High-throughput sequencing (HTS) of DNA barcodes (metabarcoding), particularly when combined with... more High-throughput sequencing (HTS) of DNA barcodes (metabarcoding), particularly when combined with standardized sampling protocols, is one of the most promising approaches for censusing overlooked cryptic invertebrate communities. We present biodiversity estimates based on sequencing of the cytochrome c oxidase subunit 1 (COI) gene for coral reefs of the Gulf of Aqaba, a semi-enclosed system in the northern Red Sea. Samples were obtained from standardized sampling devices (Autonomous Reef Monitoring Structures (ARMS)) deployed for 18 months. DNA bar-coding of non-sessile specimens >2 mm revealed 83 OTUs in six phyla, of which only 25% matched a reference sequence in public databases. Metabarcoding of the 2 mm – 500 m and sessile bulk fractions revealed 1197 OTUs in 15 animal phyla, of which only 4.9% matched reference barcodes. These results highlight the scarcity of COI data for cryptoben-thic organisms of the Red Sea. Compared with data obtained using similar methods, our results suggest that Gulf of Aqaba reefs are less diverse than two Pacific coral reefs but much more diverse than an Atlantic oyster reef at a similar latitude. The standardized approaches used here show promise for establishing baseline data on biodiversity, monitoring the impacts of environmental change, and quantifying patterns of diversity at regional and global scales. Résumé : Le séquencage a haut debit (HTS) de barcodes ADN (metabarcoding), en particulier lorsqu'il est associé a un échantillonnage standardizé, est l'une des approaches les plus prometteuses pour recenser les communautés d'invertébrés cryptiques negligées jusqu'a ` aujourd'hui. Nous présentons des estimations de biodiversité basées sur le séquencage du gene cytochrome c oxidase subunit I (COI) pour les récifs coralliens du Golfe d'Aqaba, un système semi-fermé situé au nord de la mer Rouge. Les échantillons ont été obtenus a partir de dispositifs d'échantillonage standardizés (structures autonomes de suivi des récifs (ARMS)) deployés pendant 18 mois. L'analyse des barcodes ADN des organismes non-sessiles >2 mm révéla 83 OTUs appartenant a ` six embranche-ments, parmi lesquels seulement 25 % correspondaient a ` des barcodes de référence dans les bases de données publiques. Le metabarcoding des fractions 2 mm – 500 m et sessile révéla un total de 1197 OTUs dont des représentants de 15 embranchements animaux, parmi lesquels seulement 4.9 % correspondaient a ` des barcodes de référence. Ces résultats mettent en evidence le manque de données COI pour les organismes cryptobenthiques de la mer Rouge. Comparé a ` des données obtenues avec des méthodes similaires, nos résultats suggèrent que le Golfe d'Aqaba est moins diversifié que deux récifs coralliens du Pacifique mais beaucoup plus diversifié que des récifs d'huitre a ` la même latitude. Les approches standardizées utilisées dans cette étude sont très prometteuses pour la mise en place de points de comparaison de données de biodiversité, le suivi des changements environnementaux, et pour la quantification des patrons de diversité aux échelles régionales et globales. Mots-clés : codage a ` barres de l'ADN, métacodage a ` barres, COI, structures autonomes de suivi des récifs, Golfe d'Aqaba.
Biological dinitrogen (N 2) fixation (diazotrophy, BNF) relieves marine primary producers of nitr... more Biological dinitrogen (N 2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m-2 reef d-1 , with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m-2 reef d-1. Planktonic GPP and BNF rates were respectively approximately 60-and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.
