Beyond the visual: using metabarcoding to characterize the hidden reef cryptobiome

Settlement patterns and temporal successions of coral reef cryptic communities affect diversity assessments using autonomous reef monitoring structures (ARMS)

Autonomous Reef Monitoring Structures (ARMS) are used worldwide to assess cryptic diversity, especially on coral reefs. They were developed as standardised tools, yet conditions of deployment, such as immersion duration and/or deployment and retrieval seasons, vary among studies. Here we studied temporal and seasonal variability in coral reef cryptic communities sampled with 15 ARMS on a single coral reef slope site at Reunion Island, Southwest Indian Ocean. Settlement patterns and temporal succession of Eukaryote communities were investigated across three immersion times (6 months, 1 year, 2 years), two immersion seasons (hot vs. cool) and three fractions (500-2000 μm, 106-500 μm, sessile) using two genetic markers (18S, COI). Both markers detected different taxa with different resolutions, but broadly similar patterns of community composition and structure. While OTU diversity of communities did not change with immersion time and season, these parameters significantly affected community structure and composition. Our results showed a decrease of the similarity of ARMS communities with duration of immersion, and a strong temporal turnover of species with only a small proportion of the diversity persisting over time. The small proportion of OTUs assigned at phylum level highlights the uniqueness of the Mascarene cryptobiome.

Differential spatio-temporal responses of Red Sea coral reef benthic communities to a mass bleaching event

Understanding how coral reefs respond to disturbances is fundamental to assessing their resistance and resilience, particularly in the context of climate change. Due to the escalating frequency and intensity of coral bleaching events, it is essential to evaluate spatio-temporal responses of coral reef communities to disentangle the mechanisms underlying ecological changes. Here, we used benthic data collected from 59 reefs in the Red Sea over five years (2014-2019), a period that encompasses the 2015/2016 mass bleaching event. Reefs were located within three different geographic regions with different environmental settings: north (Duba; Al Wajh), central (Jeddah; Thuwal), and south (Al Lith; Farasan Banks; Farasan Islands). Coral community responses were region-specific, with communities in the south being more promptly affected than those in the northern and central regions, with hard and soft coral cover dropping drastically in several reefs from around > 40% to < 5% two years after bleaching. Coral bleaching effects were particularly evident in the decrease of cover in branching corals. Overall, we documented a shift towards a dominance of macroalgae, turf algae, and crustose coralline algae (CCA). Using remote sensing data, we analyzed sea surface temperature (SST) regimes at the study sites to infer potential drivers of changes in benthic composition. Both SST and Degree Heating Weeks (DHW) only partially aligned with the responses of benthic communities, highlighting the need for more accurate predictors of coral bleaching in the Red Sea. In times of intense coastal development along Saudi Arabia's Red Sea coast, our study provides crucial baseline information on developments in coral reef community composition, as well as to guide decision-making, namely restoration efforts.

Seasonality of primary production explains the richness of pioneering benthic communities

A pattern of increasing species richness from the poles to the equator is frequently observed in many animal taxa. Ecological limits, determined by the abiotic conditions and biotic interactions within an environment, are one of the major factors influencing the geographical distribution of species diversity. Energy availability is often considered a crucial limiting factor, with temperature and productivity serving as empirical measures. However, these measures may not fully explain the observed species richness, particularly in marine ecosystems. Here, through a global comparative approach and standardised methodologies, such as Autonomous Reef Monitoring Structures (ARMS) and DNA metabarcoding, we show that the seasonality of primary production explains sessile animal richness comparatively or better than surface temperature or primary productivity alone. A Hierarchical Generalised Additive Model (HGAM) is validated, after a model selection procedure, and the prediction error is compared, following a cross-validation approach, with HGAMs including environmental variables commonly used to explain animal richness. Moreover, the linear effect of production magnitude on species richness becomes apparent only when considered jointly with seasonality, and, by identifying world coastal areas characterized by extreme values of both, we postulate that this effect may result in a positive relationship in environments with lower seasonality.

