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To thrive in nutrient-poor waters, coral reefs must retain and recycle materials efficiently. This review centers microbial processes in facilitating the persistence and stability of coral reefs, specifically the role of these processes in transforming and recycling the dissolved organic matter (DOM) that acts as an invisible central currency in reef production, nutrient cycling, and organismal interactions. The defining characteristics of coral reefs, including high productivity, balanced metabolism, high biodiversity, nutrient recycling efficiency, and structural complexity, are inextricably linked to microbial processing of DOM. The composition of microbes and DOM in reefs is summarized, and the spatial and temporal dynamics of biogeochemical processes carried out by microorganisms in diverse reef habitats are explored in a variety of key reef processes, including decomposition, accretion, trophic transfer, and macronutrient recycling. Finally, we examine how widespread habitat degradation of reefs is altering these important microbe-DOM interactions, creating feedbacks that reduce reef resilience to global change. Expected final online publication date for the

Significance All plants and animals are associated with communities of viruses and microbes that interact via a suite of metabolites. These components play critical roles in the success of these assemblages; however, the role of individual components (i.e., bacteria, viruses, metabolites) and how these govern ecological interactions between macroorganisms is not understood. This study investigates the role of holobiont components in coral–turf algal interactions. The data demonstrate that an emergent microbiome and metabolome form at the interface between coral and turf algae in competitive interactions. Machine learning analyses show that this emergent community predicts the outcome of these interactions. These results provide insight into rules of community assembly in microbiomes and the roles of holobiont components in mediating ecological interactions., Viruses, microbes, and host macroorganisms form ecological units called holobionts. Here, a combination of metagenomic sequencing, metabolomic profiling, and epifluorescence microscopy was used to investigate how the different components of the holobiont including bacteria, viruses, and their associated metabolites mediate ecological interactions between corals and turf algae. The data demonstrate that there was a microbial assemblage unique to the coral-turf algae interface displaying higher microbial abundances and larger microbial cells. This was consistent with previous studies showing that turf algae exudates feed interface and coral-associated microbial communities, often at the detriment of the coral. Further supporting this hypothesis, when the metabolites were assigned a nominal oxidation state of carbon (NOSC), we found that the turf algal metabolites were significantly more reduced (i.e., have higher potential energy) compared to the corals and interfaces. The algae feeding hypothesis was further supported when the ecological outcomes of interactions (e.g., whether coral was winning or losing) were considered. For example, coral holobionts losing the competition with turf algae had higher Bacteroidetes-to-Firmicutes ratios and an elevated abundance of genes involved in bacterial growth and division. These changes were similar to trends observed in the obese human gut microbiome, where overfeeding of the microbiome creates a dysbiosis detrimental to the long-term health of the metazoan host. Together these results show that there are specific biogeochemical changes at coral–turf algal interfaces that predict the competitive outcomes between holobionts and are consistent with algal exudates feeding coral-associated microbes.

Metabolites exuded by primary producers comprise a significant fraction of marine dissolved organic matter, a poorly characterized, heterogenous mixture that dictates microbial metabolism and biogeochemical cycling. We present a foundational untargeted molecular analysis of exudates released by coral reef primary producers using liquid chromatography–tandem mass spectrometry to examine compounds produced by two coral species and three types of algae (macroalgae, turfing microalgae, and crustose coralline algae [CCA]) from Mo’orea, French Polynesia. Of 10,568 distinct ion features recovered from reef and mesocosm waters, 1,667 were exuded by producers; the majority (86%) were organism specific, reflecting a clear divide between coral and algal exometabolomes. These data allowed us to examine two tenets of coral reef ecology at the molecular level. First, stoichiometric analyses show a significantly reduced nominal carbon oxidation state of algal exometabolites than coral exometabolites, illustrating one ecological mechanism by which algal phase shifts engender fundamental changes in the biogeochemistry of reef biomes. Second, coral and algal exometabolomes were differentially enriched in organic macronutrients, revealing a mechanism for reef nutrient-recycling. Coral exometabolomes were enriched in diverse sources of nitrogen and phosphorus, including tyrosine derivatives, oleoyl-taurines, and acyl carnitines. Exometabolites of CCA and turf algae were significantly enriched in nitrogen with distinct signals from polyketide macrolactams and alkaloids, respectively. Macroalgal exometabolomes were dominated by nonnitrogenous compounds, including diverse prenol lipids and steroids. This study provides molecular-level insights into biogeochemical cycling on coral reefs and illustrates how changing benthic cover on reefs influences reef water chemistry with implications for microbial metabolism.

