Culture-Independent Characterization of Bacterial Communities Associated with the Cold-Water Coral Lophelia pertusa in the Northeastern Gulf of Mexico

Bacteria are recognized as an important part of the total biology of shallow-water corals. Studies of shallow-water corals suggest that associated bacteria may benefit the corals by cycling carbon, fixing nitrogen, chelating iron, and producing antibiotics that protect the coral from other microbes. Cold-water or deep-sea corals have a fundamentally different ecology due to their adaptation to cold, dark, high-pressure environments and as such have novel microbiota. The goal of this study was to characterize the microbial associates of Lophelia pertusa in the northeastern Gulf of Mexico. This is the first study to collect the coral samples in individual insulated containers and to preserve coral samples at depth in an effort to minimize thermal shock and evaluate the effects of environmental gradients on the microbial diversity of samples. Molecular analysis of bacterial diversity showed a marked difference between the two study sites, Viosca Knoll 906/862 (VK906/862) and Viosca Knoll 826 (VK826). The bacterial communities from VK826 were dominated by a variety of unknown mycoplasmal members of the Tenericutes and Bacteroidetes, whereas the libraries from VK906/862 were dominated by members of the Proteobacteria. In addition to novel sequences, the 16S rRNA gene clone libraries revealed many bacterial sequences in common between Gulf of Mexico Lophelia corals and Norwegian fjord Lophelia corals, as well as shallow-water corals. Two Lophelia-specific bacterial groups were identified: a cluster of gammaproteobacteria related to sulfide-oxidizing gill symbionts of seep clams and a group of Mycoplasma spp. The presence of these groups in both Gulf and Norwegian Lophelia corals indicates that in spite of the geographic heterogeneity observed in Lophelia-associated bacterial communities, there are Lophelia-specific microbes.Cold-water and deep-sea corals have become a topic of interest due to conservation concerns over the impacts of trawling, exploration for oil and gas, and climate change (51, 52). Although the existence of these corals has been known since the 1800s, our knowledge of their distribution, ecology, and biology is limited due to the technical difficulties of studying them. Lophelia pertusa is a globally distributed cold-water scleractinian coral (53). In the Gulf of Mexico, Lophelia reefs occur primarily along the continental shelf break (300- to 500-m depth), providing an important complex habitat for a wide variety of fishes, crustaceans, and other invertebrates living below the photic zone (48).The microbial ecology of cold-water corals in deep water is fundamentally different from that of shallow-water corals due to the ambient environmental parameters (e.g., darkness, low temperature, and increased pressure) and the absence of symbiotic zooxanthellae. A few studies have begun to address the microbial associates of deep-sea corals, focusing on octocorals (9, 44) and on L. pertusa (27, 41, 42, 57, 72). To date, all the Lophelia studies have been conducted on the eastern side of the Atlantic: the Mediterranean basin (72), Mingulay Bay, Scotland (27), and Norwegian fjords (41, 42, 57). These studies have confirmed that the Lophelia-associated bacterial community is distinct from that of the surrounding seawater and sediments (27, 42, 57, 72). A variety of community profile methods (automated rRNA intergenic spacer analysis, terminal restriction fragment length polymorphism, and denaturing gradient gel electrophoresis [DGGE]) were used to demonstrate differences between samples within a geographic area, suggesting that the Lophelia-associated microbial community varies depending on regional environmental factors (27, 42, 57). Sequencing of 16S rRNA genes was done in only two studies, and there was no overlap between their data (42, 72). However, different methods of collection, extraction, amplification, and sequencing were employed, so the lack of commonality may be due to methodology rather than biogeography.Methodology is a concern, particularly the care with which samples need to be collected for microbial ecology studies. Deep-sea coral samples are typically collected by a trawl, net, or dredge or by a submersible/remotely operated vehicle (ROV). With these methods, many corals may be combined in a single container, which is not acceptable for microbiological studies because the microbial community of one coral could contaminate that of the other. Similarly, contact with sediment, other invertebrates, mobile fauna, or water masses between the collection point and the surface could contaminate the coral samples. Unlike the case with the northeastern Atlantic and Norwegian fjords, the temperature and salinity gradients in the Gulf of Mexico during the warm months of the year can be considerable. In the case of the Viosca Knoll sites, the bottom temperature was 8 to 11°C, compared to a surface temperature of ≥30°C. Coral samples collected in uninsulated containers in this area have been observed to be affected (e.g., polyps retracted and copious stress mucus production) compared to those in insulated containers. Viosca Knoll is also impacted by the Mississippi River plume. The surface waters at these sites were turbid and green and had a salinity of 30 practical salinity units (psu), but below the plume the waters were clear and had a salinity of 35 psu. With this in mind, we designed a sampling container that would protect the coral samples from dramatic changes in temperature and salinity by sealing them in individual insulated compartments (see Fig. S1 in the supplemental material). However, the question remained whether environmental gradients in light and pressure would have an effect on the microbial diversity of the samples. To address this question, each sample was collected in duplicate: one piece was sealed in a compartment alive, and a replicate piece was sealed in another compartment and preserved at depth with a fixative solution. Both sample types (“live” versus “fixed”) were sealed and insulated, so temperature and salinity gradients did not affect them; live samples were subject to gradients in light and pressure, while fixed samples were not.The main objective of this study was to characterize the bacterial associates of Lophelia pertusa from two sites in the northern Gulf of Mexico. Comparing multiple individual colonies from two geographic locations in the Gulf to each other and to bacterial data from Lophelia samples on the eastern side of the Atlantic will clarify whether Lophelia has a species-specific bacterial community, as has been described for shallow-water corals (49, 55). The results of this study will also better define the total microbial diversity associated with this cold-water coral. A specialized sampling device (see Fig. S1 in the supplemental material) was designed to minimize contamination and thermal shock and to allow the introduction of preservative at depth to determine if environmental gradients were affecting microbial diversity during sampling.     

