-negative and gram-positive bacteria, mycobacteria, yeasts and fungi. A total of 28 species displayed antibacterial activity, of which six also showed antifungal activity. Asparagopsis taxiformis and Cymopolia barbata were the species with the strongest activities against the broadest spectrum of target microorganisms. All the species with antibacterial activity were active against gram-positive bacteria, whereas only two species, A. taxiformis and Osmundea hybrida, were active against mycobacteria. The production of secondary metabolites with antimicrobial activities by the macroalgae was also studied under different conditions, although no common trend for bioactivity was observed.     

Competition-Mediated Antibiotic Induction in the Marine Bacterium Streptomyces tenjimariensis

Microbial competition for limiting natural resources within a community is thought to be the selective force that promotes biosynthesis of antimicrobial compounds The marine bacterium Streptomyces tenjimariensis produces the antibiotics istamycin A and B under select laboratory culture conditions; presumably these compounds serve an, ecological role under natural conditions. Here we report results of a novel marine microbial competion experiment that examined the impact of co-culture of marine bacteria on istamycin production by S. tenjimariensis. Twelve of the 53 bacterial species tested (i.e., 22.6%) induced Istamycin production; this antibiotic also inhibited growth of the competitor colonies. These results suggest that marine bacterial metabolites, serve an ecological role in countering competitive species.     

Exometabolome analysis identifies pyruvate dehydrogenase as a target for the antibiotic triphenylbismuthdichloride in multiresistant bacterial pathogens

The desperate need for new therapeutics against notoriously antibiotic-resistant bacteria has led to a quest for novel antibacterial target structures and compounds. Moreover, defining targets and modes of action of new antimicrobial compounds remains a major challenge with standard technologies. Here we characterize the antibacterial properties of triphenylbismuthdichloride (TPBC), which has recently been successfully used against device-associated infections. We demonstrate that TPBC has potent antimicrobial activity against many bacterial pathogens. Using an exometabolome profiling approach, a unique TPBC-mediated change in the metabolites of Staphylococcus aureus was identified, indicating that TPBC blocks bacterial pyruvate catabolism. Enzymatic studies showed that TPBC is a highly efficient, uncompetitive inhibitor of the bacterial pyruvate dehydrogenase complex. Our study demonstrates that metabolomics approaches can offer new avenues for studying the modes of action of antimicrobial compounds, and it indicates that inhibition of the bacterial pyruvate dehydrogenase complex may represent a promising strategy for combating multidrug-resistant bacteria.     

Bioactive stilbenes from a Bacillus sp. N strain associated with a novel rhabditid entomopathogenic nematode

Aims: The aim of the present study was to purify and characterize a natural antimicrobial compound from Bacillus sp. strain N associated with a novel rhabditid entomopathogenic nematode. Methods and Results: The cell‐free culture filtrate of a bacterium associated with a novel entomopathogenic nematode (EPN), Rhabditis (Oscheius) sp. exhibited strong antimicrobial activity. The ethyl acetate extract of the bacterial culture filtrate was purified by column chromatography, and two bioactive compounds were isolated and their chemical structures were established based on spectral analysis. The compounds were identified as 3,4′,5‐trihydroxystilbene (1) and 3,5‐dihydroxy‐4‐isopropylstilbene (2). The presence of 3,4′,5‐trihydroxystilbene (resveratrol) is reported for the first time in bacteria. Compound 1 showed antibacterial activity against all the four test bacteria, whereas compound 2 was effective against the Gram‐positive bacteria only. Compounds 1 and 2 were active against all the five fungi tested and are more effective than bavistin, the standard fungicide. The antifungal activity of the compounds against the plant pathogenic fungi, Rhizoctonia solani is reported for the first time. Conclusions: Cell‐free extract of the bacterium and isolated stilbenes demonstrated high antibacterial activity against bacteria and fungi especially against plant pathogenic fungi. We conclude that the bacterium‐associated EPN are promising sources of natural bioactive secondary metabolites. Significance and Impact of the Study: Stilbene compounds can be used for the control of fungi and bacteria.     