The giant diatom Ethmodiscus was examined along an east–west transect at 28–301N during 2002 and ... more The giant diatom Ethmodiscus was examined along an east–west transect at 28–301N during 2002 and 2003 to determine if abundance, chemical composition or physiological status of this largest of diatoms varied on the scale of 100's–1000's of km in North Pacific gyre. Abundance ranged from o0.1–42.0 cells m À3 and supported the notion of an abundance mosaic reported previously. However, there was only minimal support for the relationship between abundance and nutrient concentration at 125 m reported previously. Cellular chlorophyll varied little along the transect (7.3–10.9 ng chl cell À1) except at the westernmost station. Cellular N and P quotas co-varied 3–4.5 fold (mean ¼ 50.873.7 and 3.770.8 nmol N and P cell À1) and yielded N:P ratios that closely clustered around the Redfield ratio (average ¼ 14.671.1). Only low levels of chlorophyll-normalized alkaline phosphatase (APase) activity were observed (0.4–2.5 nmol P mg chl À1 h À1) with APase activity lower than that in either the bulk water, or co-occurring Trichodesmium spp. and Pyrocystis noctiluca. The active fluorescence parameter F v :F m , a property sensitive to Fe stress, was uniformly high at all stations (average ¼ 0.7370.04 for 2003, and 0.6970.05 for 2002), indicating sufficient Fe for optimum photosynthetic competence. These results contrasted sharply with results from Rhizosolenia mats reported along the same transect where there was a significant decline westward in F v :F m. Both ferredoxin (Fd) and flavodoxin accumulated in cells of Ethmodiscus, resulting in Fd Index values ofo0.6. Iron cell quotas ranged from 0.7–5.1 pmol Fe cell À1. When normalized to cytoplasmic volume, the Fe mm À3 was comparable to that of Escherichia coli. We note that the disproportionate contribution of the vacuole (with its high organic content) to total volume typical of large diatoms is a potentially significant source of error in Fe:C ratios and suggest that Fe should be normalized to cytoplasmic volume whenever possible to permit valid intercomparisons between studies. The composition, F v :F m data and Fe:C ratio suggest a relatively uniform population experiencing little N, P or Fe stress. The uncoupling of the Fd Index from these measures is consistent with previous findings showing that the expression of flavodoxin can be characterized as an early stress response and that its accumulation is not necessarily correlated with physiological deficit. Ethmodiscus appears to be well adapted to some of the most oligotrophic waters in the ocean. Because it is an important sedimentary marker, the biology of living Ethmodiscus provides insights into the source of extensive ARTICLE IN PRESS
Potentially Toxic Substances (PTS) in soils are of increasingly growing concern worldwide. Heavy ... more Potentially Toxic Substances (PTS) in soils are of increasingly growing concern worldwide. Heavy metals are acting as one of the most serious groups of environmental contaminants, and their release into the environment has strongly increased over the last decades. Heavy metals can cause acute and long-term toxic effects on both human health and the ecosystems around. Toxic effects of heavy metals reach soil biota in general and affect the microbial community biomass and metabolic activities related to such communities. Although all members of the soil biota respond relatively to soil pollution, microbial communities are considered to be the first and most swift responders to such environmental pollutants. This study focused on the state of the art of developing a consortium of different enzymes and how their collective activities could be used for the assessment and monitoring of soil in response to heavy metal pollution. By measuring microbial community biomass and activity from so...
Biological dinitrogen (N2) fixation (diazotrophy,
BNF) relieves marine primary producers of nitro... more Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, andNis therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m-2 reef d-1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m-2 reef d-1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.
Functional traits define species by their ecological role in the ecosystem. Animals themselves ar... more Functional traits define species by their ecological role in the ecosystem. Animals themselves are host–microbe ecosystems (holobionts), and the application of ecophysiological approaches can help to understand their functioning. In hard coral holobionts, communities of dinitrogen (N2)-fixing prokaryotes (diazotrophs) may contribute a functional trait by providing bioavailable nitrogen (N) that could sustain coral productivity under oligotrophic conditions. This study quantified N2 fixation by diazotrophs associated with four genera of hermatypic corals on a northern Red Sea fringing reef exposed to high seasonality. We found N2 fixation activity to be 5- to 10-fold higher in summer, when inorganic nutrient concentrations were lowest and water temperature and light availability highest. Concurrently, coral gross primary productivity remained stable despite lower Symbiodinium densities and tissue chlorophyll a contents. In contrast, chlorophyll a content per Symbiodinium cell increased from spring to summer, suggesting that algal cells overcame limitation of N, an essential element for chlorophyll synthesis. In fact, N2 fixation was positively correlated with coral productivity in summer, when its contribution was estimated to meet 11% of the Symbiodinium N requirements. These results provide evidence of an important functional role of diazotrophs in sustaining coral productivity when alternative external N sources are scarce.
