Function and stability of mesophotic coral reefs.

The function and stability of mesophotic coral ecosystems (MCEs) have been extensively studied in recent years. These deep reefs are characterized by local physical processes, particularly the steep gradient in irradiance with increasing depth, and their impact on trophic resources. Mesophotic reefs exhibit distinct zonation patterns that segregate shallow reef biodiversity from ecologically unique deeper communities of endemic species. While mesophotic reefs are hypothesized as relatively stable refuges from anthropogenic stressors and a potential seed bank for degraded shallow reefs, these are site-specific features, if they occur at all. Mesophotic reefs are now known to be susceptible to many of the same stressors that are degrading shallow reefs, suggesting that they require their own specific conservation and management strategies.

Mesophotic corals in Hawai'i maintain autotrophy to survive low-light conditions.

In mesophotic coral ecosystems, reef-building corals and their photosynthetic symbionts can survive with less than 1% of surface irradiance. How depth-specialist corals rely upon autotrophically and heterotrophically derived energy sources across the mesophotic zone remains unclear. We analysed the stable carbon (δ13C) and nitrogen (δ15N) isotope values of a Leptoseris community from the 'Au'au Channel, Maui, Hawai'i (65-125 m) including four coral host species living symbiotically with three algal haplotypes. We characterized the isotope values of hosts and symbionts across species and depth to compare trophic strategies. Symbiont δ13C was consistently 0.5‰ higher than host δ13C at all depths. Mean colony host and symbiont δ15N differed by up to 3.7‰ at shallow depths and converged at deeper depths. These results suggest that both heterotrophy and autotrophy remained integral to colony survival across depth. The increasing similarity between host and symbiont δ15N at deeper depths suggests that nitrogen is more efficiently shared between mesophotic coral hosts and their algal symbionts to sustain autotrophy. Isotopic trends across depth did not generally vary by host species or algal haplotype, suggesting that photosynthesis remains essential to Leptoseris survival and growth despite low light availability in the mesophotic zone.

Organellar genomic characterization of Anunuuluaehu liula representing a new genus and species of Phyllophoraceae (Gigartinales, Rhodophyta) from the mesophotic zone of Hawai'i.

Over the last 2 decades, routine collections in the Hawaiian Archipelago have expanded to mesophotic reefs, leading to the discovery of a new red algal genus and species, here described as Anunuuluaehu liula gen. et sp. nov. This study provides a detailed genus and species description and characterizes chloroplast and mitochondrial organellar genomes. The new genus, Anunuuluaehu, shares many characteristics with the family Phyllophoraceae and shows close similarities to Archestennogramma and Stenogramma, including habit morphology, nemathecia forming proliferations at the outer cortex with terminal chains of tetrasporangia, and carposporophytes with multi-layered pericarps. The single species in this genus exhibits distinctive features within the Phyllophoraceae: the presence of single-layer construction of large medullary cells and the development of long, tubular gonimoblastic filaments. Multi-gene phylogenetic analyses confirmed it as a unique, monophyletic lineage within the family. Cis-splicing genes, interrupted by intron-encoded proteins within group II introns, are present in both the chloroplast and mitochondrial genomes of A. liula. Notably, a specific region of the coxI group II intron exhibits similarity to fungal introns. Anunuuluaehu liula is presumed to be endemic to the Hawaiian Archipelago and thus far is known to live solely at mesophotic depths from Holaniku to Kaho'olawe ranging from 54 to 201 m, which is the deepest collection record of any representative in the family. Overall, this study enhances our understanding of the genomic and taxonomic complexities of red algae in mesophotic habitats, emphasizing the significance of continued research in this area to uncover further insights into evolutionary processes and biogeographic patterns.

Characterization of macroalgal-associated microbial communities from shallow to mesophotic depths at Manawai, Papahanaumokuakea Marine National Monument, Hawai'i.

The Papahanaumokuakea Marine National Monument, Hawai'i, is one of the most isolated and protected archipelagos in the world, making it a natural laboratory to examine macroalgal-microbial diversity because of limited direct anthropogenic impacts. We collected the most abundant macroalgae from nine sites ranging from shallow subtidal (1.5 m) to mesophotic (75 m) depths around Manawai (Pearl and Hermes Atoll). We characterized the macroalgal bacterial communities via high-throughput amplicon sequencing and compared the influence of host phylum, species, site, and depth on these relationships at a single atoll. Ochrophyta species had the lowest bacterial diversity compared to Chlorophyta and Rhodophyta. Site and/or depth may influence the microbial community structure associated with Microdictyon setchellianum, indicating a possible disconnect of these microbial communities among habitats. Chondria tumulosa, a cryptogenic species with invasive traits, differed in associated microbiota compared to the native Laurencia galtsoffii, an alga from the same family collected at the same site and depth. While there was overlap of bacterial communities across sites for some algal species, the majority had minimal macroalgal-microbial community connectivity across Manawai. This mesophotic system, therefore, did not appear to be refugia for shallow water coral reefs at microscopic scales. Additional studies are required to identify other significant influences on microbial community variation.

