Genomic diversification of giant enteric symbionts reflects host dietary lifestyles

Fecal bacterial communities of the platypus (Ornithorhynchus anatinus) reflect captivity status-Implications for conservation and management

The duck-billed platypus (Ornithorhynchus anatinus) is currently listed as near-threatened. A key part of the conservation strategy for this species is its captive maintenance; however, captive animals often have dysbiotic gut bacterial microbiomes. Here, for the first time, we characterize the gut microbiome of wild platypus via fecal samples using high-throughput sequencing of the bacterial 16S rRNA gene and identify microbial biomarkers of captivity in this species. At the phylum level, Firmicutes (50.4%) predominated among all platypuses, followed by Proteobacteria (28.7%), Fusobacteria (13.4%), and Bacteroidota (6.9%), with 21 "core" bacteria identified. Captive individuals did not differ in their microbial α-diversity compared to wild platypus but had significantly different community composition (β-diversity) and exhibited higher abundances of Enterococcus, which are potential pathogenic bacteria. Four taxa were identified as biomarkers of wild platypus, including Rickettsiella, Epulopiscium, Clostridium, and Cetobacterium. This contrast in gut microbiome composition between wild and captive platypus is an essential insight for guiding conservation management, as the rewilding of captive animal microbiomes is a new and emerging tool to improve captive animal health, maximize captive breeding efforts, and give reintroduced or translocated animals the best chance of survival.

Refocusing the microbial rare biosphere concept through a functional lens

The influential concept of the rare biosphere in microbial ecology has underscored the importance of taxa occurring at low abundances yet potentially playing key roles in communities and ecosystems. Here, we refocus the concept of rare biosphere through a functional trait-based lens and provide a framework to characterize microbial functional rarity, a combination of numerical scarcity across space or time and trait distinctiveness. We demonstrate how this novel interpretation of the rare biosphere, rooted in microbial functions, can enhance our mechanistic understanding of microbial community structure. It also sheds light on functionally distinct microbes, directing conservation efforts towards taxa harboring rare yet ecologically crucial functions.

Bacteroidia and Clostridia are equipped to degrade a cascade of polysaccharides along the hindgut of the herbivorous fish Kyphosus sydneyanus

The gut microbiota of the marine herbivorous fish Kyphosus sydneyanus are thought to play an important role in host nutrition by supplying short-chain fatty acids (SCFAs) through fermentation of dietary red and brown macroalgae. Here, using 645 metagenome-assembled genomes (MAGs) from wild fish, we determined the capacity of different bacterial taxa to degrade seaweed carbohydrates along the gut. Most bacteria (99%) were unclassified at the species level. Gut communities and CAZyme-related transcriptional activity were dominated by Bacteroidia and Clostridia. Both classes possess genes CAZymes acting on internal polysaccharide bonds, suggesting their role initiating glycan depolymerization, followed by rarer Gammaproteobacteria and Verrucomicrobiae. Results indicate that Bacteroidia utilize substrates in both brown and red algae, whereas other taxa, namely, Clostridia, Bacilli, and Verrucomicrobiae, utilize mainly brown algae. Bacteroidia had the highest CAZyme gene densities overall, and Alistipes were especially enriched in CAZyme gene clusters (n = 73 versus just 62 distributed across all other taxa), pointing to an enhanced capacity for macroalgal polysaccharide utilization (e.g., alginate, laminarin, and sulfated polysaccharides). Pairwise correlations of MAG relative abundances and encoded CAZyme compositions provide evidence of potential inter-species collaborations. Co-abundant MAGs exhibited complementary degradative capacities for specific substrates, and flexibility in their capacity to source carbon (e.g., glucose- or galactose-rich glycans), possibly facilitating coexistence via niche partitioning. Results indicate the potential for collaborative microbial carbohydrate metabolism in the K. sydneyanus gut, that a greater variety of taxa contribute to the breakdown of brown versus red dietary algae, and that Bacteroidia encompass specialized macroalgae degraders.