The marine green alga Brilliantia kiribatiensis gen. et sp. nov. is described from samples collected during two expeditions (2009, 2013) from the coral reefs of the Southern Line Islands, Republic of Kiribati, Pacific Ocean. Phylogenetic analysis of sequences of the large- and small-subunit rDNA and the rDNA internal transcribed spacer region revealed that Brilliantia is a member of the Boodleaceae (Cladophorales),containing the genera Apjohnia, Boodlea, Cladophoropsis, Chamaedoris, Phyllodictyon and Struvea. Within this clade it formed a distinct lineage, sister to Struvea elegans , but more distantly related to the bona-fide Struvea species (including the type S. plumosa). Brilliantia differs from the other genera by having a very simple architecture forming upright, unbranched, single-celled filaments attached to the substratum by a rhizoidal mat. Cell division occurs by segregative cell division only atthe onset of reproduction. Based on current sample collection, B. kiribatiensis seems to be largely restricted to the Southern Line Islands, although it was also observed on neighboring islands, including Orona Atoll in the Phoenix Islands of Kiribati, and the Rangiroa and Takapoto Atolls in the Tuamotus of French Polynesia. This discovery highlights the likeliness that there is still much biodiversity yet to be discovered from these remote and pristine reefs of the central Pacific.

Reef-building corals are ecosystem engineers that compete with other benthic organisms for space and resources. Corals harvest energy through their surface by photosynthesis and heterotrophic feeding, and they divert part of this energy to defend their outer colony perimeter against competitors. Here, we hypothesized that corals with a larger space-filling surface and smaller perimeters increase energy gain while reducing the exposure to competitors. This predicted an association between these two geometric properties of corals and the competitive outcome against other benthic organisms. To test the prediction, fifty coral colonies from the Caribbean island of Curaçao were rendered using digital 3D and 2D reconstructions. The surface areas, perimeters, box-counting dimensions (as a proxy of surface and perimeter space-filling), and other geometric properties were extracted and analyzed with respect to the percentage of the perimeter losing or winning against competitors based on the coral tissue apparent growth or damage. The increase in surface space-filling dimension was the only significant single indicator of coral winning outcomes, but the combination of surface space-filling dimension with perimeter length increased the statistical prediction of coral competition outcomes. Corals with larger surface space-filling dimensions (Ds > 2) and smaller perimeters displayed more winning outcomes, confirming the initial hypothesis. We propose that the space-filling property of coral surfaces complemented with other proxies of coral competitiveness, such as life history traits, will provide a more accurate quantitative characterization of coral competition outcomes on coral reefs. This framework also applies to other organisms or ecological systems that rely on complex surfaces to obtain energy for competition.

Primary producers release oxygen as the by-product of photosynthetic light reactions during the day. However, a prevalent, globally-occurring nighttime spike in dissolved oxygen in the absence of light challenges the traditional assumption that biological oxygen production is limited to daylight hours, particularly in tropical coral reefs. Here we show: 1) the widespread nature of this phenomenon, 2) its reproducibility across tropical marine ecosystems, 3) the influence of biotic and abiotic factors on this phenomenon across numerous datasets, and 4) the observation of nighttime oxygen spikesin vitrofrom incubations of coral reef benthic organisms. The data from this study demonstrate that in addition to physical forcing, biological processes are likely responsible for increasing dissolved oxygen at night. Additionally, we demonstrate an association between these nighttime oxygen spikes and measures of both net community calcification and net community production. These results suggest that nighttime oxygen spikes are likely a biological response associated with increased respiration and are most prominent in communities dominated by calcifying organisms.