Dinitrogen (N2) fixation by specialized prokaryotes (diazotrophs) represents an important source ... more Dinitrogen (N2) fixation by specialized prokaryotes (diazotrophs) represents an important source of bioavailable nitrogen (N) in the ocean. In coral reefs, several substrates and organisms are associated with diazotrophs, but potential N2 fixation activity by zooxanthellate soft corals has not yet been investigated. Such soft corals may contribute importantly to the input of new N into the reef ecosystem, as they can cover substantial benthic areas in today's coral reefs. Therefore, this study investigated N2 fixation of 2 dominant zooxanthellate soft coral groups (Sarcophyton sp. and Xeniidae) in a northern Red Sea fringing reef during all 4 seasons of 1 yr. This was supplemented by respirometry incubations and in situ monitoring of key environmental parameters. Findings revealed detectable N2 fixation for both soft corals during all seasons. Annual N2 fixation by Sarcophyton sp. was 3 times higher than that by Xeniidae, but both soft corals exhibited similar seasonal patterns. N2 fixation significantly increased during summer, when water temperature and light intensity were highest and inorganic nutrient availability was lowest. Coral respiration also peaked during summer and was positively correlated to N2 fixation, while photosynthesis revealed maximum rates during the nutrient-enriched spring season. Given the importance of N for reproduction and growth, N2 fixation may be a key component of soft coral nutrition during summer, when inorganic nutrient availability in the water column is lowest and likely not sufficient to sustain the high metabolic demand of soft corals.
Permeable sediments are highly bioactive compartments in coral reefs. The associated dense microb... more Permeable sediments are highly bioactive compartments in coral reefs. The associated dense microbial communities sustain fast degradation of organic matter, thereby playing a key role in nutrient recycling within the reef. Besides nutrient recycling, new nutrients (i.e. nitrogen) are acquired by dinitrogen (N2) fixing microbial communities, but knowledge about the influence of sand mineralogy and key environmental factors on this process is scarce. Therefore, this study quantified seasonal N2 fixation (via acetylene reduction) along with gross photosynthesis (via O2 fluxes) by adjacent carbonate and silicate sands in a Northern Red Sea coral reef. Findings revealed significantly higher N2 fixation in carbonate than in silicate sands (2.88 and 1.52 nmol C2H4 cm-2 h-1, respectively) and a more pronounced seasonal response in the former, likely caused by its higher permeability, grain size and microbial abundance. Ambient light and organic matter availability were the main controlling environmental factors for sand-associated N2 fixation. Carbonate and silicate sands showed similar gross photosynthesis rates (270 and 233 nmol O2 cm-2 h-1) that positively (carbonate sands) or negatively (silicate sands) correlated with N2 fixation, likely due to different diazotrophic communities. Seasonal appearance of microbial mats on carbonate sands increased N2 fixation and gross photosynthesis by up to one order of mag nitude. On an annual average, carbonate and silicate sands obtain ∼8% and microbial mat communities obtain ∼13% of their photo-metabolic N demand via N2 fixation.