The exceptional form and function of the giant bacterium Ca. Epulopiscium viviparus revolves around its sodium motive force

Epulopiscium spp. are the largest known heterotrophic bacteria; a large cigar-shaped individual is a million times the volume of Escherichia coli. To better understand the metabolic potential and relationship of Epulopiscium sp. type B with its host Naso tonganus, we generated a high-quality draft genome from a population of cells taken from a single fish. We propose the name Candidatus Epulopiscium viviparus to describe populations of this best-characterized Epulopiscium species. Metabolic reconstruction reveals more than 5% of the genome codes for carbohydrate active enzymes, which likely degrade recalcitrant host-diet algal polysaccharides into substrates that may be fermented to acetate, the most abundant short-chain fatty acid in the intestinal tract. Moreover, transcriptome analyses and the concentration of sodium ions in the host intestinal tract suggest that the use of a sodium motive force (SMF) to drive ATP synthesis and flagellar rotation is integral to symbiont metabolism and cellular biology. In natural populations, genes encoding both F-type and V-type ATPases and SMF generation via oxaloacetate decarboxylation are among the most highly expressed, suggesting that ATPases synthesize ATP and balance ion concentrations across the cell membrane. High expression of these and other integral membrane proteins may allow for the growth of its extensive intracellular membrane system. Further, complementary metabolism between microbe and host is implied with the potential provision of nitrogen and B vitamins to reinforce this nutritional symbiosis. The few features shared by all bacterial behemoths include extreme polyploidy, polyphosphate synthesis, and thus far, they have all resisted cultivation in the lab.

The phenotype and genotype of fermentative prokaryotes

Fermentation is a type of metabolism pervasive in oxygen-deprived environments. Despite its importance, we know little about the range and traits of organisms that carry out this metabolism. Our study addresses this gap with a comprehensive analysis of the phenotype and genotype of fermentative prokaryotes. We assembled a dataset with phenotypic records of 8350 organisms plus 4355 genomes and 13.6 million genes. Our analysis reveals fermentation is both widespread (in ~30% of prokaryotes) and complex (forming ~300 combinations of metabolites). Furthermore, it points to previously uncharacterized proteins involved in this metabolism. Previous studies suggest that metabolic pathways for fermentation are well understood, but metabolic models built in our study show gaps in our knowledge. This study demonstrates the complexity of fermentation while showing that there is still much to learn about this metabolism. All resources in our study can be explored by the scientific community with an online, interactive tool.

Gill-associated bacteria are homogeneously selected in amphibious mangrove crabs to sustain host intertidal adaptation

BACKGROUND

The transition from water to air is a key event in the evolution of many marine organisms to access new food sources, escape water hypoxia, and exploit the higher and temperature-independent oxygen concentration of air. Despite the importance of microorganisms in host adaptation, their contribution to overcoming the challenges posed by the lifestyle changes from water to land is not well understood. To address this, we examined how microbial association with a key multifunctional organ, the gill, is involved in the intertidal adaptation of fiddler crabs, a dual-breathing organism.

RESULTS

Electron microscopy revealed a rod-shaped bacterial layer tightly connected to the gill lamellae of the five crab species sampled across a latitudinal gradient from the central Red Sea to the southern Indian Ocean. The gill bacterial community diversity assessed with 16S rRNA gene amplicon sequencing was consistently low across crab species, and the same actinobacterial group, namely Ilumatobacter, was dominant regardless of the geographic location of the host. Using metagenomics and metatranscriptomics, we detected that these members of actinobacteria are potentially able to convert ammonia to amino acids and may help eliminate toxic sulphur compounds and carbon monoxide to which crabs are constantly exposed.

CONCLUSIONS

These results indicate that bacteria selected on gills can play a role in the adaptation of animals in dynamic intertidal ecosystems. Hence, this relationship is likely to be important in the ecological and evolutionary processes of the transition from water to air and deserves further attention, including the ontogenetic onset of this association. Video Abstract.