Soil Bacteria are Differentially Affected by the Resin of the Medicinal Plant Pseudognaphalium vira vira and Its Main Component Kaurenoic Acid

The diterpenoid kaurenoic acid is the main component of the resin from the medicinal plant Pseudognaphalium vira vira. As some diterpenoids have antimicrobial properties, the effect of this resin and the kaurenoic acid on soil bacteria was studied. The resin of P. vira vira and purified kaurenoic acid were two to four times more effective as antibacterial agents with Gram-positive than with Gram-negative soil isolates. The chemical stability of kaurenoic acid and the antibacterial activity of both the resin and the diterpenoid were studied in microcosms containing plant-associated soil. After 15 days of incubation, the diterpenoid was stable, as determined by ¹H nuclear magnetic resonance and thin-layer chromatography, and soil extracts still exhibited antibacterial activity. However, after 30 days of incubation, loss of antibacterial activity of soil extracts correlated with removal or chemical modification of kaurenoic acid. The effect of the resin or this diterpenoid on the soil bacteria community was analyzed by the terminal restriction fragment length polymorphisms technique. After 15 days of incubation, the resin and the pure compound caused significant changes in the soil bacterial community. The relative abundance of specific bacterial groups was differentially affected by the resin components, being the effects with the resin stronger than with the kaurenoic acid. After 30 days of incubation, these changes mostly reverted. These results indicate that a plant resin containing diterpenoid compounds plays a significant role controlling specific groups of microorganisms in the soil associated with the plant.     

Propolis as a novel antibacterial agent

Propolis (bee glue) is a bee glue, sticky resinous material released from various plant sources such as bud exudates, flowers, and leaves modified by bee secretions and wax propolis is composed of resins, waxes, polyphenols, polysaccharides, volatile materials, and secondary metabolites that are responsible for various bioactivity such as antibacterial, anti-angiogenic, antiulcer, anti-inflammatory, antioxidant, and anti-viral activities. The physico-chemical characteristics and the natural properties of various kinds of propolis have been studied for the past decade. Novel active anti-microbial compounds have been identified in propolis. Those compounds positively modulated the antimicrobial resistance of multidrug resistant bacteria. Published research has indicated that propolis and its derivatives has many natural antimicrobial compounds with a broad spectrum against different types of bacteria and that it enhanced the efficacy of conventional antibiotics. Besides, the combination of propolis with other compounds such as honey has been studied whereby, such combinations have a synergistic effect against bacterial strains such as Escherichia coli and Staphylococcus aureus. The activity of propolis is very much dependent on seasonal and regional factors, and Middle Eastern propolis have shown best antibacterial efficacy. Propolis and its main flavonoids ingredients should not be overlooked and should be evaluated in clinical trials to better elucidate their potential application in various fields of medicine. Clinical antibacterial potential and its use in new drugs of biotechnological products should be conducted. This review aims at highlighting some of the recent scientific findings associated with the antibacterial properties of propolis and its components.     

Phylogeny and bioactivity of epiphytic Gram-positive bacteria isolated from three co-occurring antarctic macroalgae

Marine macroalgae are emerging as an untapped source of novel microbial diversity and, therefore, of new bioactive secondary metabolites. This study was aimed at assessing the diversity and antimicrobial activity of the culturable Gram-positive bacteria associated with the surface of three co-occurring Antarctic macroalgae. Specimens of Adenocystis utricularis (brown alga), Iridaea cordata (red alga) and Monostroma hariotii (green alga) were collected from the intertidal zone of King George Island, Antarctica. Gram-positive bacteria were investigated by cultivation-based methods and 16S rRNA gene sequencing, and screened for antimicrobial activity against a panel of pathogenic microorganisms. Isolates were found to belong to 12 families, with a dominance of Microbacteriaceae and Micrococcaceae. Seventeen genera of Actinobacteria and 2 of Firmicutes were cultured from the three macroalgae, containing 29 phylotypes. Three phylotypes within Actinobacteria were regarded as potentially novel species. Sixteen isolates belonging to the genera Agrococcus, Arthrobacter, Micrococcus, Pseudarthrobacter, Pseudonocardia, Sanguibacter, Staphylococcus, Streptomyces and Tessaracoccus exhibited antibiotic activity against at least one of the indicator strains. The bacterial phylotype composition was distinct among the three macroalgae species, suggesting that these macroalgae host species-specific Gram-positive associates. The results highlight the importance of Antarctic macroalgae as a rich source of Gram-positive bacterial diversity and potentially novel species, and a reservoir of bacteria producing biologically active compounds with pharmacological potential.     