The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is r... more The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is responsible for up to half of the global mesopelagic fixed nitrogen (N) loss from the ocean via denitrification and anammox. Dinitrogen (N2) fixation is usually attributed to cyanobacteria in the surface ocean. Model prediction and physiological inhibition of N2 fixation by oxygen, however, suggest that N2 fixation should be enhanced near the oxygen-deficient zone (ODZ) of the Arabian Sea. N2 fixation and cyanobacterial nifH genes (the gene encoding dinitrogenase reductase) have been reported in surface waters overlying the Arabian Sea ODZ. Here, water samples from depths above and within the Arabian Sea ODZ were examined to explore the distribution, diversity, and expression of nifH genes. In surface waters, nifH DNA and cDNA sequences related to Trichodesmium, a diazotroph known to occur and fix N2 in the Arabian Sea, were detected. Proteobacterial nifH phylotypes (DNA but not cDNA) were also detected in surface waters. Proteobacterial nifH DNA and cDNA sequences, as well as nifH DNA and cDNA sequences related to strictly anaerobic N-fixers, were obtained from oxygen-deficient depths. This first report of nifH gene expression in subsurface low-oxygen waters suggests that there is potential for active N2 fixation by several phylogenetically and potentially metabolically diverse microorganisms in pelagic OMZs.
High-throughput sequencing (HTS) of DNA barcodes (metabarcoding), particularly when combined with... more High-throughput sequencing (HTS) of DNA barcodes (metabarcoding), particularly when combined with standardized sampling protocols, is one of the most promising approaches for censusing overlooked cryptic invertebrate communities. We present biodiversity estimates based on sequencing of the cytochrome c oxidase subunit 1 (COI) gene for coral reefs of the Gulf of Aqaba, a semi-enclosed system in the northern Red Sea. Samples were obtained from standardized sampling devices (Autonomous Reef Monitoring Structures (ARMS)) deployed for 18 months. DNA bar-coding of non-sessile specimens >2 mm revealed 83 OTUs in six phyla, of which only 25% matched a reference sequence in public databases. Metabarcoding of the 2 mm – 500 m and sessile bulk fractions revealed 1197 OTUs in 15 animal phyla, of which only 4.9% matched reference barcodes. These results highlight the scarcity of COI data for cryptoben-thic organisms of the Red Sea. Compared with data obtained using similar methods, our results suggest that Gulf of Aqaba reefs are less diverse than two Pacific coral reefs but much more diverse than an Atlantic oyster reef at a similar latitude. The standardized approaches used here show promise for establishing baseline data on biodiversity, monitoring the impacts of environmental change, and quantifying patterns of diversity at regional and global scales. Résumé : Le séquencage a haut debit (HTS) de barcodes ADN (metabarcoding), en particulier lorsqu'il est associé a un échantillonnage standardizé, est l'une des approaches les plus prometteuses pour recenser les communautés d'invertébrés cryptiques negligées jusqu'a ` aujourd'hui. Nous présentons des estimations de biodiversité basées sur le séquencage du gene cytochrome c oxidase subunit I (COI) pour les récifs coralliens du Golfe d'Aqaba, un système semi-fermé situé au nord de la mer Rouge. Les échantillons ont été obtenus a partir de dispositifs d'échantillonage standardizés (structures autonomes de suivi des récifs (ARMS)) deployés pendant 18 mois. L'analyse des barcodes ADN des organismes non-sessiles >2 mm révéla 83 OTUs appartenant a ` six embranche-ments, parmi lesquels seulement 25 % correspondaient a ` des barcodes de référence dans les bases de données publiques. Le metabarcoding des fractions 2 mm – 500 m et sessile révéla un total de 1197 OTUs dont des représentants de 15 embranchements animaux, parmi lesquels seulement 4.9 % correspondaient a ` des barcodes de référence. Ces résultats mettent en evidence le manque de données COI pour les organismes cryptobenthiques de la mer Rouge. Comparé a ` des données obtenues avec des méthodes similaires, nos résultats suggèrent que le Golfe d'Aqaba est moins diversifié que deux récifs coralliens du Pacifique mais beaucoup plus diversifié que des récifs d'huitre a ` la même latitude. Les approches standardizées utilisées dans cette étude sont très prometteuses pour la mise en place de points de comparaison de données de biodiversité, le suivi des changements environnementaux, et pour la quantification des patrons de diversité aux échelles régionales et globales. Mots-clés : codage a ` barres de l'ADN, métacodage a ` barres, COI, structures autonomes de suivi des récifs, Golfe d'Aqaba.