Herbivorous Fish Microbiome Adaptations to Sulfated Dietary Polysaccharides

Marine herbivorous fish that feed primarily on macroalgae, such as those from the genus Kyphosus, are essential for maintaining coral health and abundance on tropical reefs. Here, deep metagenomic sequencing and assembly of gut compartment-specific samples from three sympatric, macroalgivorous Hawaiian kyphosid species have been used to connect host gut microbial taxa with predicted protein functional capacities likely to contribute to efficient macroalgal digestion. Bacterial community compositions, algal dietary sources, and predicted enzyme functionalities were analyzed in parallel for 16 metagenomes spanning the mid- and hindgut digestive regions of wild-caught fishes. Gene colocalization patterns of expanded carbohydrate (CAZy) and sulfatase (SulfAtlas) digestive enzyme families on assembled contigs were used to identify likely polysaccharide utilization locus associations and to visualize potential cooperative networks of extracellularly exported proteins targeting complex sulfated polysaccharides. These insights into the gut microbiota of herbivorous marine fish and their functional capabilities improve our understanding of the enzymes and microorganisms involved in digesting complex macroalgal sulfated polysaccharides. IMPORTANCE This work connects specific uncultured bacterial taxa with distinct polysaccharide digestion capabilities lacking in their marine vertebrate hosts, providing fresh insights into poorly understood processes for deconstructing complex sulfated polysaccharides and potential evolutionary mechanisms for microbial acquisition of expanded macroalgal utilization gene functions. Several thousand new marine-specific candidate enzyme sequences for polysaccharide utilization have been identified. These data provide foundational resources for future investigations into suppression of coral reef macroalgal overgrowth, fish host physiology, the use of macroalgal feedstocks in terrestrial and aquaculture animal feeds, and the bioconversion of macroalgae biomass into value-added commercial fuel and chemical products.

Microbial circadian clocks: host-microbe interplay in diel cycles

BACKGROUND

Circadian rhythms, observed across all domains of life, enable organisms to anticipate and prepare for diel changes in environmental conditions. In bacteria, a circadian clock mechanism has only been characterized in cyanobacteria to date. These clocks regulate cyclical patterns of gene expression and metabolism which contribute to the success of cyanobacteria in their natural environments. The potential impact of self-generated circadian rhythms in other bacterial and microbial populations has motivated extensive research to identify novel circadian clocks.

MAIN TEXT

Daily oscillations in microbial community composition and function have been observed in ocean ecosystems and in symbioses. These oscillations are influenced by abiotic factors such as light and the availability of nutrients. In the ocean ecosystems and in some marine symbioses, oscillations are largely controlled by light-dark cycles. In gut systems, the influx of nutrients after host feeding drastically alters the composition and function of the gut microbiota. Conversely, the gut microbiota can influence the host circadian rhythm by a variety of mechanisms including through interacting with the host immune system. The intricate and complex relationship between the microbiota and their host makes it challenging to disentangle host behaviors from bacterial circadian rhythms and clock mechanisms that might govern the daily oscillations observed in these microbial populations.

CONCLUSIONS

While the ability to anticipate the cyclical behaviors of their host would likely be enhanced by a self-sustained circadian rhythm, more evidence and further studies are needed to confirm whether host-associated heterotrophic bacteria possess such systems. In addition, the mechanisms by which heterotrophic bacteria might respond to diel cycles in environmental conditions has yet to be uncovered.

Hatchery tanks induce intense reduction in microbiota diversity associated with gills and guts of two endemic species of the São Francisco River

The São Francisco River (SFR), one of the main Brazilian rivers, has suffered cumulative anthropogenic impacts, leading to ever-decreasing fish stocks and environmental, economic, and social consequences. Rhinelepis aspera and Prochilodus argenteus are medium-sized, bottom-feeding, and rheophilic fishes from the SFR that suffer from these actions. Both species are targeted for spawning and restocking operations due to their relevance in artisanal fisheries, commercial activities, and conservation concerns. Using high-throughput sequencing of the 16S rRNA gene, we characterized the microbiome present in the gills and guts of these species recruited from an impacted SFR region and hatchery tanks (HT). Our results showed that bacterial diversity from the gill and gut at the genera level in both fish species from HT is 87% smaller than in species from the SFR. Furthermore, only 15 and 29% of bacterial genera are shared between gills and guts in R. aspera and P. argenteus from SFR, respectively, showing an intimate relationship between functional differences in organs. In both species from SFR, pathogenic, xenobiont-degrading, and cyanotoxin-producer bacterial genera were found, indicating the critical pollution scenario in which the river finds itself. This study allowed us to conclude that the conditions imposed on fish in the HT act as important modulators of microbial diversity in the analyzed tissues. It also raises questions regarding the effects of these conditions on hatchery spawn fish and their suitability for restocking activities, aggravated by the narrow genetic diversity associated with such freshwater systems.