Complete chloroplast genome of Chondria tumulosa (Ceramiales, Rhodophyta), a recently described cryptogenic species with invasive traits from Papahanaumokuakea Marine National Monument, Hawai'i.

The complete chloroplast genome of Chondria tumulosa, a red alga from Manawai (Pearl and Hermes Atoll), Hawai'i, was determined and analyzed using next-generation sequencing and de novo assembly approaches. The chloroplast genome sequence of C. tumulosa was 172,617 bp and contained 231 genes, consisting of 197 protein-coding genes, 29 transfer RNA genes, three ribosomal RNA genes, one transfer-messenger RNA gene, one non-coding RNA gene, and one intron inserted into the trnM gene. The number of genes and genome structure was largely similar to other members of the family Rhodomelaceae. The phylogenomic analysis of 32 complete cpDNA from the red algal order Ceramiales showed that C. tumulosa is a distinct species within the Chondrieae tribe, and is a diverging early relative to the other three available Chondria chloroplast genomes.

Microbiota-Macroalgal Relationships at a Hawaiian Intertidal Bench Are Influenced by Macroalgal Phyla and Associated Thallus Complexity.

The ocean represents the largest biome on earth; however, we have only begun to understand the diversity and function of the marine microbial inhabitants and their interactions with macroalgal species. Macroalgae play an integral role in overall ocean biome health and serve both as major primary producers and foundation species in the ecosystem. Previous studies have been limited, focusing on the microbiome of a single algal species or its interaction with selected microbes. This project aimed to understand overall biodiversity of microbial communities associated with five common macroalgal species and to determine the drivers of these communities at 'Ewa Beach, O'ahu, HI. Representative species of Chlorophyta (green), Ochrophyta (brown), and Rhodophyta (red) algae, each species having various levels of calcification, thallus complexity, and status as native or invasive species, were collected from an intertidal bench in May 2019. A portion of the V3-V4 variable region of the small-subunit rRNA gene was amplified for high-throughput sequencing using universal bacterial primers to elucidate the core and variable algal microbiome. Significant differences in bacterial community composition were only partially explained by host species, whether the host was native or invasive, and thallus complexity. Macroalgal phylum explained the most variation in associated microbial communities at 'Ewa Beach. This study advances our understanding of microbial-macroalgal interactions and their connectivity by producing insight into factors that influence the community structure of macroalga-associated microbiota. IMPORTANCE Generally, most eukaryotic organisms form relationships with microbes that are important in mediating host organismal health. Macroalgae are a diverse group of photosynthetic eukaryotic organisms that serve as primary producers and foundational species in many ecosystems. However, little is known about their microbial counterparts across a wide range of macroalgal morphologies, phylogenies, and calcification levels. Thus, to further understand the factors involved in bacterial community composition associated with macroalgal species at one point in time, representative samples were collected across phyla. Here, we show that both host macroalga phyla and morphology influenced the associated microbial community. Additionally, we show that the invasive species Avrainvillea lacerata does not have a unique microbial community on this intertidal bench, further supporting the idea that host phylum strongly influences microbial community composition.

Taxonomic determination of the cryptogenic red alga, Chondria tumulosa sp. nov., (Rhodomelaceae, Rhodophyta) from Papahanaumokuakea Marine National Monument, Hawai'i, USA: A new species displaying invasive characteristics.

Survey cruises by the National Oceanic and Atmospheric Administration (NOAA) in 2016 and 2019 yielded specimens of an undetermined red alga that rapidly attained alarming levels of benthic coverage at Pearl and Hermes Atoll, Papahanaumokuakea Marine National Monument, Hawai'i. By 2019 the seaweed had covered large expanses on the northeast side of the atoll with mat-like, extensive growth of entangled thalli. Specimens were analyzed using light microscopy and molecular analysis, and were compared to morphological descriptions in the literature for closely related taxa. Light microscopy demonstrated that the specimens likely belonged to the rhodomelacean genus Chondria, yet comparisons to taxonomic literature revealed no morphological match. DNA sequence analyses of the mitochondrial COI barcode marker, the plastidial rbcL gene, and the nuclear SSU gene confirmed its genus-level placement and demonstrated that this alga was unique compared to all other available sequences. Based on these data, this cryptogenic seaweed is here proposed as a new species: Chondria tumulosa A.R.Sherwood & J.M.Huisman sp. nov. Chondria tumulosa is distinct from all other species of Chondria based on its large, robust thalli, a mat-forming tendency, large axial diameter in mature branches (which decreases in diameter with subsequent orders of branching), terete axes, and bluntly rounded apices. Although C. tumulosa does not meet the criteria for the definition of an invasive species given that it has not been confirmed as introduced to Pearl and Hermes Atoll, this seaweed is not closely related to any known Hawaiian native species and is of particular concern given its sudden appearance and rapid increase in abundance in the Papahanaumokuakea Marine National Monument; an uninhabited, remote, and pristine island chain to the northwest of the Main Hawaiian Islands.