Biodiversity patterns of the coral reef cryptobiota around the Arabian Peninsula

The Arabian Peninsula accounts for approximately 6% of the world's coral reefs. Some thrive in extreme environments of temperature and salinity. Using 51 Autonomous Reef Monitoring Structure (ARMS), a standardized non-destructive monitoring device, we investigated the spatial patterns of coral reef cryptobenthic diversity in four ecoregions around the Arabian Peninsula and analyzed how geographical and/or environmental drivers shape those patterns. The mitochondrial cytochrome c oxidase subunit I (COI) gene was used to identify Amplicon Sequence Variants and assign taxonomy of the cryptobenthic organisms collected from the sessile and mobile fractions of each ARMS. Cryptobenthic communities sampled from the two ecoregions in the Red Sea showed to be more diverse than those inhabiting the Arabian (Persian) Gulf and the Gulf of Oman. Geographic distance revealed a stronger relationship with beta diversity in the Mantel partial correlation than environmental distance. However, the two mobile fractions (106-500 µm and 500-2000 µm) also had a significant correlation between environmental distance and beta diversity. In our study, dispersal limitations explained the beta diversity patterns in the selected reefs, supporting the neutral theory of ecology. Still, increasing differences in environmental variables (environmental filtering) also had an effect on the distribution patterns of assemblages inhabiting reefs within short geographic distances. The influence of geographical distance in the cryptofauna assemblages makes these relevant, yet usually ignored, communities in reef functioning vulnerable to large scale coastal development and should be considered in ecosystem management of such projects.

Life goes on: Spatial heterogeneity promotes biodiversity in an urbanized coastal marine ecosystem

Both human populations and marine biodiversity are concentrated along coastlines, with growing conservation interest in how these ecosystems can survive intense anthropogenic impacts. Tropical urban centres provide valuable research opportunities because these megacities are often adjacent to mega-diverse coral reef systems. The Pearl River Delta is a prime exemplar, as it encompasses one of the most densely populated and impacted regions in the world and is located just northwest of the Coral Triangle. However, the spatial and taxonomic complexity of this biodiversity, most of which is small, cryptic in habitat and poorly known, make comparative analyses challenging. We deployed standardized settlement structures at seven sites differing in the intensity of human impacts and used COI metabarcoding to characterize benthic biodiversity, with a focus on metazoans. We found a total of 7184 OTUs, with an average of 665 OTUs per sampling unit; these numbers exceed those observed in many previous studies using comparable methods, despite the location of our study in an urbanized environment. Beta diversity was also high, with 52% of the OTUs found at just one site. As expected, we found that the sites close to point sources of pollution had substantially lower diversity (44% less) relative to sites bathed in less polluted oceanic waters. However, the polluted sites contributed substantially to the total animal diversity of the region, with 25% of all OTUs occurring only within polluted sites. Further analysis of Arthropoda, Annelida and Mollusca showed that phylogenetic clustering within a site was common, suggesting that environmental filtering reduced biodiversity to a subset of lineages present within the region, a pattern that was most pronounced in polluted sites and for the Arthropoda. The water quality gradients surrounding the PRD highlight the unique role of in situ studies for understanding the impacts of complex urbanization pressures on biodiversity.

Seq' and ARMS shall find: DNA (meta)barcoding of Autonomous Reef Monitoring Structures across the tree of life uncovers hidden cryptobiome of tropical urban coral reefs