Intestinal Microbiome Richness of Coral Reef Damselfishes (Actinopterygii: Pomacentridae)

Fish gastro-intestinal system harbors diverse microbiomes that affect the host's digestion, nutrition, and immunity. Despite the great taxonomic diversity of fish, little is understood about fish microbiome and the factors that determine its structure and composition. Damselfish are important coral reef species that play pivotal roles in determining algae and coral population structures of reefs. Broadly, damselfish belong to either of two trophic guilds based on whether they are planktivorous or algae-farming. In this study, we used 16S rRNA gene sequencing to investigate the intestinal microbiome of 5 planktivorous and 5 algae-farming damselfish species (Pomacentridae) from the Great Barrier Reef. We detected Gammaproteobacteria ASVs belonging to the genus Actinobacillus in 80% of sampled individuals across the 2 trophic guilds, thus, bacteria in this genus can be considered possible core members of pomacentrid microbiomes. Algae-farming damselfish had greater bacterial alpha-diversity, a more diverse core microbiome and shared 35 ± 22 ASVs, whereas planktivorous species shared 7 ± 3 ASVs. Our data also highlight differences in microbiomes associated with both trophic guilds. For instance, algae-farming damselfish were enriched in Pasteurellaceae, whilst planktivorous damselfish in Vibrionaceae. Finally, we show shifts in bacterial community composition along the intestines. ASVs associated with the classes Bacteroidia, Clostridia, and Mollicutes bacteria were predominant in the anterior intestinal regions while Gammaproteobacteria abundance was higher in the stomach. Our results suggest that the richness of the intestinal bacterial communities of damselfish reflects host species diet and trophic guild.

The gut microbiome variability of a butterflyfish increases on severely degraded Caribbean reefs

Environmental degradation has the potential to alter key mutualisms that underlie the structure and function of ecological communities. How microbial communities associated with fishes vary across populations and in relation to habitat characteristics remains largely unknown despite their fundamental roles in host nutrition and immunity. We find significant differences in the gut microbiome composition of a facultative coral-feeding butterflyfish (Chaetodon capistratus) across Caribbean reefs that differ markedly in live coral cover (∼0–30%). Fish gut microbiomes were significantly more variable at degraded reefs, a pattern driven by changes in the relative abundance of the most common taxa potentially associated with stress. We also demonstrate that fish gut microbiomes on severely degraded reefs have a lower abundance of Endozoicomonas and a higher diversity of anaerobic fermentative bacteria, which may suggest a less coral dominated diet. The observed shifts in fish gut bacterial communities across the habitat gradient extend to a small set of potentially beneficial host associated bacteria (i.e., the core microbiome) suggesting essential fish-microbiome interactions may be vulnerable to severe coral degradation.

The gut microbiome composition of the coral-feeding butterflyfish across Caribbean reefs is more variable at degraded reefs. These microbiomes have a lower abundance of Endozoicomonas and a higher diversity of anaerobic fermentative bacteria.

Substrate degradation pathways, conserved functions and community composition of the hindgut microbiota in the herbivorous marine fish Kyphosus sydneyanus

Symbiotic gut microbiota in the herbivorous marine fish Kyphosus sydneyanus play an important role in digestion by converting refractory algal carbohydrate into short-chain fatty acids. Here we characterised community composition using both 16S rRNA gene amplicon sequencing and shotgun-metagenome sequencing. Sequencing was carried out on lumen and mucosa samples (radial sections) from three axial sections taken from the hindgut of wild-caught fish. Both lumen and mucosa communities displayed distinct distributions along the hindgut, likely an effect of the differing selection pressures within these hindgut locations, as well as considerable variation among individual fish. In contrast, metagenomic sequences displayed a high level of functional similarity between individual fish and gut sections in the relative abundance of genes (based on sequencing depth) that encoded enzymes involved in algal-derived substrate degradation. These results suggest that the host gut environment selects for functional capacity in symbionts rather than taxonomic identity. Functional annotation of the enzymes encoded by the gut microbiota was carried out to infer the metabolic pathways used by the gut microbiota for the degradation of important dietary substrates: mannitol, alginate, laminarin, fucoidan and galactan (e.g. agar and carrageenan). This work provides the first evidence of the genomic potential of K. sydneyanus hindgut microbiota to convert highly refractory algal carbohydrates into metabolically useful short-chain fatty acids.