The microbialization of coral reefs predicts that microbial oxygen consumption will cause reef deoxygenation. Here we tested this hypothesis by analyzing reef microbial and primary producer oxygen metabolisms. Metagenomic data and in vitro incubations of bacteria with primary producer exudates showed that fleshy algae stimulate incomplete carbon oxidation metabolisms in heterotrophic bacteria. These metabolisms lead to increased cell sizes and abundances, resulting in bacteria consuming 10 times more oxygen than in coral incubations. Experiments probing the dissolved and gaseous oxygen with primary producers and bacteria together indicated the loss of oxygen through ebullition caused by heterogenous nucleation on algae surfaces. A model incorporating experimental production and loss rates predicted that microbes and ebullition can cause the loss of up to 67% of gross benthic oxygen production. This study indicates that microbial respiration and ebullition are increasingly relevant to reef deoxygenation as reefs become dominated by fleshy algae., eLife digest Rising water temperatures, pollution and other factors are increasingly threatening corals and the entire reef ecosystems they build. The potential for corals to resist and recover from the stress these factors cause ultimately depends on their ability to compete against fast-growing fleshy algae that can rapidly take over the reefs. Living on the fleshy algae, the coral and in the surrounding water are communities of bacteria and other microbes that help maintain the health of the coral reef. Both corals and algae modify the chemical and physical environment of the reef to alter the composition of the microbial communities for their own benefit. Algae, for instance, release large amounts of sugars and other molecules of organic carbon into the water. These carbon molecules are then taken up by the bacteria, along with oxygen, to produce chemical energy via a process called respiration. This could cause the levels of oxygen in the water to decrease, potentially damaging the corals and creating more open space for the algae. Previous studies have revealed how communities of microbes on coral reefs use organic carbon, but it remains unclear how they affect the levels of oxygen in the reefs. To address this question, Silveira et al. used an approach called metagenomics to analyze the bacteria in samples of water from 87 reefs across the Pacific and the Caribbean, and also performed experiments with reef bacteria grown in the laboratory. The experiments showed that bacteria growing in the presence of fleshy algae became larger and more abundant than bacteria growing near corals, resulting in the water containing lower levels of oxygen. Furthermore, the fleshy algae produced bubbles of oxygen that were released from the water. Silveira et al. developed a mathematical model that predicted that these bubbles, combined with the respiration of bacteria that live near algae, caused the loss of 67% of the oxygen in the water surrounding the reef. These findings represent a fundamental step towards understanding how changes in the levels of oxygen in water affect the ability of coral reefs to resist and recover from stress.

On coral reefs, microorganisms are essential for recycling nutrients to primary producers through the remineralization of benthic-derived organic matter. Diel investigations of reef processes are required to holistically understand the functional roles of microbial players in these ecosystems. Here we report a metagenomic analysis characterizing microbial communities in the water column overlying 16 remote forereef sites over a diel cycle. Our results show that microbial community composition is more dissimilar between day and night samples collected from the same site than between day or night samples collected across geographically distant reefs. Diel community differentiation is largely driven by the flux of Psychrobacter sp., which is two-orders of magnitude more abundant during the day. Nighttime communities are enriched with species of Roseobacter, Halomonas, and Alteromonas encoding a greater variety of pathways for carbohydrate catabolism, further illustrating temporal patterns of energetic provisioning between different marine microbes. Dynamic diel fluctuations of microbial populations could also support the efficient trophic transfer of energy posited in coral reef food webs., Microbes structure biogeochemical cycles and food webs in the marine environment. Here, the authors sample coral reef-associated microbes across a 24-hour period, showing clear day–night patterns of microbial populations and thus calling for more studies to consider temporal variation in microbiomes at this scale.

Microbial viruses can control host abundances via density-dependent lytic predator-prey dynamics. Less clear is how temperate viruses, which coexist and replicate with their host, influence microbial communities. Here we show that virus-like particles are relatively less abundant at high host densities. This suggests suppressed lysis where established models predict lytic dynamics are favoured. Meta-analysis of published viral and microbial densities showed that this trend was widespread in diverse ecosystems ranging from soil to freshwater to human lungs. Experimental manipulations showed viral densities more consistent with temperate than lytic life cycles at increasing microbial abundance. An analysis of 24 coral reef viromes showed a relative increase in the abundance of hallmark genes encoded by temperate viruses with increased microbial abundance. Based on these four lines of evidence, we propose the Piggyback-the-Winner model wherein temperate dynamics become increasingly important in ecosystems with high microbial densities; thus 'more microbes, fewer viruses'.