Phenotype-specific bacterial communities in the cold-water coral Lophelia pertusa (Scleractinia) and their implications for the coral's nutrition, health, and distribution

The pseudocolonial coral Lophelia pertusa (Scleractinia, Caryophylliidae) is a eurybathic, stenothermal cosmopolitan cold-water species. It occurs in two color varieties, white and red. L. pertusa builds vast cold-water coral reefs along the continental margins, which are among the most diverse deep-sea habitats. Microbiology of L. pertusa has been in scientific focus for only a few years, but the question of whether the coral holds a host-specific bacterial community has not been finally answered. Bacteria on coral samples from the Trondheimsfjord (Norway) were characterized by the culture-independent 16S rRNA gene-based techniques terminal restriction fragment length polymorphism and sequence analysis. L. pertusa revealed a high microbial richness. Clone sequences were dominated by members of the Alpha- and Gammaproteobacteria. Other abundant taxa were Bacteroidetes, Actinobacteria, Verrucomicrobia, Firmicutes, and Planctomycetes. The bacterial community of L. pertusa not only differed conspicuously from that of the environment but also varied with both the location and color variety of its host. Therefore, the microbial colonization cannot be termed "specific" sensu stricto. However, similarities to other coral-bacterium associations suggest the existence of "cold-water coral-specific" bacterial groups sensu lato. L. pertusa-associated bacteria appear to play a significant role in the nutrition of their host by degradation of sulfur compounds, cellulose, chitin, and end products of the coral's anaerobic metabolism. Some coral-associated microbes were regarded as opportunistic pathogens. Dominance of mixotrophic members of the Rhodobacteraceae in white L. pertusa could explain the wider dispersal of this phenotype by supplementary nutrition.     

Mitogenome rearrangement in the cold-water scleractinian coral Lophelia pertusa (Cnidaria, Anthozoa) involves a long-term evolving group I intron