Shift and homogenization of gut microbiome during invasion in marine fishes

Biological invasion is one of the main components of global changes in aquatic ecosystems. Unraveling how establishment in novel environments affects key biological features of animals is a key step towards understanding invasion. Gut microbiome of herbivorous animals is important for host health but has been scarcely assessed in invasive species. Here, we characterized the gut microbiome of two invasive marine herbivorous fishes (Siganus rivulatus and Siganus luridus) in their native (Red Sea) and invaded (Mediterranean Sea) ranges. The taxonomic and phylogenetic diversity of the microbiome increased as the fishes move away from the native range and its structure became increasingly different from the native microbiome. These shifts resulted in homogenization of the microbiome in the invaded range, within and between the two species. The shift in microbial diversity was associated with changes in its functions related with the metabolism of short-chain fatty acids. Altogether, our results suggest that the environmental conditions encountered by Siganidae during their expansion in Mediterranean ecosystems strongly modifies the composition of their gut microbiome along with its putative functions. Further studies should pursue to identify the precise determinants of these modifications (e.g. changes in host diet or behavior, genetic differentiation) and whether they participate in the ecological success of these species.

Fine scale transitions of the microbiota and metabolome along the gastrointestinal tract of herbivorous fishes

Background

Gut microorganisms aid in the digestion of food by providing exogenous metabolic pathways to break down organic compounds. An integration of longitudinal microbial and chemical data is necessary to illuminate how gut microorganisms supplement the energetic and nutritional requirements of animals. Although mammalian gut systems are well-studied in this capacity, the role of microbes in the breakdown and utilization of recalcitrant marine macroalgae in herbivorous fish is relatively understudied and an emerging priority for bioproduct extraction. Here we use a comprehensive survey of the marine herbivorous fish gut microbial ecosystem via parallel 16S rRNA gene amplicon profiling (microbiota) and untargeted tandem mass spectrometry (metabolomes) to demonstrate consistent transitions among 8 gut subsections across five fish of the genus of Kyphosus.

Results

Integration of microbial phylogenetic and chemical diversity data reveals that microbial communities and metabolomes covaried and differentiated continuously from stomach to hindgut, with the midgut containing multiple distinct and previously uncharacterized microenvironments and a distinct hindgut community dominated by obligate anaerobes. This differentiation was driven primarily by anaerobic gut endosymbionts of the classes Bacteroidia and Clostridia changing in concert with bile acids, small peptides, and phospholipids: bile acid deconjugation associated with early midgut microbiota, small peptide production associated with midgut microbiota, and phospholipid production associated with hindgut microbiota.

Conclusions

The combination of microbial and untargeted metabolomic data at high spatial resolution provides a new view of the diverse fish gut microenvironment and serves as a foundation to understand functional partitioning of microbial activities that contribute to the digestion of complex macroalgae in herbivorous marine fish.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-022-00182-z.

Diverse events have transferred genes for edible seaweed digestion from marine to human gut bacteria

Humans harbor numerous species of colonic bacteria that digest fiber polysaccharides in commonly consumed terrestrial plants. More recently in history, regional populations have consumed edible macroalgae seaweeds containing unique polysaccharides. It remains unclear how extensively gut bacteria have adapted to digest these nutrients. Here, we show that the ability of gut bacteria to digest seaweed polysaccharides is more pervasive than previously appreciated. Enrichment-cultured Bacteroides harbor previously discovered genes for seaweed degradation, which have mobilized into several members of this genus. Additionally, other examples of marine bacteria-derived genes, and their mobile DNA elements, are involved in gut microbial degradation of seaweed polysaccharides, including genes in gut-resident Firmicutes. Collectively, these results uncover multiple separate events that have mobilized the genes encoding seaweed-degrading-enzymes into gut bacteria. This work further underscores the metabolic plasticity of the human gut microbiome and global exchange of genes in the context of dietary selective pressures.