An overall synthesis of biology and non-equilibrium thermodynamics remains a challenge at the interface between the physical and life sciences. Herein, theorems from finite-time and control thermodynamics are applied to biological processes to indicate which biological strategies will succeed over different time scales. In general, living systems maximize power at the expense of efficiency during the early stages of their development while proceeding at slower rates to maximize efficiency over longer time scales. The exact combination of yield and power depends upon the constraints on the system, the degrees of freedom in question, and the time scales of the processes. It is emphasized that biological processes are not driven by entropy production but, rather, by informed exergy flow. The entropy production is the generalized friction that is minimized insofar as the constraints allow. Theorems concerning thermodynamic path length and entropy production show that there is a direct tradeoff between the efficiency of a process and the process rate. To quantify this tradeoff, the concepts of compensated heat and waste heat are introduced. Compensated heat is the exergy dissipated, which is necessary for a process to satisfy constraints. Conversely, waste heat is exergy that is dissipated as heat, but does not provide a compensatory increase in rate or other improvement. We hypothesize that it is waste heat that is minimized through natural selection. This can be seen in the strategies employed at several temporal and spatial scales, including organismal development, ecological succession, and long-term evolution. Better understanding the roles of compensated heat and waste heat in biological processes will provide novel insight into the underlying thermodynamic mechanisms involved in metabolism, ecology, and evolution.

Corals have built reefs on the benthos for millennia, becoming an essential element in marine ecosystems. Climate change and human impact, however, are favoring the invasion of non-calcifying benthic algae and reducing coral coverage. Corals rely on energy derived from photosynthesis and heterotrophic feeding, which depends on their surface area, to defend their outer perimeter. But the relation between geometric properties of corals and the outcome of competitive coral-algal interactions is not well known. To address this, 50 coral colonies interacting with algae were sampled in the Caribbean island of Curaçao. 3D and 2D digital models of corals were reconstructed to measure their surface area, perimeter, and polyp sizes. A box counting algorithm was applied to calculate their fractal dimension. The perimeter and surface dimensions were statistically non-fractal, but differences in the mean surface fractal dimension captured relevant features in the structure of corals. The mean fractal dimension and surface area were negatively correlated with the percentage of losing perimeter and positively correlated with the percentage of winning perimeter. The combination of coral perimeter, mean surface fractal dimension, and coral species explained 19% of the variability of losing regions, while the surface area, perimeter, and perimeter-to-surface area ratio explained 27% of the variability of winning regions. Corals with surface fractal dimensions smaller than two and small perimeters displayed the highest percentage of losing perimeter, while corals with large surface areas and low perimeter-to-surface ratios displayed the largest percentage of winning perimeter. This study confirms the importance of fractal surface dimension, surface area, and perimeter of corals in coral-algal interactions. In combination with non-geometrical measurements such as microbial composition, this approach could facilitate environmental conservation and restoration efforts on coral reefs.

Understanding the release and remineralization of organic matter by benthic macroorganisms provides insight into nutrient cycling and microbial metabolism in coral reef environments. The release rate of particulate (POC) and dissolved organic carbon (DOC) by the soft coral Lobophytum crassum was quantified and subsequent bacterial growth rates determined in response to this resource, and compared with results from those of the common hard coral Acropora intermedia . The results of this study show that the soft coral released more DOC than POC into the surrounding seawater , similar to what was measured for the hard coral species. However, the soft coral-derived organic matter fostered a lower microbial growth rate with a lower growth efficiency compared to DOC and POC of hard corals, likely due to the lower C:N ratio of the organic matter derived from soft corals. These results suggest that soft coral exudates are relatively refractory compared to the mucus of hard corals. Possible phase shifts from hard to soft corals on degraded reefs may represent very different changes in microbial community dynamics and metabolism as compared to the widely studied coral-algal phase shifts.

Human impacts are causing ecosystem phase shifts from coral- to algal-dominated reef systems on a global scale. As these ecosystems undergo transition, there is an increased incidence of coral-macroalgal interactions. Mounting evidence indicates that the outcome of these interaction events is, in part, governed by microbially mediated dynamics. The allocation of available energy through different trophic levels, including the microbial food web, determines the outcome of these interactions and ultimately shapes the benthic community structure. However, little is known about the underlying thermodynamic mechanisms involved in these trophic energy transfers. This study utilizes a novel combination of methods including calorimetry, flow cytometry, and optical oxygen measurements, to provide a bioenergetic analysis of coral-macroalgal interactions in a controlled aquarium setting. We demonstrate that the energetic demands of microbial communities at the coral-algal interaction interface are higher than in the communities associated with either of the macroorganisms alone. This was evident through higher microbial power output (energy use per unit time) and lower oxygen concentrations at interaction zones compared to areas distal from the interface. Increases in microbial power output and lower oxygen concentrations were significantly correlated with the ratio of heterotrophic to autotrophic microbes but not the total microbial abundance. These results suggest that coral-algal interfaces harbor higher proportions of heterotrophic microbes that are optimizing maximal power output, as opposed to yield. This yield to power shift offers a possible thermodynamic mechanism underlying the transition from coral- to algal-dominated reef ecosystems currently being observed worldwide. As changes in the power output of an ecosystem are a significant indicator of the current state of the system, this analysis provides a novel and insightful means to quantify microbial impacts on reef health.