Coral reefs are among the richest marine ecosystems on Earth, but there remains much diversity hidden within cavities of complex reef structures awaiting discovery. While the abundance of corals and other macroinvertebrates are known to influence the diversity of other reef-associated organisms, much remains unknown on the drivers of cryptobenthic diversity. A combination of standardized sampling with 12 units of the Autonomous Reef Monitoring Structure (ARMS) and high-throughput sequencing was utilized to uncover reef cryptobiome diversity across the equatorial reefs in Singapore. DNA barcoding and metabarcoding of mitochondrial cytochrome c oxidase subunit I, nuclear 18S and bacterial 16S rRNA genes revealed the taxonomic composition of the reef cryptobiome, comprising 15,356 microbial ASVs from over 50 bacterial phyla, and 971 MOTUs across 15 metazoan and 19 non-metazoan eukaryote phyla. Environmental factors across different sites were tested for relationships with ARMS diversity. Differences among reefs in diversity patterns of metazoans and other eukaryotes, but not microbial communities, were associated with biotic (coral cover) and abiotic (distance, temperature and sediment) environmental variables. In particular, ARMS deployed at reefs with higher coral cover had greater metazoan diversity and encrusting plate cover, with larger-sized non-coral invertebrates influencing spatial patterns among sites. Our study showed that DNA barcoding and metabarcoding of ARMS constitute a valuable tool for quantifying cryptobenthic diversity patterns and can provide critical information for the effective management of coral reef ecosystems.

Evaluating biodiversity for coral reef reformation and monitoring on complex 3D structures using environmental DNA (eDNA) metabarcoding

Quantifying coral reef biodiversity is challenging for cryptofauna and organisms in early life stages. We demonstrate the utility of eDNA metabarcoding as a tool for comprehensively evaluating invertebrate communities on complex 3D structures for reef reformation, and the role these structures play in provisioning habitat for organisms. 3D design and printing were used to create 18 complex tiles, which were used to form artificial reef structures. eDNA was collected from scraping tile surfaces for organismal biomass and from seawater samples around the artificial reefs in the Gulf of Eilat/Aqaba, Red Sea. Metabarcoding targeted the mitochondrial COI gene with specific primers for marine biodiversity. We provide the first eDNA biodiversity baseline for the Gulf of Eilat/Aqaba, capturing extensive information on species abundance, richness, and diversity. Tile tops had higher phylogenetic diversity and richness, despite a higher abundance of organisms on tile bottoms, highlighting the detection of cryptic organisms with eDNA. We recommend eDNA metabarcoding for reef restoration initiatives, especially for complex marine structures, to improve success and evaluation of biodiversity.

Be positive: customized reference databases and new, local barcodes balance false taxonomic assignments in metabarcoding studies

Background

In metabarcoding analyses, the taxonomic assignment is crucial to place sequencing data in biological and ecological contexts. This fundamental step depends on a reference database, which should have a good taxonomic coverage to avoid unassigned sequences. However, this goal is rarely achieved in many geographic regions and for several taxonomic groups. On the other hand, more is not necessarily better, as sequences in reference databases belonging to taxonomic groups out of the studied region/environment context might lead to false assignments.

Methods

We investigated the effect of using several subsets of a cytochrome c oxidase subunit I (COI) reference database on taxonomic assignment. Published metabarcoding sequences from the Mediterranean Sea were assigned to taxa using COInr, which is a comprehensive, non-redundant and recent database of COI sequences obtained both from BOLD and NCBI, and two of its subsets: (i) all sequences except insects (COInr-WO-Insecta), which represent the overwhelming majority of COInr database, but are irrelevant for marine samples, and (ii) all sequences from taxonomic families present in the Mediterranean Sea (COInr-Med). Four different algorithms for taxonomic assignment were employed in parallel to evaluate differences in their output and data consistency.

Results

The reduction of the database to more specific custom subsets increased the number of unassigned sequences. Nevertheless, since most of them were incorrectly assigned by the less specific databases, this is a positive outcome. Moreover, the taxonomic resolution (the lowest taxonomic level to which a sequence is attributed) of several sequences tended to increase when using customized databases. These findings clearly indicated the need for customized databases adapted to each study. However, the very high proportion of unassigned sequences points to the need to enrich the local database with new barcodes specifically obtained from the studied region and/or taxonomic group. Including novel local barcodes to the COI database proved to be very profitable: by adding only 116 new barcodes sequenced in our laboratory, thus increasing the reference database by only 0.04%, we were able to improve the resolution for ca. 0.6-1% of the Amplicon Sequence Variants (ASVs).