Distinct microbiota composition and fermentation products indicate functional compartmentalization in the hindgut of a marine herbivorous fish

Many marine herbivorous fishes harbour diverse microbial communities in the hindgut that can play important roles in host health and nutrition. Kyphosus sydneyanus is a temperate marine herbivorous fish that feeds predominantly on brown seaweeds. We employed 16S rRNA gene amplicon sequencing and gas chromatography to characterize microbial communities and their metabolites in different hindgut regions of six K. sydneyanus. Measurements were confined to three distal sections of the intestine, labelled III, IV and V from anterior to posterior. A total of 625 operational taxonomic units from 20 phyla and 123 genera were obtained. Bacteroidota, Firmicutes and Proteobacteria were the major phyla in mean relative abundance, which varied along the gut. Firmicutes (76%) was the most dominant group in section III, whereas Bacteroidota (69.3%) dominated section V. Total short‐chain fatty acid (SCFA) concentration was highest in sections IV and V, confirming active fermentation in these two most distal sections. The abundance of Bacteroidota correlated with propionate concentration in section V, while Firmicutes positively correlated with formate in sections III and IV. Acetate levels were highest in sections IV and V, which correlated with abundance of Bacteroidota. Despite differences in gut microbial community composition, SCFA profiles were consistent between individual fish in the different hindgut regions of K. sydneyanus, although proportions of SCFAs differed among gut sections. These findings demonstrate functional compartmentalization of the hindgut microbial community, highlighting the need for regional sampling when interpreting overall microbiome function. These results support previous work suggesting that hindgut microbiota in marine herbivorous fish are important to nutrition in some host species by converting dietary carbohydrates into metabolically useful SCFAs.

Microbiomes and Obligate Symbiosis of Deep-Sea Animals

Microbial communities associated with deep-sea animals are critical to the establishment of novel biological communities in unusual environments. Over the past few decades, rapid exploration of the deep sea has enabled the discovery of novel microbial communities, some of which form symbiotic relationships with animal hosts. Symbiosis in the deep sea changes host physiology, behavior, ecology, and evolution over time and space. Symbiont diversity within a host is often aligned with diverse metabolic pathways that broaden the environmental niche for the animal host. In this review, we focus on microbiomes and obligate symbionts found in different deep-sea habitats and how they facilitate survival of the organisms that live in these environments. In addition, we discuss factors that govern microbiome diversity, host specificity, and biogeography in the deep sea. Finally, we highlight the current limitations of microbiome research and draw a road map for future directions to advance our knowledge of microbiomes in the deep sea. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Recent progress in the understanding of the gut microbiota of marine fishes

As the significance of the gut microbiota has become increasingly realized, a large number of related studies have emerged. With respect to the gut microbial composition of fish, the predominant gut microbes and core gut microbiota have been reported by many researchers. Our understanding of fish gut microbiota, especially its functional roles, has fallen far behind that of terrestrial vertebrates, although previous studies using gnotobiotic zebrafish models have revealed that the gut microbiota performs a significant role in gut development, nutrient metabolism and immune responses. Given that environmental factors of marine habitats are very different from those of freshwater habitats, a distinct difference may exist in the gut microbiota between freshwater and marine fish. Therefore, this review aims to address the advances in marine fish gut microbiota in terms of methodologies, the gut microbial composition, and gnotobiotic models of marine fish, the important factors (host genotype and three environmental factors: temperature, salinity and diet) that drive marine fish gut microbiota, and significant roles of the gut microbiota in marine fish.

Histology and ultrastructure of the gastrointestinal tract in four temperate marine herbivorous fishes