Primary production due to photosynthesis results in daytime oxygen production across marine and freshwater ecosystems. However, a prevalent, globally-occurring nighttime spike in dissolved oxygen (DO) challenges our traditional assumption that oxygen production is limited to daylight hours, particularly in tropical coral reefs. When considered in the context of ecosystem oxygen budget estimates, these nocturnal spikes in DO could account for up to 24 percent of the daytime oxygen production. Here we show, 1) the widespread nature of this phenomenon, 2) the reproducibility across tropical marine ecosystems, 3) the lack of a consistent abiotic mechanism across all datasets we examined, and 4) the observation of nighttime DO spikes in vitro from incubations of coral reef benthic samples. Our study suggests that in addition to physical forcing, biological processes may be responsible for the production of oxygen at night, a finding that demands additional research.

Temperate bacterial viruses (phages) may enter a symbiosis with their host cell, forming a unit called a lysogen. Infection and viral replication are disassociated in lysogens until an induction event such as DNA damage occurs, triggering viral-mediated lysis. The lysogen-lytic viral reproduction switch is central to viral ecology, with diverse ecosystem impacts. It has been argued that lysogeny is favoured in phages at low host densities. This paradigm is based on the fraction of chemically inducible cells (FCIC) lysogeny proxy determined using DNA-damaging mitomycin C inductions. Contrary to the established paradigm, a survey of 39 inductions publications found FCIC to be highly variable and pervasively insensitive to bacterial host density at global, within-environment and within-study levels. Attempts to determine the source(s) of variability highlighted the inherent complications in using the FCIC proxy in mixed communities, including dissociation between rates of lysogeny and FCIC values. Ultimately, FCIC studies do not provide robust measures of lysogeny or consistent evidence of either positive or negative host density dependence to the lytic-lysogenic switch. Other metrics are therefore needed to understand the drivers of the lytic-lysogenic decision in viral communities and to test models of the host density-dependent viral lytic-lysogenic switch.

Benthic marine primary producers affect the chemistry of their surrounding environment through metabolic processes. Photosynthesis and respiration will elevate or depress the concentration of oxygen in the diffusive boundary layer. Likewise, acid–base regulation and biomineralization/dissolution for calcifying species can alter the relative concentration of inorganic carbon species and thus pH. Here, we measured the relative ability of several common benthic primary producers from coral reef systems of the central Pacific and the Caribbean to simultaneously affect seawater oxygen concentration and pH values. Repeated measurements over a diel cycle confirmed that several primary producers substantially alter surrounding seawater chemistry over time. The majority of fleshy algae exhibited a stoichiometric ratio of oxygen to hydrogen ions not significantly different from one during daylight hours. In contrast, calcifiers exhibited significantly lower oxygen to hydrogen ion ratios that were unique for each species and were inversely correlated with known rates of calcification. These data provide the first quantitative estimates of the simultaneous influence of several species of benthic primary producers on water column oxygen concentrations and pH across different tropical reef systems. Finally, because more productive fleshy taxa have the potential to raise both oxygen and pH during the day to a greater extent than calcified species, our results suggest that some fleshy taxa may provide a buffering capacity to future ocean acidification scenarios.