Red Sea Atlas of Coral-Associated Bacteria Highlights Common Microbiome Members and Their Distribution across Environmental Gradients-A Systematic Review

The Red Sea is a suitable model for studying coral reefs under climate change due to its strong environmental gradient that provides a window into future global warming scenarios. For instance, corals in the southern Red Sea thrive at temperatures predicted to occur at the end of the century in other biogeographic regions. Corals in the Red Sea thrive under contrasting thermal and environmental regimes along their latitudinal gradient. Because microbial communities associated with corals contribute to host physiology, we conducted a systematic review of the known diversity of Red Sea coral-associated bacteria, considering geographic location and host species. Our assessment comprises 54 studies of 67 coral host species employing cultivation-dependent and cultivation-independent techniques. Most studies have been conducted in the central and northern Red Sea, while the southern and western regions remain largely unexplored. Our data also show that, despite the high diversity of corals in the Red Sea, the most studied corals were Pocillopora verrucosa, Dipsastraea spp., Pleuractis granulosa, and Stylophora pistillata. Microbial diversity was dominated by bacteria from the class Gammaproteobacteria, while the most frequently occurring bacterial families included Rhodobacteraceae and Vibrionaceae. We also identified bacterial families exclusively associated with each of the studied coral orders: Scleractinia (n = 125), Alcyonacea (n = 7), and Capitata (n = 2). This review encompasses 20 years of research in the Red Sea, providing a baseline compendium for coral-associated bacterial diversity.

Discovering marine biodiversity in the 21st century

We review the current knowledge of the biodiversity of the ocean as well as the levels of decline and threat for species and habitats. The lack of understanding of the distribution of life in the ocean is identified as a significant barrier to restoring its biodiversity and health. We explore why the science of taxonomy has failed to deliver knowledge of what species are present in the ocean, how they are distributed and how they are responding to global and regional to local anthropogenic pressures. This failure prevents nations from meeting their international commitments to conserve marine biodiversity with the results that investment in taxonomy has declined in many countries. We explore a range of new technologies and approaches for discovery of marine species and their detection and monitoring. These include: imaging methods, molecular approaches, active and passive acoustics, the use of interconnected databases and citizen science. Whilst no one method is suitable for discovering or detecting all groups of organisms many are complementary and have been combined to give a more complete picture of biodiversity in marine ecosystems. We conclude that integrated approaches represent the best way forwards for accelerating species discovery, description and biodiversity assessment. Examples of integrated taxonomic approaches are identified from terrestrial ecosystems. Such integrated taxonomic approaches require the adoption of cybertaxonomy approaches and will be boosted by new autonomous sampling platforms and development of machine-speed exchange of digital information between databases.

Functional and phylogenetic responses of motile cryptofauna to habitat degradation

Biodiversity of terrestrial and marine ecosystems, including coral reefs, is dominated by small, often cryptic, invertebrate taxa that play important roles in ecosystem structure and functioning. While cryptofauna community structure is determined by strong small-scale microhabitat associations, the extent to which ecological and environmental factors shape these communities are largely unknown, as is the relative importance of particular microhabitats in supporting reef trophodynamics from the bottom up. The goal of this study was to address these knowledge gaps, provided coral reefs are increasingly exposed to multiple disturbances and environmental gradients that influence habitat complexity, condition and ecosystem functioning. We compared the density, biomass, size range, phylogenetic diversity and functional roles of motile cryptofauna in Palau, Western Micronesia, among four coral-derived microhabitats representing various states of degradation (live coral [Acropora and Pocillopora], dead coral and coral rubble) from reefs along a gradient of effluent exposure. In total, 122 families across ten phyla were identified, dominated by the Arthropoda (Crustacea) and Mollusca. Cryptofauna biomass was greatest in live Pocillopora, while coral rubble contained the greatest density and diversity. Size ranges were broader in live corals than both dead coral and rubble. From a bottom-up perspective, effluent exposure had mixed effects on cryptic communities including a decline in total biomass in rubble. From a top-down perspective, cryptofauna were generally unaffected by predator biomass. Our data show that, as coral reef ecosystems continue to decline in response to more frequent and severe disturbances, habitats other than live coral may become increasingly important in supporting coral reef biodiversity and food webs.