While alimentary tract anatomy in many terrestrial herbivorous vertebrates is well documented, the digestive systems of marine herbivorous fishes are poorly characterised. The gastrointestinal tract (GIT) of four species of marine herbivorous fishes from northeastern New Zealand, butterfish Odax pullus (Labridae), marblefish Aplodactylus arctidens (Aplodactylidae), notch-head marblefish A. etheridgii (Aplodactylidae) and silver drummer Kyphosus sydneyanus (Kyphosidae), were examined using histology and transmission electron microscopy (TEM) to provide a detailed histological and ultrastructural description of gut anatomy. Gastric glands were distributed over rugae in the stomach of A. arctidens, A. etheridgii and K. sydneyanus. The luminal surface of the stomach of these three species was lined by columnar mucous cells, and oxynticopeptic cells lined the glands in the stomach. Villi were present along the length of the intestine in all four species. The anterior intestine had thin musculature, and was lined by absorptive cells with long microvilli and numerous small vesicles in the apical cytoplasm. The posterior intestine was lined by absorptive columnar cells with long microvilli, invaginations between microvilli with electron-dense membranes, and pinocytotic vesicles. Surface area generally decreased from the anterior to posterior intestine. Histological and ultrastructural results were consistent with lipid absorption occurring in the anterior GIT and protein absorption in the posterior GIT. The results of this study indicate clear differences in GIT structure among the study species, and digestion models based on chemical reactor theory were developed to characterise these differences.

Microbial Shift in the Enteric Bacteriome of Coral Reef Fish Following Climate-Driven Regime Shifts

Replacement of coral by macroalgae in post-disturbance reefs, also called a “coral-macroalgal regime shift”, is increasing in response to climate-driven ocean warming. Such ecosystem change is known to impact planktonic and benthic reef microbial communities but few studies have examined the effect on animal microbiota. In order to understand the consequence of coral-macroalgal shifts on the coral reef fish enteric bacteriome, we used a metabarcoding approach to examine the gut bacteriomes of 99 individual fish representing 36 species collected on reefs of the Inner Seychelles islands that, following bleaching, had either recovered to coral domination, or shifted to macroalgae. While the coral-macroalgal shift did not influence the diversity, richness or variability of fish gut bacteriomes, we observed a significant effect on the composition (R2 = 0.02; p = 0.001), especially in herbivorous fishes (R2 = 0.07; p = 0.001). This change is accompanied by a significant increase in the proportion of fermentative bacteria (Rikenella, Akkermensia, Desulfovibrio, Brachyspira) and associated metabolisms (carbohydrates metabolism, DNA replication, and nitrogen metabolism) in relation to the strong turnover of Scarinae and Siganidae fishes. Predominance of fermentative metabolisms in fish found on macroalgal dominated reefs indicates that regime shifts not only affect the taxonomic composition of fish bacteriomes, but also have the potential to affect ecosystem functioning through microbial functions.

Metatranscriptomics for the Human Microbiome and Microbial Community Functional Profiling

Shotgun metatranscriptomics (MTX) is an increasingly practical way to survey microbial community gene function and regulation at scale. This review begins by summarizing the motivations for community transcriptomics and the history of the field. We then explore the principles, best practices, and challenges of contemporary MTX workflows: beginning with laboratory methods for isolation and sequencing of community RNA, followed by informatics methods for quantifying RNA features, and finally statistical methods for detecting differential expression in a community context. In thesecond half of the review, we survey important biological findings from the MTX literature, drawing examples from the human microbiome, other (nonhuman) host-associated microbiomes, and the environment. Across these examples, MTX methods prove invaluable for probing microbe-microbe and host-microbe interactions, the dynamics of energy harvest and chemical cycling, and responses to environmental stresses. We conclude with a review of open challenges in the MTX field, including making assays and analyses more robust, accessible, and adaptable to new technologies; deciphering roles for millions of uncharacterized microbial transcripts; and solving applied problems such as biomarker discovery and development of microbial therapeutics.

Lists of names of prokaryotic Candidatus taxa

We here present annotated lists of names of Candidatus taxa of prokaryotes with ranks between subspecies and class, proposed between the mid-1990s, when the provisional status of Candidatus taxa was first established, and the end of 2018. Where necessary, corrected names are proposed that comply with the current provisions of the International Code of Nomenclature of Prokaryotes and its Orthography appendix. These lists, as well as updated lists of newly published names of Candidatus taxa with additions and corrections to the current lists to be published periodically in the International Journal of Systematic and Evolutionary Microbiology, may serve as the basis for the valid publication of the Candidatus names if and when the current proposals to expand the type material for naming of prokaryotes to also include gene sequences of yet-uncultivated taxa is accepted by the International Committee on Systematics of Prokaryotes.