Increasing algal cover on tropical reefs worldwide may be maintained through feedbacks whereby algae outcompete coral by altering microbial activity. We hypothesized that algae and coral release compositionally distinct exudates that differentially alter bacterioplankton growth and community structure. We collected exudates from the dominant hermatypic coral holobiont Porites spp. and three dominant macroalgae (one each Ochrophyta, Rhodophyta and Chlorophyta) from reefs of Mo'orea, French Polynesia. We characterized exudates by measuring dissolved organic carbon (DOC) and fractional dissolved combined neutral sugars (DCNSs) and subsequently tracked bacterioplankton responses to each exudate over 48 h, assessing cellular growth, DOC/DCNS utilization and changes in taxonomic composition (via 16S rRNA amplicon pyrosequencing). Fleshy macroalgal exudates were enriched in the DCNS components fucose (Ochrophyta) and galactose (Rhodophyta); coral and calcareous algal exudates were enriched in total DCNS but in the same component proportions as ambient seawater. Rates of bacterioplankton growth and DOC utilization were significantly higher in algal exudate treatments than in coral exudate and control incubations with each community selectively removing different DCNS components. Coral exudates engendered the smallest shift in overall bacterioplankton community structure, maintained high diversity and enriched taxa from Alphaproteobacteria lineages containing cultured representatives with relatively few virulence factors (VFs) (Hyphomonadaceae and Erythrobacteraceae). In contrast, macroalgal exudates selected for less diverse communities heavily enriched in copiotrophic Gammaproteobacteria lineages containing cultured pathogens with increased VFs (Vibrionaceae and Pseudoalteromonadaceae). Our results demonstrate that algal exudates are enriched in DCNS components, foster rapid growth of bacterioplankton and select for bacterial populations with more potential VFs than coral exudates.

Immunity is mostly studied in a few model organisms, leaving the majority of immune systems on the planet unexplored. To characterize the immune systems of non-model organisms alternative approaches are required. Viruses manipulate host cell biology through the expression of proteins that modulate the immune response. We hypothesized that metagenomic sequencing of viral communities would be useful to identify both known and unknown host immune proteins. To test this hypothesis, a mock human virome was generated and compared to the human proteome using tBLASTn, resulting in 36 proteins known to be involved in immunity. This same pipeline was then applied to reef-building coral, a non-model organism that currently lacks traditional molecular tools like transgenic animals, gene-editing capabilities, and in vitro cell cultures. Viromes isolated from corals and compared with the predicted coral proteome resulted in 2503 coral proteins, including many proteins involved with pathogen sensing and apoptosis. There were also 159 coral proteins predicted to be involved with coral immunity but currently lacking any functional annotation. The pipeline described here provides a novel method to rapidly predict host immune components that can be applied to virtually any system with the potential to discover novel immune proteins.

Holobionts are assemblages of microbial symbionts and their macrobial host. As extant representatives of some of the oldest macro-organisms, corals and algae are important for understanding how holobionts develop and interact with one another. Using untargeted metabolomics, we show that non-self interactions altered the coral metabolome more than self-interactions (i.e. different or same genus, respectively). Platelet activating factor (PAF) and Lyso-PAF, central inflammatory modulators in mammals, were major lipid components of the coral holobionts. When corals were damaged during competitive interactions with algae, PAF increased along with expression of the gene encoding Lyso-PAF acetyltransferase; the protein responsible for converting Lyso-PAF to PAF. This shows that self and non-self recognition among some of the oldest extant holobionts involve bioactive lipids identical to those in highly derived taxa like humans. This further strengthens the hypothesis that major players of the immune response evolved during the pre-Cambrian.

Coral reef-associated benthic algae can control important metabolic processes in reef ecosystems via organic matter release. However, information about the chemical composition of macroalgae exudates is very limited. We therefore quantified total and dissolved organic carbon (TOC, DOC) and total and dissolved organic nitrogen (TON, DON) of exudates released by 2 cosmopolitan coral reef-associated green algae species, Halimeda opuntia and Caulerpa serrulata. Glycosyl com- position and content analyses, along with protein, lipid and chlorophyll a (chl a) quantifications of algae exudates were conducted. Findings revealed that organic matter released from both algae pre- dominately consisted of carbohydrates (59 ± 5%) and proteins (32 ± 7%), which mainly (89 to 93%) dissolved in surrounding waters. Traces of fatty acids (C16:0; C18:0) and chl a were also found in algae incubation waters, but in a quantitatively negligible amount. Carbohydrate analysis further showed that glucose was the dominant glycosyl released by algae, accounting for 77 ± 8% of the carbohydrate fraction and 42 ± 8% of TOC. Galactose (9 ± 4% of carbohydrate fraction), mannose (6 ± 3%), xylose (4 ± 1%), rhamnose (3 ± 1%) and fucose (2 ± 1%) were also detected in all incubation water samples. High glucose and protein contents of algae exudates found in the present study con- firm assumptions on the fast microbial degradability of these exudates, with ensuing potential nega- tive effects on O2 availability and on interactions between corals and algae in coral reef ecosystems.