A new record of Avrainvillea cf. erecta (Berkeley) A. Gepp & E. S. Gepp (Bryopsidales, Chlorophyta) from urbanized estuaries in the Hawaiian Islands.

BACKGROUND: A second species in the siphonous green algal genus Avrainvillea was recently discovered off the island of O'ahu in the Main Hawaiian Islands. Specimens were collected from Honolulu Harbor, including its entrance channel, and near Ke'ehi Harbor. These locations are both in Malama Bay on O'ahu's south shore in or adjacent to urbanized estuaries, respectively. In situ observations, morphological and molecular assessments were conducted to examine the alga's habit and distribution, as well as to assess its putative species identification. NEW INFORMATION: The alga occurred in sand as single individuals or in clusters of several individuals at both sites, and near or within seagrass beds (Halophila decipiens) and algal meadows composed of the green alga Halimeda kanaloana and an unidentified Udotea species at the Ke'ehi Harbor site. All analyses supported both populations as representative of the same taxa, reported until further investigation in the broad Pacific as Avrainvillea cf. erecta based on morphological and molecular analyses. This record of a second Avrainvillea species in Hawai'i is of particular concern considering that an alga recognized as A. amadelpha, first observed in 1981 from two locales on O'ahu's south shore, has become invasive in Hawai'i's intertidal to mesophotic environments.

Fungi associated with mesophotic macroalgae from the 'Au'au Channel, west Maui are differentiated by host and overlap terrestrial communities.

Mesophotic coral ecosystems are an almost entirely unexplored and undocumented environment that likely contains vast reservoirs of undescribed biodiversity. Twenty-four macroalgae samples, representing four genera, were collected from a Hawaiian mesophotic reef at water depths between 65 and 86 m in the 'Au'au Channel, Maui, Hawai'i. Algal tissues were surveyed for the presence and diversity of fungi by sequencing the ITS1 gene using Illumina technology. Fungi from these algae were then compared to previous fungal surveys conducted in Hawaiian terrestrial ecosystems. Twenty-seven percent of the OTUs present on the mesophotic coral ecosystem samples were shared between the marine and terrestrial environment. Subsequent analyses indicated that host species of algae significantly differentiate fungal community composition. This work demonstrates yet another understudied habitat with a moderate diversity of fungi that should be considered when estimating global fungal diversity.

A survey of fishes associated with Hawaiian deep-water Halimeda kanaloana (Bryopsidales: Halimedaceae) and Avrainvillea sp. (Bryopsidales: Udoteaceae) meadows.

The invasive macroalgal species Avrainvillea sp. and native species Halimeda kanaloana form expansive meadows that extend to depths of 80 m or more in the waters off of O'ahu and Maui, respectively. Despite their wide depth distribution, comparatively little is known about the biota associated with these macroalgal species. Our primary goals were to provide baseline information on the fish fauna associated with these deep-water macroalgal meadows and to compare the abundance and diversity of fishes between the meadow interior and sandy perimeters. Because both species form structurally complex three-dimensional canopies, we hypothesized that they would support a greater abundance and diversity of fishes when compared to surrounding sandy areas. We surveyed the fish fauna associated with these meadows using visual surveys and collections made with clove-oil anesthetic. Using these techniques, we recorded a total of 49 species from 25 families for H. kanaloana meadows and surrounding sandy areas, and 28 species from 19 families for Avrainvillea sp. habitats. Percent endemism was 28.6% and 10.7%, respectively. Wrasses (Family Labridae) were the most speciose taxon in both habitats (11 and six species, respectively), followed by gobies for H. kanaloana (six species). The wrasse Oxycheilinus bimaculatus and cardinalfish Apogonichthys perdix were the most frequently-occurring species within the H. kanaloana and Avrainvillea canopies, respectively. Obligate herbivores and food-fish species were rare in both habitats. Surprisingly, the density and abundance of small epibenthic fishes were greater in open sand than in the meadow canopy. In addition, species richness was also higher in open sand for Avrainvillea sp. We hypothesize that the dense holdfasts and rhizoids present within the meadow canopy may impede benthic-dwelling or bioturbator species, which accounted for 86% and 57% of individuals collected in sand adjacent to H. kanaloana and Avrainvillea sp. habitats, respectively. Of the 65 unique species recorded in this study, 16 (25%) were detected in clove oil stations alone, illustrating the utility of clove-oil anesthetic in assessing the diversity and abundance of small-bodied epibenthic fishes.