Group I introns are genetic insertion elements that invade host genomes in a wide range of organisms. In metazoans, however, group I introns are extremely rare, so far only identified within mitogenomes of hexacorals and some sponges. We sequenced the complete mitogenome of the cold-water scleractinian coral Lophelia pertusa, the dominating deep sea reef-building coral species in the North Atlantic Ocean. The mitogenome (16,150 bp) has the same gene content but organized in a unique gene order compared to that of other known scleractinian corals. A complex group I intron (6460 bp) inserted in the ND5 gene (position 717) was found to host seven essential mitochondrial protein genes and one ribosomal RNA gene. Phylogenetic analysis supports a vertical inheritance pattern of the ND5-717 intron among hexacoral mitogenomes with no examples of intron loss. Structural assessments of the Lophelia intron revealed an unusual organization that lacks the universally conserved ωG at the 3′ end, as well as a highly compact RNA core structure with overlapping ribozyme and protein coding capacities. Based on phylogenetic and structural analyses we reconstructed the evolutionary history of ND5-717, from its ancestral protist origin, through intron loss in some early metazoan lineages, and into a compulsory feature with functional implications in hexacorals.     

Partitioning of Bacterial Communities between Seawater and Healthy, Black Band Diseased, and Dead Coral Surfaces

Distinct partitioning has been observed in the composition and diversity of bacterial communities inhabiting the surface and overlying seawater of three coral species infected with black band disease (BBD) on the southern Caribbean island of Curaçao, Netherlands Antilles. PCR amplification and sequencing of bacterial 16S rRNA genes (rDNA) with universally conserved primers have identified over 524 unique bacterial sequences affiliated with 12 bacterial divisions. The molecular sequences exhibited less than 5% similarity in bacterial community composition between seawater and the healthy, black band diseased, and dead coral surfaces. The BBD bacterial mat rapidly migrates across and kills the coral tissue. Clone libraries constructed from the BBD mat were comprised of eight bacterial divisions and 13% unknowns. Several sequences representing bacteria previously found in other marine and terrestrial organisms (including humans) were isolated from the infected coral surfaces, including Clostridium spp., Arcobacter spp., Campylobacter spp., Cytophaga fermentans, Cytophaga columnaris, and Trichodesmium tenue.     

Observations of Lophelia pertusa and the surficial geology at a deep-water site in the northeastern Gulf of Mexico

Authigenic carbonate, precipitated in conjunction with biogeochemical activity associated with hydrocarbon and related fluid seepage, provides exposed and buried hard substrate on the crest and flanks of a low-relief mound located on the upper De Soto Slope in the northeastern Gulf of Mexico. Lophelia pertusa has successfully colonized some of this carbonate material. Individual colonies range in size from a few centimeters to over 1.5 m in diameter while aggregations of closely associated colonies with linear orientations were observed to attain 1.5–2 m in height and width and 3–4 m in length. Many of the aggregated colonies appear to be in the first phase of the `thicket' building stage described by Squires (1964). Colonies less than 50–75 cm in diameter were nearly always completely pure white. Larger colonies and the aggregated colonies are often light to dark brown in coloration at their base and center with many having only white terminal branches and some with no white corallum at all.     

Characterization of 13 microsatellite loci for the deep-sea coral, Lophelia pertusa (Linnaeus 1758), from the western North Atlantic Ocean and Gulf of Mexico

A suite of 13 polymorphic tri- and tetranucleotide microsatellite loci were isolated from the ahermatypic deep-sea coral, Lophelia pertusa. Among 51 individuals collected from three disjunct oceanic regions, allelic diversity ranged from six to 38 alleles and averaged 9.1 alleles per locus. Observed heterozygosity ranged from 9.1 to 96.8% and averaged 62.3% in the Gulf of Mexico population. For some loci, amplification success varied among collections, suggesting regional variation in priming site sequences. Four loci showed departures from Hardy-Weinberg equilibrium in certain collections which may reflect nonrandom mating.     

Mapping Cold-Water Coral Habitats at Different Scales within the Northern Ionian Sea (Central Mediterranean): An Assessment of Coral Coverage and Associated Vulnerability