Antimicrobial activity in sub-Arctic marine invertebrates

In the marine environment, any living or non-living surface is exposed to bacterial colonization. Many invertebrate species in temperate, tropical and Antarctic regions have demonstrated chemical defences against the formation of microbial films. In the present study, the antimicrobial activity of sub-Arctic invertebrates was investigated for the first time. Crude extracts of abundant invertebrates belonging to several taxonomic groups were tested for their inhibitory effects on the growth of five sympatric phylogenetically diverse bacterial strains. Six out of 18 (33%) crude extracts inhibited bacterial growth at natural extract concentrations. The crude extract of the sponge Haliclona viscosa inhibited growth of all five bacterial strains, suggesting the presence of metabolites with broad-spectrum activity. Three active compounds were isolated from H. viscosa having antibacterial properties similar to those of the crude extract. Our data indicate that antibacterial secondary metabolites are present in sub-Arctic marine invertebrates but are less abundant than in temperate, tropical or Antarctic species.     

Ecological role of a seaweed secondary metabolite for a colonizing bacterial community

Bacteria associated with seaweeds can both harm and benefit their hosts. Many seaweed species are known to produce compounds that inhibit growth of bacterial isolates, but the ecological role of seaweed metabolites for the associated bacterial community structure is not well understood. In this study the response of a colonizing bacterial community to the secondary metabolite (1,1,3,3-tetrabromo-2-heptanone) from the red alga Bonnemaisonia hamifera was investigated by using field panels coated with the metabolite at a range of concentrations covering those measured at the algal surface. The seaweed metabolite has previously been shown to have antibacterial effects. The metabolite significantly affected the natural fouling community by (i) altering the composition, (ii) altering the diversity by increasing the evenness and (iii) decreasing the density, as measured by terminal restriction fragment length polymorphism in conjunction with clone libraries of the 16S rRNA genes and by bacterial enumeration. No single major bacterial taxon (phylum, class) was particularly affected by the metabolite. Instead changes in community composition were observed at a more detailed phylogenetic level. This indicates a broad specificity of the seaweed metabolite against bacterial colonization, which is supported by the observation that the bacterial density was significantly affected at a lower concentration (0.02 μg cm^?2) than the composition (1–2.5 μg cm^?2) and the evenness (5 μg cm^?2) of the bacterial communities. Altogether, the results emphasize the role of secondary metabolites for control of the density and structure of seaweed-associated bacterial communities.     

Ecological role of a seaweed secondary metabolite for a colonizing bacterial community

Bacteria associated with seaweeds can both harm and benefit their hosts. Many seaweed species are known to produce compounds that inhibit growth of bacterial isolates, but the ecological role of seaweed metabolites for the associated bacterial community structure is not well understood. In this study the response of a colonizing bacterial community to the secondary metabolite (1,1,3,3-tetrabromo-2-heptanone) from the red alga Bonnemaisonia hamifera was investigated by using field panels coated with the metabolite at a range of concentrations covering those measured at the algal surface. The seaweed metabolite has previously been shown to have antibacterial effects. The metabolite significantly affected the natural fouling community by (i) altering the composition, (ii) altering the diversity by increasing the evenness and (iii) decreasing the density, as measured by terminal restriction fragment length polymorphism in conjunction with clone libraries of the 16S rRNA genes and by bacterial enumeration. No single major bacterial taxon (phylum, class) was particularly affected by the metabolite. Instead changes in community composition were observed at a more detailed phylogenetic level. This indicates a broad specificity of the seaweed metabolite against bacterial colonization, which is supported by the observation that the bacterial density was significantly affected at a lower concentration (0.02 μg cm?2) than the composition (1-2.5 μg cm?2) and the evenness (5 μg cm?2) of the bacterial communities. Altogether, the results emphasize the role of secondary metabolites for control of the density and structure of seaweed-associated bacterial communities.     