Mucus, a complex composed primarily of carbohydrates, is released in similar quantities by scleractinian warm- and cold-water reef corals, and can function as an important carrier of organic material from corals to a range of consumers, microbes in particular. However, information about mucus chemical composition is rare for warm-water corals and non-existent for cold-water corals. This study therefore presents comparative carbohydrate composition analyses of mucus released by the dominant and cosmopolitan warm- and cold-water coral genera. Arabinose was the major mucus carbohydrate component for the genus Acropora, but was not found in cold-water coral mucus. Mucus derived from corals of the genus Fungia contained significantly more fucose than the mucus of all other coral genera. However, comparison of mucus carbohydrate composition for the warm- and cold-water corals in the present study and in the literature revealed no significant differences. This indicates use of similar carbohydrate components (with the exception of arabinose) during mucus synthesis by scleractinian corals, largely irrespective of zooxanthellate or azooxanthellate carbon supply mechanisms.

Coral reef lagoon benthic primary producers may control various processes important for ecosystem functioning, predominately via the release of organic matter, but comparative data are rare. This study therefore comparatively investigated the quantity of particulate and dissolved organic matter released by different benthic primary producers (seagrasses, macroalgae and sclerac- tinian corals) from the coral reef lagoon of Puerto Morelos, Mexican Caribbean. Microbial degrad- ability of the released organic matter was determined along with diurnal in situ measurements of O2 concentrations at lagoon sites dominated by different primary producers. Particulate organic carbon (POC) release was highest for corals (8.2 ± 4.2 mg m -2 h -1 ), followed by benthic algae (3.9 ± 0.7 mg m -2 h -1 ) and seagrasses (3.1 ± 2.0 mg m -2 h -1 ). Dissolved organic carbon (DOC) release rates were highest for seagrasses (15.8 ± 6.0 mg m -2 h -1 ), followed by algae (1.9 ± 2.0 mg m -2 h -1 ), whereas corals displayed net DOC uptake. Benthic algae-derived organic matter stimulated planktonic microbial O2 consumption significantly more than seagrass- or coral-derived organic matter. In situ O2 loggers revealed significantly lower average O2 concentrations, particularly during the night, at algae-domi- nated sites compared to other benthic lagoon environments. This indicates effects of algae-derived organic matter on in situ O2 availability. We therefore suggest that shifts in benthic primary producer dominance affect ecosystem functioning owing to differences in quantity, composition and microbial degradability of the released organic matter.

article Recent research indicates that coral reef associated benthic algae may control important metabolic processes in reef ecosystems via organic matter release. Yet little information is available about quantity and chemical composition of these algae-derived exudates. Therefore first comprehensive studies on algal organic matter release were conducted at a fringing reef ecosystem in the Northern Red Sea. Dissolved organic carbon (DOC), particulate organic carbon (POC) and nitrogen (PN) release by dominant reef associated benthic algae (Caulerpa serrulata, Peyssonnelia capensis, turf algae assemblages) were quantified during 4 seasonally resolved expeditions. Additionally, 4 seasonal blooming (Ulva lactuca, Enteromorpha flexuosa, Liagora turneri, Hydroclathrus clathratus) and 2 patchy growing algae species (Lobophora variegata, Saragassum dentifolium) were included in these investigations. To complement the dataset organic matter release by Caulerpa was studied under different light conditions, simulating water depths of 1, 5, 10 and 20 m. Environmental parameters (temperature, light availability, inorganic nutrient concentrations) were monitored simulta- neously to assess potential effects on algal organic matter release. All 9 investigated genera of benthic algae exuded DOC and POC in amounts of 12.2 ± 2.1 and 4.2 ± 0.3 mg organic C m − ² algae surface area h − 1 , respectively. Resident algae, primarily turf algae assemblages, displayed highest and seasonal blooming algae lowest organic matter release rates. Results therefore indicate that organic matter release rates are rather influenced by functional properties (growth form, life strategy) of algae than by taxonomic affiliation. Quantities of organic matter release showed seasonal and depth-mediated variations and were positively correlated with temperature and light availability within photosynthetically active radiation intensities of 0 to 300 μmol quanta m − 2

Coastal underwater soundscapes typically contain signals from soniferous, biological processes. Identifying the sources that contribute to oftentimes acoustically complex soundscapes would facilitate a noninvasive, remote, and volumetrically integrative method to survey underwater ecological state, as has been demonstrated on land. To date the lack of knowledge on these sources has posed a challenge for extracting meaningful ecological information from underwater biological soundscape recordings. We have observed that photosynthesis by macroalgae is an acoustically active process, driven by oxygen bubbles separating from the algal surface and ringing at the Minnaert frequency. The resultant soundscapes correlate with benthic macroalgal cover across ecological gradients on shallow Hawaiian reefs during periods of daylight. Bubble size, production rate, and sound exposure level follow the concentration of dissolved oxygen, which in turn rises and falls with the availability of photosynthetically active radi...