In this study, we mapped the distribution of Cold-Water Coral (CWC) habitats on the northern Ionian Margin (Mediterranean Sea), with an emphasis on assessing coral coverage at various spatial scales over an area of 2,000 km² between 120 and 1,400 m of water depth. Our work made use of a set of data obtained from ship-based research surveys. Multi-scale seafloor mapping data, video inspections, and previous results from sediment samples were integrated and analyzed using Geographic Information System (GIS)-based tools. Results obtained from the application of spatial and textural analytical techniques to acoustic meso-scale maps (i.e. a Digital Terrain Model (DTM) of the seafloor at a 40 m grid cell size and associated terrain parameters) and large-scale maps (i.e. Side-Scan Sonar (SSS) mosaics of 1 m in resolution ground-truthed using underwater video observations) were integrated and revealed that, at the meso-scale level, the main morphological pattern (i.e. the aggregation of mound-like features) associated with CWC habitat occurrences was widespread over a total area of 600 km². Single coral mounds were isolated from the DTM and represented the geomorphic proxies used to model coral distributions within the investigated area. Coral mounds spanned a total area of 68 km² where different coral facies (characterized using video analyses and mapped on SSS mosaics) represent the dominant macro-habitat. We also mapped and classified anthropogenic threats that were identifiable within the examined videos, and, here, discuss their relationship to the mapped distribution of coral habitats and mounds. The combined results (from multi-scale habitat mapping and observations of the distribution of anthropogenic threats) provide the first quantitative assessment of CWC coverage for a Mediterranean province and document the relevant role of seafloor geomorphology in influencing habitat vulnerability to different types of human pressures.     

Effects of Plant Biomass, Plant Diversity, and Water Content on Bacterial Communities in Soil Lysimeters: Implications for the Determinants of Bacterial Diversity

Soils may comprise tens of thousands to millions of bacterial species. It is still unclear whether this high level of diversity is governed by functional redundancy or by a multitude of ecological niches. In order to address this question, we analyzed the reproducibility of bacterial community composition after different experimental manipulations. Soil lysimeters were planted with four different types of plant communities, and the water content was adjusted. Group-specific phylogenetic fingerprinting by PCR-denaturing gradient gel electrophoresis revealed clear differences in the composition of Alphaproteobacteria, Betaproteobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, and Verrucomicrobia populations in soils without plants compared to that of populations in planted soils, whereas no influence of plant species composition on bacterial diversity could be discerned. These results indicate that the presence of higher plant species affects the species composition of bacterial groups in a reproducible manner and even outside of the rhizosphere. In contrast, the environmental factors tested did not affect the composition of Acidobacteria, Actinobacteria, Archaea, and Firmicutes populations. One-third (52 out of 160) of the sequence types were found to be specifically and reproducibly associated with the absence or presence of plants. Unexpectedly, this was also true for numerous minor constituents of the soil bacterial assemblage. Subsequently, one of the low-abundance phylotypes (beta10) was selected for studying the interdependence under particular experimental conditions and the underlying causes in more detail. This so-far-uncultured phylotype of the Betaproteobacteria species represented up to 0.18% of all bacterial cells in planted lysimeters compared to 0.017% in unplanted systems. A cultured representative of this phylotype exhibited high physiological flexibility and was capable of utilizing major constituents of root exudates. Our results suggest that the bacterial species composition in soil is determined to a significant extent by abiotic and biotic factors, rather than by mere chance, thereby reflecting a multitude of distinct ecological niches.     

Complexity of Bacterial Communities in a River-Floodplain System (Danube, Austria)

Natural floodplains play an essential role in the processing and decomposition of organic matter and in the self-purification ability of rivers, largely due to the activity of bacteria. Knowledge about the composition of bacterial communities and its impact on organic-matter cycling is crucial for the understanding of ecological processes in river-floodplain systems. Particle-associated and free-living bacterial assemblages from the Danube River and various floodplain pools with different hydrological characteristics were investigated using terminal restriction fragment length polymorphism analysis. The particle-associated bacterial community exhibited a higher number of operational taxonomic units (OTUs) and was more heterogeneous in time and space than the free-living community. The temporal dynamics of the community structure were generally higher in isolated floodplain pools. The community structures of the river and the various floodplain pools, as well as those of the particle-associated and free-living bacteria, differed significantly. The compositional dynamics of the planktonic bacterial communities were related to changes in the algal biomass, temperature, and concentrations of organic and inorganic nutrients. The OTU richness of the free-living community was correlated with the concentration and origin of organic matter and the concentration of inorganic nutrients, while no correlation with the OTU richness of the particle-associated assemblage was found. Our results demonstrate the importance of the river-floodplain interactions and the influence of damming and regulation on the bacterial-community composition.     