Intestinal microbes: an axis of functional diversity among large marine consumers

Microbes are ubiquitous throughout the world's oceans, yet the manner and extent of their influence on the ecology and evolution of large, mobile fauna remains poorly understood. Here, we establish the intestinal microbiome as a hidden, and potentially important, 'functional trait' of tropical herbivorous fishes-a group of large consumers critical to coral reef resilience. Using field observations, we demonstrate that five common Caribbean fish species display marked differences in where they feed and what they feed on. However, in addition to space use and feeding behaviour-two commonly measured functional traits-we find that interspecific trait differences are even more pronounced when considering the herbivore intestinal microbiome. Microbiome composition was highly species specific. Phylogenetic comparison of the dominant microbiome members to all known microbial taxa suggest that microbiomes are comprised of putative environmental generalists, animal-associates and fish specialists (resident symbionts), the latter of which mapped onto host phylogeny. These putative symbionts are most similar to-among all known microbes-those that occupy the intestines of ecologically and evolutionarily related herbivorous fishes in more distant ocean basins. Our findings therefore suggest that the intestinal microbiome may be an important functional trait among these large-bodied consumers.

Host-associated microbiomes drive structure and function of marine ecosystems

The significance of symbioses between eukaryotic hosts and microbes extends from the organismal to the ecosystem level and underpins the health of Earth’s most threatened marine ecosystems. Despite rapid growth in research on host-associated microbes, from individual microbial symbionts to host-associated consortia of significantly relevant taxa, little is known about their interactions with the vast majority of marine host species. We outline research priorities to strengthen our current knowledge of host–microbiome interactions and how they shape marine ecosystems. We argue that such advances in research will help predict responses of species, communities, and ecosystems to stressors driven by human activity and inform future management strategies.

The significance of symbioses between eukaryotic hosts and microbes extends from the organismal to the ecosystem level and underpins the health of Earth’s most threatened marine ecosystems. This Perspective article outlines research priorities to strengthen our current knowledge of host-microbiome interactions, to help predict responses to anthropogenic stressors and to inform future management strategies.

The Gills of Reef Fish Support a Distinct Microbiome Influenced by Host-Specific Factors

Teleost fish represent the most diverse of the vertebrate groups and play important roles in food webs, as ecosystem engineers, and as vectors for microorganisms. However, the microbial ecology of fishes remains underexplored for most host taxa and for certain niches on the fish body. This is particularly true for the gills, the key sites of respiration and waste exchange in fishes. Here we provide a comprehensive analysis of the gill microbiome. We focus on ecologically diverse taxa from coral reefs around Moorea, sampling the gills and intestines of adults and juveniles representing 15 families. The gill microbiome composition differed significantly from that of the gut for both adults and juveniles, with fish-associated niches having lower alpha diversity values and higher beta diversity values than those for seawater, sediment, and alga-associated microbiomes. Of ∼45,000 operational taxonomic units (OTUs) detected across all samples, 11% and 13% were detected only in the gill and the intestine, respectively. OTUs most enriched in the gill included members of the gammaproteobacterial genus Shewanella and the family Endozoicimonaceae In adult fish, both gill and intestinal microbiomes varied significantly among host species grouped by diet category. Gill and intestinal microbiomes from the same individual were more similar to one another than to gill and intestinal microbiomes from different individuals. These results demonstrate that distinct body sites are jointly influenced by host-specific organizing factors operating at the level of the host individual. The results also identify taxonomic signatures unique to the gill and the intestine, confirming fish-associated niches as distinct reservoirs of marine microbial diversity.IMPORTANCE Fish breathe and excrete waste through their gills. The gills are also potential sites of pathogen invasion and colonization by other microbes. However, we know little about the microbial communities that live on the gill and the factors shaping their diversity. Focusing on ecologically distinct types of coral reef fish, we provide a comprehensive analysis of the fish gill microbiome. By comparison to microbiomes of the gut and the surrounding environment, we identify microbes unique to the gill niche. These microbes may be targets for further studies to determine the contribution of the microbiome to waste exchange or host immunity. We also show that despite exhibiting a unique taxonomic signature, the gill microbiome is influenced by factors that also influence the gut microbiome. These factors include the specific identity of the host individual. These results suggest basic principles describing how association with fishes structures the composition of microbial communities.