Biological dinitrogen (N 2) fixation (diazotrophy, BNF) relieves marine primary producers of nitr... more Biological dinitrogen (N 2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m-2 reef d-1 , with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m-2 reef d-1. Planktonic GPP and BNF rates were respectively approximately 60-and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.
The giant diatom Ethmodiscus was examined along an east–west transect at 28–301N during 2002 and ... more The giant diatom Ethmodiscus was examined along an east–west transect at 28–301N during 2002 and 2003 to determine if abundance, chemical composition or physiological status of this largest of diatoms varied on the scale of 100's–1000's of km in North Pacific gyre. Abundance ranged from o0.1–42.0 cells m À3 and supported the notion of an abundance mosaic reported previously. However, there was only minimal support for the relationship between abundance and nutrient concentration at 125 m reported previously. Cellular chlorophyll varied little along the transect (7.3–10.9 ng chl cell À1) except at the westernmost station. Cellular N and P quotas co-varied 3–4.5 fold (mean ¼ 50.873.7 and 3.770.8 nmol N and P cell À1) and yielded N:P ratios that closely clustered around the Redfield ratio (average ¼ 14.671.1). Only low levels of chlorophyll-normalized alkaline phosphatase (APase) activity were observed (0.4–2.5 nmol P mg chl À1 h À1) with APase activity lower than that in either the bulk water, or co-occurring Trichodesmium spp. and Pyrocystis noctiluca. The active fluorescence parameter F v :F m , a property sensitive to Fe stress, was uniformly high at all stations (average ¼ 0.7370.04 for 2003, and 0.6970.05 for 2002), indicating sufficient Fe for optimum photosynthetic competence. These results contrasted sharply with results from Rhizosolenia mats reported along the same transect where there was a significant decline westward in F v :F m. Both ferredoxin (Fd) and flavodoxin accumulated in cells of Ethmodiscus, resulting in Fd Index values ofo0.6. Iron cell quotas ranged from 0.7–5.1 pmol Fe cell À1. When normalized to cytoplasmic volume, the Fe mm À3 was comparable to that of Escherichia coli. We note that the disproportionate contribution of the vacuole (with its high organic content) to total volume typical of large diatoms is a potentially significant source of error in Fe:C ratios and suggest that Fe should be normalized to cytoplasmic volume whenever possible to permit valid intercomparisons between studies. The composition, F v :F m data and Fe:C ratio suggest a relatively uniform population experiencing little N, P or Fe stress. The uncoupling of the Fd Index from these measures is consistent with previous findings showing that the expression of flavodoxin can be characterized as an early stress response and that its accumulation is not necessarily correlated with physiological deficit. Ethmodiscus appears to be well adapted to some of the most oligotrophic waters in the ocean. Because it is an important sedimentary marker, the biology of living Ethmodiscus provides insights into the source of extensive ARTICLE IN PRESS
Potentially Toxic Substances (PTS) in soils are of increasingly growing concern worldwide. Heavy ... more Potentially Toxic Substances (PTS) in soils are of increasingly growing concern worldwide. Heavy metals are acting as one of the most serious groups of environmental contaminants, and their release into the environment has strongly increased over the last decades. Heavy metals can cause acute and long-term toxic effects on both human health and the ecosystems around. Toxic effects of heavy metals reach soil biota in general and affect the microbial community biomass and metabolic activities related to such communities. Although all members of the soil biota respond relatively to soil pollution, microbial communities are considered to be the first and most swift responders to such environmental pollutants. This study focused on the state of the art of developing a consortium of different enzymes and how their collective activities could be used for the assessment and monitoring of soil in response to heavy metal pollution. By measuring microbial community biomass and activity from so...
Biological dinitrogen (N2) fixation (diazotrophy,
BNF) relieves marine primary producers of nitro... more Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, andNis therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m-2 reef d-1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m-2 reef d-1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.