Particulate organic matter (POM) and dissolved organic carbon (DOC) release by six dominant hermatypic coral genera (Acropora, Fungia, Goniastrea, Millepora, Pocillopora and Stylophora) were measured under undisturbed conditions by laboratory incubations during four seasonal expeditions to the Northern Red Sea. In addition, the influence of environmental factors (water temperature, light availability and ambient inorganic nutrient concentrations) was evaluated. Particulate organic carbon (POC) and particulate nitrogen (PN) release were always detectable and genus-specific, with Stylophora releasing most POM (6.5 mg POC and 0.5 mg PN m−2 coral surface area h−1) during all seasons. The fire coral Millepora released significantly less POM (0.3 mg POC and 0.04 mg PN m−2 coral surface area h−1) than all investigated anthozoan genera. The average POC:PN ratio of POM released by all coral genera was 12 ± 1, indicating high carbon/low nitrogen content of coral-derived organic matter. POM release showed little seasonal variation, but average values of POC and PN release rates correlated with water temperature, light availability and ambient nitrate concentrations. DOC net release and elevated DOC:POC ratios were detectable for Acropora, Goniastrea and Millepora, revealing maximum values for Acropora (30.7 mg DOC m−2 coral surface area h−1), whilst predominant DOC uptake was observed for Pocillopora, Fungia and Stylophora. Depth-mediated light availability influenced DOC fluxes of Acropora and Fungia, while fluctuations in water temperature and ambient inorganic nutrient concentrations showed no correlation. These comprehensive data provide an important basis for the understanding of coral reef organic matter dynamics and relevant environmental factors.

Many coral reef ecosystems experience shifts in benthic community composition from scleractinian corals to algae. However, consequences of such phase shifts on O2 availability, important for many reef organisms, are unresolved. This study therefore comparatively investigated potential in situ effects of different benthic cover by reef macroalgae and scleractinian corals on water column O2 concentrations in a Northern Red Sea fringing reef. Findings revealed that mean daily O2 concentrations at algae-dominated sites were significantly lower compared to coral-dominated sites. Minimum O2 concentrations were significantly negatively correlated, while diurnal variability in O2 concentration was significantly positively correlated, with increasing benthic cover by algae. In contrast, no correlation with coral cover was found. These results indicate that shifts from corals to benthic algae may likely affect both in situ O2 availability and variability. This may be particularly pronounced in reef systems with low water exchange (e.g. closed lagoons) or under calm weather conditions and suggests potential O2-mediated effects on reef organisms.

This paper presents seasonal in situ monitoring data on benthic coverage and coral–algae interactions in high-latitude fringing reefs of the Northern Red Sea over a period of 19 months. More than 30% of all hermatypic corals were involved in interaction with benthic reef algae during winter compared to 17% during summer, but significant correlation between the occurrence of coral–algae interactions and monitored environmental factors such as temperature and inorganic nutrient availability was not detected. Between 5 and 10-m water depth, the macroalgae Caulerpa serrulata, Peyssonnelia capensis and filamentous turf algae represented almost 100% of the benthic algae involved in interaction with corals. Turf algae were most frequently (between 77 and 90% of all interactions) involved in interactions with hermatypic corals and caused most tissue damage to them. Maximum coral tissue loss of 0.75% day−1 was observed for Acropora-turf algae interaction during fall, while an equilibrium between both groups of organisms appeared during summer. Slow-growing massive corals were more resistant against negative algal influence than fast-growing branching corals. Branching corals of the genus Acropora partly exhibited a newly observed phenotypic plasticity mechanism, by development of a bulge towards the competing organism, when in interaction with algae. These findings may contribute to understand the dynamics of phase shifts in coral reefs by providing seasonally resolved in situ monitoring data on the abundance and the competitive dynamic of coral–algae interactions.