东亚粗叶木属(茜草科)植物的分布及其生物地理意义

茜草科粗叶木属植物是亚洲热带原始林下优势地位明显的一类灌木植物.依据标本资料和分类学修订,研究了东亚产粗叶木属植物33个种的地理分布式样,并将其划分为热带亚洲、东亚和中国特有3个分布区类型,其中热带亚洲分布型可以进一步划分为印度(喜马拉雅)至马来西亚分布、印度(喜马拉雅)至中国南部和大陆东南亚分布及中国南部至大陆东南亚分布3个亚型.中国粗叶木属植物中热带亚洲分布型占总种数的72.7%,显示了中国热带地区植物区系的热带亚洲亲缘.一些粗叶木属植物种类的分布式样暗示了中国-日本、中国-喜马拉雅森林植物区系的分区及物种形成,喜马拉雅(横断山)-台湾山地植物区系的联系及台湾-琉球-日本物种迁移通道.海南、台湾植物区系缺少特有种反映了它们的植物区系大陆性很强. 粗叶木属植物种类的分布式样对中国热带植物种分布区类型的划分提供了参考.     

Electrophoretic Polymorphisms and Their Taxonomic Implications in Callitrichini (Primates, Platyrrhini)

Five hundred forty-three blood samples from 15populations of the four genera of callitrichin primateswere studied electrophoretically. Polymorphism andgenetic distances were estimated for 20 loci, 13 of which were polymorphic. The lion tamarin(Leontopithecus) studied here exhibited theleast variability for these loci, while the monospecificCebuella showed the most. The genetic distancesobserved between Callithrix andCebuella genera support previous evidenceindicating a close taxonomic relationship between them.Genetic distance values obtained in this study alsosupport the synonimization of the kuhli form with Callithrix jacchuspenicillata.     

Temporal Dynamics of South End Tidal Creek (Sapelo Island, Georgia) Bacterial Communities

Bacterial community dynamics in South End tidal creek, Sapelo Island, GA, were studied over a 74-h, five-tidal-cycle period. Observations were made hourly for the first consecutive 24 hours, every 3 hours on the second day, and every 6 hours on the third day. Tide most strongly influenced bacterial community composition (high-tide versus low-tide community analysis of similarities, R = 0.41, P < 0.03). Dissolved oxygen concentration and conductivity were important proximate drivers. However, after accounting for tide and environmental variables colinear with tide, cumulative time became more important in describing community variation. In-stream physical processes, including particulate suspension and sedimentation, may explain tide-associated trends in the bacterial community composition observed.     

Moonmilk Deposits Originate from Specific Bacterial Communities in Altamira Cave (Spain)

The influence of bacterial communities on the formation of carbonate deposits such as moonmilk was investigated in Altamira Cave (Spain). The study focuses on the relationship between the bacterial communities at moonmilk deposits and those forming white colonizations, which develop sporadically throughout the cave. Using molecular fingerprinting of the metabolically active bacterial communities detected through RNA analyses, the development of white colonizations and moonmilk deposits showed similar bacterial profiles. White colonizations were able to raise the pH as a result of their metabolism (reaching in situ pH values above 8.5), which was proportional to the nutrient supply. Bacterial activity was analyzed by nanorespirometry showing higher metabolic activity from bacterial colonizations than uncolonized areas. Once carbonate deposits were formed, bacterial activity decreased drastically (down to 5.7% of the white colonization activity). This study reports on a specific type of bacterial community leading to moonmilk deposit formation in a cave environment as a result of bacterial metabolism. The consequence of this process is a macroscopic phenomenon of visible carbonate depositions and accumulation in cave environments.     