Inter-annual variability patterns of reef cryptobiota in the central Red Sea across a shelf gradient

The combination of molecular tools, standard surveying techniques, and long-term monitoring programs are relevant to understanding environmental and ecological changes in coral reef communities. Here we studied temporal variability in cryptobenthic coral reef communities across the continental shelf in the central Red Sea spanning 6 years (three sampling periods: 2013-2019) and including the 2015 mass bleaching event. We used a combination of molecular tools (barcoding and metabarcoding) to assess communities on Autonomous Reef Monitoring Structures (ARMS) as a standardized sampling approach. Community composition associated with ARMS for both methodologies (barcoding and metabarcoding) was statistically different across reefs (shelf position) and time periods. The partition of beta diversity showed a higher turnover and lower nestedness between pre-bleaching and post-bleaching samples than between the two post-bleaching periods, revealing a community shift from the bleaching event. However, a slight return to the pre-bleaching community composition was observed in 2019 suggesting a recovery trajectory. Given the predictions of decreasing time between bleaching events, it is concerning that cryptobenthic communities may not fully recover and communities with new characteristics will emerge. We observed a high turnover among reefs for all time periods, implying a homogenization of the cryptobiome did not occur across the cross shelf following the 2015 bleaching event. It is possible that dispersal limitations and the distinct environmental and benthic structures present across the shelf maintained the heterogeneity in communities among reefs. This study has to the best of our knowledge presented for the first time a temporal aspect into the analysis of ARMS cryptobenthic coral reef communities and encompasses a bleaching event. We show that these structures can detect cryptic changes associated with reef degradation and provides support for these being used as long-term monitoring tools.

Integration of DNA-Based Approaches in Aquatic Ecological Assessment Using Benthic Macroinvertebrates

Benthic macroinvertebrates are among the most used biological quality elements for assessing the condition of all types of aquatic ecosystems worldwide (i.e., fresh water, transitional, and marine). Current morphology-based assessments have several limitations that may be circumvented by using DNA-based approaches. Here, we present a comprehensive review of 90 publications on the use of DNA metabarcoding of benthic macroinvertebrates in aquatic ecosystems bioassessments. Metabarcoding of bulk macrozoobenthos has been preferentially used in fresh waters, whereas in marine waters, environmental DNA (eDNA) from sediment and bulk communities from deployed artificial structures has been favored. DNA extraction has been done predominantly through commercial kits, and cytochrome c oxidase subunit I (COI) has been, by far, the most used marker, occasionally combined with others, namely, the 18S rRNA gene. Current limitations include the lack of standardized protocols and broad-coverage primers, the incompleteness of reference libraries, and the inability to reliably extrapolate abundance data. In addition, morphology versus DNA benchmarking of ecological status and biotic indexes are required to allow general worldwide implementation and higher end-user confidence. The increased sensitivity, high throughput, and faster execution of DNA metabarcoding can provide much higher spatial and temporal data resolution on aquatic ecological status, thereby being more responsive to immediate management needs.

Translational Molecular Ecology in practice: Linking DNA-based methods to actionable marine environmental management

Molecular-based approaches can provide timely biodiversity assessments, showing an immense potential to facilitate decision-making in marine environmental management. However, the uptake of molecular data into environmental policy remains minimal. Here, we showcase a selection of local to global scale studies applying molecular-based methodologies for environmental management at various stages of implementation. Drawing upon lessons learned from these case-studies, we provide a roadmap to facilitate applications of DNA-based methods to marine policies and to overcome the existing challenges. The main impediment identified is the need for standardized protocols to guarantee data comparison across spatial and temporal scales. Adoption of Translational Molecular Ecology - the sustained collaboration between molecular ecologists and stakeholders, will enhance consensus with regards to the objectives, methods, and outcomes of environmental management projects. Establishing a sustained dialogue among stakeholders is key to accelerating the adoption of molecular-based approaches for marine monitoring and assessment.