Functional traits define species by their ecological role in the ecosystem. Animals themselves ar... more Functional traits define species by their ecological role in the ecosystem. Animals themselves are host–microbe ecosystems (holobionts), and the application of ecophysiological approaches can help to understand their functioning. In hard coral holobionts, communities of dinitrogen (N2)-fixing prokaryotes (diazotrophs) may contribute a functional trait by providing bioavailable nitrogen (N) that could sustain coral productivity under oligotrophic conditions. This study quantified N2 fixation by diazotrophs associated with four genera of hermatypic corals on a northern Red Sea fringing reef exposed to high seasonality. We found N2 fixation activity to be 5- to 10-fold higher in summer, when inorganic nutrient concentrations were lowest and water temperature and light availability highest. Concurrently, coral gross primary productivity remained stable despite lower Symbiodinium densities and tissue chlorophyll a contents. In contrast, chlorophyll a content per Symbiodinium cell increased from spring to summer, suggesting that algal cells overcame limitation of N, an essential element for chlorophyll synthesis. In fact, N2 fixation was positively correlated with coral productivity in summer, when its contribution was estimated to meet 11% of the Symbiodinium N requirements. These results provide evidence of an important functional role of diazotrophs in sustaining coral productivity when alternative external N sources are scarce.
Dinitrogen (N2) fixation by specialized prokaryotes (diazotrophs) represents an important source ... more Dinitrogen (N2) fixation by specialized prokaryotes (diazotrophs) represents an important source of bioavailable nitrogen (N) in the ocean. In coral reefs, several substrates and organisms are associated with diazotrophs, but potential N2 fixation activity by zooxanthellate soft corals has not yet been investigated. Such soft corals may contribute importantly to the input of new N into the reef ecosystem, as they can cover substantial benthic areas in today's coral reefs. Therefore, this study investigated N2 fixation of 2 dominant zooxanthellate soft coral groups (Sarcophyton sp. and Xeniidae) in a northern Red Sea fringing reef during all 4 seasons of 1 yr. This was supplemented by respirometry incubations and in situ monitoring of key environmental parameters. Findings revealed detectable N2 fixation for both soft corals during all seasons. Annual N2 fixation by Sarcophyton sp. was 3 times higher than that by Xeniidae, but both soft corals exhibited similar seasonal patterns. N2 fixation significantly increased during summer, when water temperature and light intensity were highest and inorganic nutrient availability was lowest. Coral respiration also peaked during summer and was positively correlated to N2 fixation, while photosynthesis revealed maximum rates during the nutrient-enriched spring season. Given the importance of N for reproduction and growth, N2 fixation may be a key component of soft coral nutrition during summer, when inorganic nutrient availability in the water column is lowest and likely not sufficient to sustain the high metabolic demand of soft corals.
Permeable sediments are highly bioactive compartments in coral reefs. The associated dense microb... more Permeable sediments are highly bioactive compartments in coral reefs. The associated dense microbial communities sustain fast degradation of organic matter, thereby playing a key role in nutrient recycling within the reef. Besides nutrient recycling, new nutrients (i.e. nitrogen) are acquired by dinitrogen (N2) fixing microbial communities, but knowledge about the influence of sand mineralogy and key environmental factors on this process is scarce. Therefore, this study quantified seasonal N2 fixation (via acetylene reduction) along with gross photosynthesis (via O2 fluxes) by adjacent carbonate and silicate sands in a Northern Red Sea coral reef. Findings revealed significantly higher N2 fixation in carbonate than in silicate sands (2.88 and 1.52 nmol C2H4 cm-2 h-1, respectively) and a more pronounced seasonal response in the former, likely caused by its higher permeability, grain size and microbial abundance. Ambient light and organic matter availability were the main controlling environmental factors for sand-associated N2 fixation. Carbonate and silicate sands showed similar gross photosynthesis rates (270 and 233 nmol O2 cm-2 h-1) that positively (carbonate sands) or negatively (silicate sands) correlated with N2 fixation, likely due to different diazotrophic communities. Seasonal appearance of microbial mats on carbonate sands increased N2 fixation and gross photosynthesis by up to one order of mag nitude. On an annual average, carbonate and silicate sands obtain ∼8% and microbial mat communities obtain ∼13% of their photo-metabolic N demand via N2 fixation.