The green macroalga Dictyosphaeria ocellata influences the structure of the bacterioplankton community through differential effects on individual bacterial phylotypes

Marine macroalgae are subjected to large numbers of bacteria in their environment. These bacteria have the potential to affect the health and ecology of algae in a variety of ways and can be both beneficial and harmful to the algae. Therefore, algae have likely evolved mechanisms to differentially regulate the growth of bacterial species. In this study, we examined the effects of the green alga Dictyosphaeria ocellata on the bacterioplankton community in field enclosure experiments and on individual, naturally co-occurring bacterial strains in laboratory co-culture experiments. In field experiments, we compared the bacterioplankton communities of enclosures with and without D. ocellata using denaturing gradient gel electrophoresis and found that the alga significantly changed the bacterial community composition. Seven bacterial phylotypes were eliminated in the presence of the alga and five were found exclusively with the alga. We also examined the effects of algal-treated water on the development of the bacterial community within enclosures and found no change in the community composition. Laboratory co-culture experiments revealed that D. ocellata and D. ocellata extracts affect the growth of individual bacterial strains in a species-specific manner and that the mechanisms responsible for these effects also differed by bacterial species.     

Antifouling properties of 10beta-formamidokalihinol-A and kalihinol A isolated from the marine sponge Acanthella cavernosa

Many soft-bodied sessile marine invertebrates such as sponges and soft corals defend themselves against fouling directly through the production of antifouling compounds, or indirectly through regulating the epibiotic microbes that affect larval settlement. In this study, 10beta-formamidokalihinol-A and kalihinol A were isolated and purified from the marine sponge Acanthella cavernosa (Dendy). The results indicated that both compounds inhibited the growth of bacteria isolated from the natural environment whereas kalihinol A suppressed larval settlement of a major fouling polychaete, Hydroides elegans with an EC50 of 0.5 microg ml(-1). Kalihinol A was incorporated in Phytagel that was exposed to the bacterial consortia in natural seawater for biofilm formation. Biofilms that developed on the Phytagel surfaces were analysed for bacterial abundance and bacterial species composition using a DNA fingerprinting technique, terminal restriction fragment length polymorphism (T-RFLP). The results showed that kalihinol A only slightly reduced bacterial abundance (t-test, p = 0.0497), but modified the bacterial species composition of the biofilms. Inhibition of H. elegans larval settlement was observed when biofilms developed under the influence of kalihinol A were exposed to larvae, suggesting that compounds like kalihinol A from the sponge A. cavernosa may change bacterial community composition on the sponge surface, which in turn, modulates larval settlement of fouling organisms.     

A Dissolved Oxygen Threshold for Shifts in Bacterial Community Structure in a Seasonally Hypoxic Estuary

Pelagic ecosystems can become depleted of dissolved oxygen as a result of both natural processes and anthropogenic effects. As dissolved oxygen concentration decreases, energy shifts from macrofauna to microorganisms, which persist in these hypoxic zones. Oxygen-limited regions are rapidly expanding globally; however, patterns of microbial communities associated with dissolved oxygen gradients are not yet well understood. To assess the effects of decreasing dissolved oxygen on bacteria, we examined shifts in bacterial community structure over space and time in Hood Canal, Washington, USA−a glacial fjord-like water body that experiences seasonal low dissolved oxygen levels known to be detrimental to fish and other marine organisms. We found a strong negative association between bacterial richness and dissolved oxygen. Bacterial community composition across all samples was also strongly associated with the dissolved oxygen gradient, and significant changes in bacterial community composition occurred at a dissolved oxygen concentration between 5.18 and 7.12 mg O₂ L^-1. This threshold value of dissolved oxygen is higher than classic definitions of hypoxia (<2.0 mg O₂ L^-1), suggesting that changes in bacterial communities may precede the detrimental effects on ecologically and economically important macrofauna. Furthermore, bacterial taxa responsible for driving whole community changes across the oxygen gradient are commonly detected in other oxygen-stressed ecosystems, suggesting that the patterns we uncovered in Hood Canal may be relevant in other low oxygen ecosystems.