Pan-regional marine benthic cryptobiome biodiversity patterns revealed by metabarcoding Autonomous Reef Monitoring Structures

Autonomous Reef Monitoring Structures (ARMS) have been applied worldwide to characterize the critical yet frequently overlooked biodiversity patterns of marine benthic organisms. In order to disentangle the relevance of environmental factors in benthic patterns, here, through standardized metabarcoding protocols, we analyse sessile and mobile (<2 mm) organisms collected using ARMS deployed across six regions with different environmental conditions (3 sites × 3 replicates per region): Baltic, Western Mediterranean, Adriatic, Black and Red Seas, and the Bay of Biscay. A total of 27,473 Amplicon Sequence Variants (ASVs) were observed ranging from 1,404 in the Black Sea to 9,958 in the Red Sea. No ASVs were shared among all regions. The highest number of shared ASVs was between the Western Mediterranean and the Adriatic Sea (116) and Bay of Biscay (115). Relatively high numbers of ASVs (103), mostly associated with the genus Amphibalanus, were also shared between the lower salinity seas (Baltic and Black Seas). We found that compositional differences in spatial patterns of rocky-shore benthos are determined slightly more by dispersal limitation than environmental filtering. Dispersal limitation was similar between sessile and mobile groups, while the sessile group had a larger environmental niche breadth than the mobile group. Further, our study can provide a foundation for future evaluations of biodiversity patterns in the cryptobiome, which can contribute up to 70% of the local biodiversity.

Epifaunal invertebrate assemblages associated with branching Pocilloporids in Moorea, French Polynesia

Reef-building corals can harbour high abundances of diverse invertebrate epifauna. Coral characteristics and environmental conditions are important drivers of community structure of coral-associated invertebrates; however, our current understanding of drivers of epifaunal distributions is still unclear. This study tests the relative importance of the physical environment (current flow speed) and host quality (e.g., colony height, surface area, distance between branches, penetration depth among branches, and background partial mortality) in structuring epifaunal communities living within branching Pocillopora colonies on a back reef in Moorea, French Polynesia. A total of 470 individuals belonging to four phyla, 16 families and 39 genera were extracted from 36 Pocillopora spp. colonies. Decapods were the most abundant epifaunal organisms (accounting for 84% of individuals) found living in Pocillopora spp. While coral host characteristics and flow regime are very important, these parameters were not correlated with epifaunal assemblages at the time of the study. Epifaunal assemblages associated with Pocillopora spp. were consistent and minimally affected by differences in host characteristics and flow regime. The consistency in abundance and taxon richness among colonies (regardless of habitat characteristics) highlighted the importance of total habitat availability. With escalating effects of climate change and other localized disturbances, it is critical to preserve branching corals to support epifaunal communities.

Disentangling the complex microbial community of coral reefs using standardized Autonomous Reef Monitoring Structures (ARMS)

Autonomous Reef Monitoring Structures (ARMS) have been applied worldwide to describe eukaryotic cryptic reef fauna. Conversely, bacterial communities, which are critical components of coral reef ecosystem functioning, remain largely overlooked. Here we deployed 56 ARMS across the 2,000-km spread of the Red Sea to assay biodiversity, composition and inferred underlying functions of coral reef-associated bacterial communities via 16S rRNA gene sequencing. We found that bacterial community structure and diversity aligned with environmental differences. Indeed, sea surface temperature and macroalgae cover were key in explaining bacterial relative abundance. Importantly, taxonomic and functional alpha diversity decreased under more extreme environmental conditions (e.g., higher temperatures) in the southern Red Sea. This may imply a link between bacterial community diversity and functional capabilities, with implications for conservation management. Our study demonstrates the utility of ARMS to investigate the response of coral reef-associated bacterial communities to environmental change.