A comprehensive investigation of mesophotic coral ecosystems in the Hawaiian Archipelago.

Although the existence of coral-reef habitats at depths to 165 m in tropical regions has been known for decades, the richness, diversity, and ecological importance of mesophotic coral ecosystems (MCEs) has only recently become widely acknowledged. During an interdisciplinary effort spanning more than two decades, we characterized the most expansive MCEs ever recorded, with vast macroalgal communities and areas of 100% coral cover between depths of 50-90 m extending for tens of km2 in the Hawaiian Archipelago. We used a variety of sensors and techniques to establish geophysical characteristics. Biodiversity patterns were established from visual and video observations and collected specimens obtained from submersible, remotely operated vehicles and mixed-gas SCUBA and rebreather dives. Population dynamics based on age, growth and fecundity estimates of selected fish species were obtained from laser-videogrammetry, specimens, and otolith preparations. Trophic dynamics were determined using carbon and nitrogen stable isotopic analyses on more than 750 reef fishes. MCEs are associated with clear water and suitable substrate. In comparison to shallow reefs in the Hawaiian Archipelago, inhabitants of MCEs have lower total diversity, harbor new and unique species, and have higher rates of endemism in fishes. Fish species present in shallow and mesophotic depths have similar population and trophic (except benthic invertivores) structures and high genetic connectivity with lower fecundity at mesophotic depths. MCEs in Hawai'i are widespread but associated with specific geophysical characteristics. High genetic, ecological and trophic connectivity establish the potential for MCEs to serve as refugia for some species, but our results question the premise that MCEs are more resilient than shallow reefs. We found that endemism within MCEs increases with depth, and our results do not support suggestions of a global faunal break at 60 m. Our findings enhance the scientific foundations for conservation and management of MCEs, and provide a template for future interdisciplinary research on MCEs worldwide.

New Ulvaceae (Ulvophyceae, Chlorophyta) from mesophotic ecosystems across the Hawaiian Archipelago.

Ulvalean algae (Chlorophyta) are most commonly described from intertidal and shallow subtidal marine environments worldwide, but are less well known from mesophotic environments. Their morphological simplicity and phenotypic plasticity make accurate species determinations difficult, even at the generic level. Here, we describe the mesophotic Ulvales species composition from 13 locations across 2,300 km of the Hawaiian Archipelago. Twenty-eight representative Ulvales specimens from 64 to 125 m depths were collected using technical diving, submersibles, and remotely operated vehicles. Morphological and molecular characters suggest that mesophotic Ulvales in Hawaiian waters form unique communities comprising four species within the genera Ulva and Umbraulva, each with discrete geographic and/or depth-related distributional patterns. Three genetically distinct taxa are supported by both plastid (rbcL and tufA) and nuclear (ITS1) markers, and are presented here as new species: Umbraulva kaloakulau, Ulva ohiohilulu, and Ulva iliohaha. We also propose a new Umbraulva species (Umbraulva kuaweuweu), which is closely related to subtidal records from New Zealand and Australia, but not formally described. To our knowledge, these are the first marine species descriptions from Hawai'i resulting from the collaboration of traditional Hawaiian nomenclature specialists, cultural practitioners and scientists. The difficulty of finding reliable diagnostic morphological characters for these species reflects a common problem worldwide of achieving accurate identification of ulvalean taxa using solely morphological criteria. Mesophotic Ulvales appear to be distinct from shallow-water populations in Hawai'i, but their degree of similarity to mesophotic floras in other locations in the Pacific remains unknown.

The coral core microbiome identifies rare bacterial taxa as ubiquitous endosymbionts.

Despite being one of the simplest metazoans, corals harbor some of the most highly diverse and abundant microbial communities. Differentiating core, symbiotic bacteria from this diverse host-associated consortium is essential for characterizing the functional contributions of bacteria but has not been possible yet. Here we characterize the coral core microbiome and demonstrate clear phylogenetic and functional divisions between the micro-scale, niche habitats within the coral host. In doing so, we discover seven distinct bacterial phylotypes that are universal to the core microbiome of coral species, separated by thousands of kilometres of oceans. The two most abundant phylotypes are co-localized specifically with the corals' endosymbiotic algae and symbiont-containing host cells. These bacterial symbioses likely facilitate the success of the dinoflagellate endosymbiosis with corals in diverse environmental regimes.