The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is r... more The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is responsible for up to half of the global mesopelagic fixed nitrogen (N) loss from the ocean via denitrification and anammox. Dinitrogen (N2) fixation is usually attributed to cyanobacteria in the surface ocean. Model prediction and physiological inhibition of N2 fixation by oxygen, however, suggest that N2 fixation should be enhanced near the oxygen-deficient zone (ODZ) of the Arabian Sea. N2 fixation and cyanobacterial nifH genes (the gene encoding dinitrogenase reductase) have been reported in surface waters overlying the Arabian Sea ODZ. Here, water samples from depths above and within the Arabian Sea ODZ were examined to explore the distribution, diversity, and expression of nifH genes. In surface waters, nifH DNA and cDNA sequences related to Trichodesmium, a diazotroph known to occur and fix N2 in the Arabian Sea, were detected. Proteobacterial nifH phylotypes (DNA but not cDNA) were also detected in surface waters. Proteobacterial nifH DNA and cDNA sequences, as well as nifH DNA and cDNA sequences related to strictly anaerobic N-fixers, were obtained from oxygen-deficient depths. This first report of nifH gene expression in subsurface low-oxygen waters suggests that there is potential for active N2 fixation by several phylogenetically and potentially metabolically diverse microorganisms in pelagic OMZs.
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Papers by Mamoon Al-Rshaidat
BNF) relieves marine primary producers of nitrogen
(N) limitation in a large part of the world oceans. N
concentrations are particularly low in tropical regions
where coral reefs are located, andNis therefore
a key limiting nutrient for these productive ecosystems.
In this context, the importance of diazotrophy
for reef productivity is still not resolved, with studies
up to now lacking organismal and seasonal resolution.
Here, we present a budget of gross primary
production (GPP) and BNF for a highly seasonal Red
Sea fringing reef, based on ecophysiological and
benthic cover measurements combined with
geospatial analyses. Benthic GPP varied from 215 to
262 mmol C m-2 reef d-1, with hard corals making
the largest contribution (41–76%). Diazotrophy was
omnipresent in space and time, and benthic BNF
varied from 0.16 to 0.92 mmol N m-2 reef d-1.
Planktonic GPP and BNF rates were respectively
approximately 60- and 20-fold lower than those of
the benthos, emphasizing the importance of the
benthic compartment in reef biogeochemical cycling.
BNF showed higher sensitivity to seasonality
than GPP, implying greater climatic control on reef
BNF. Up to about 20% of net reef primary production
could be supported by BNF during summer,
suggesting a strong biogeochemical coupling between
diazotrophy and the reef carbon cycle.
BNF) relieves marine primary producers of nitrogen
(N) limitation in a large part of the world oceans. N
concentrations are particularly low in tropical regions
where coral reefs are located, andNis therefore
a key limiting nutrient for these productive ecosystems.
In this context, the importance of diazotrophy
for reef productivity is still not resolved, with studies
up to now lacking organismal and seasonal resolution.
Here, we present a budget of gross primary
production (GPP) and BNF for a highly seasonal Red
Sea fringing reef, based on ecophysiological and
benthic cover measurements combined with
geospatial analyses. Benthic GPP varied from 215 to
262 mmol C m-2 reef d-1, with hard corals making
the largest contribution (41–76%). Diazotrophy was
omnipresent in space and time, and benthic BNF
varied from 0.16 to 0.92 mmol N m-2 reef d-1.
Planktonic GPP and BNF rates were respectively
approximately 60- and 20-fold lower than those of
the benthos, emphasizing the importance of the
benthic compartment in reef biogeochemical cycling.
BNF showed higher sensitivity to seasonality
than GPP, implying greater climatic control on reef
BNF. Up to about 20% of net reef primary production
could be supported by BNF during summer,
suggesting a strong biogeochemical coupling between
diazotrophy and the reef carbon cycle.