Demography and Population Dynamics of Massive Coral Communities in Adjacent High Latitude Regions (United Arab Emirates)

Individual massive coral colonies, primarily faviids and poritids, from three distinct assemblages within the southeastern Arabian Gulf and northwestern Gulf of Oman (United Arab Emirates) were studied from 2006–2009. Annual photographic censuses of approximately 2000 colonies were used to describe the demographics (size class frequencies, abundance, area cover) and population dynamics under “normal” environmental conditions. Size class transitions included growth, which occurred in 10–20% of the colonies, followed in decending order by partial mortality (3–16%), colony fission (<5%) and ramet fusion (<3%). Recruitment and whole colony mortality rates were low (<0.7 colonies/m²) with minimal interannual variation. Transition matrices indicated that the Arabian Gulf assemblages have declining growth rates (λ<1) whereas the massive coral population is stable (λ = 1) in the Gulf of Oman. Projection models indicated that (i) the Arabian Gulf population and area cover declines would be exacerbated under 10-year and 16-year disturbance scenarios as the vital rates do not allow for recovery to pre-disturbance levels during these timeframes, and (ii) the Gulf of Oman assemblage could return to its pre-disturbance area cover but its overall population size would not fully recover under the same scenarios.     

Spatial Distribution of Bacterial Communities and Phenanthrene Degradation in the Rhizosphere of Lolium perenne L.

Rhizodegradation of organic pollutants, such as polycyclic aromatic hydrocarbons, is based on the effect of root-produced compounds, known as exudates. These exudates constitute an important and constant carbon source that selects microbial populations in the plant rhizosphere, modifying global as well as specific microbial activities. We conducted an experiment in two-compartment devices to show the selection of bacterial communities by root exudates and phenanthrene as a function of distance to roots. Using direct DNA extraction, PCR amplification, and thermal gradient gel electrophoresis screening, bacterial population profiles were analyzed in parallel to bacterial counts and quantification of phenanthrene biodegradation in three layers (0 to 3, 3 to 6, and 6 to 9 mm from root mat) of unplanted-polluted (phenanthrene), planted-polluted, and planted-unpolluted treatments. Bacterial community differed as a function of the distance to roots, in both the presence and the absence of phenanthrene. In the planted and polluted treatment, biodegradation rates showed a strong gradient with higher values near the roots. In the nonplanted treatment, bacterial communities were comparable in the three layers and phenanthrene biodegradation was high. Surprisingly, no biodegradation was detected in the section of planted polluted treatment farthest from the roots, where the bacterial community structure was similar to those of the nonplanted treatment. We conclude that root exudates and phenanthrene induce modifications of bacterial communities in polluted environments and spatially modify the activity of degrading bacteria.     

Nutrients and Other Abiotic Factors Affecting Bacterial Communities in an Ohio River (USA)

Nitrogen and phosphorus additions from anthropogenic sources can alter the nutrient pool of aquatic systems, both through increased nutrient concentrations and changes in stoichiometry. Because bacteria are important in nutrient cycling and aquatic food webs, information about how nutrients affect bacterial communities enhances our understanding of how changes in nutrient concentrations and stoichiometry potentially affect aquatic ecosystems as a whole. In this study, bacterial communities were examined in biofilms from cobbles collected across seasons at three sites along the Mahoning River (Ohio) with differing levels of inorganic nutrient inputs. Members of the alpha-, beta-, and gamma-proteobacteria, the Cytophaga–Flavobacteria cluster, and the Domain Bacteria were enumerated using fluorescent in situ hybridization. Detrended canonical correspondence analysis (DCCA) revealed that stoichiometric ratios, especially the dissolved inorganic nitrogen (DIN):soluble reactive phosphorus (SRP) molar ratio (NO₂/NO₃ + NH₄:soluble reactive phosphorus), were correlated with abundance of the various bacterial taxa. However, the patterns were complicated by correlations with single nutrient concentrations and seasonal changes in temperature. Seasonal cycles appeared to play an important role in structuring the community, as there were distinct winter communities and temperature was negatively correlated with abundance of both alpha-proteobacteria and Cytophaga–Flavobacteria. However, nutrients and stoichiometry also appeared to affect the community. Numbers of cells hybridizing the Domain Bacteria probe were correlated with the DOC:DIN ratio, the beta-proteobacteria had a negative correlation with soluble reactive phosphorus concentrations and a positive correlation with the DIN:SRP ratio, and the Cytophaga–Flavobacteria had a significant negative partial correlation with the DIN:SRP ratio. Abundances of the alpha- or gamma-proteobacteria were not directly correlated to nutrient concentrations or stoichiometry. It appears that nutrient stoichiometry may be an important factor structuring bacterial communities; however, it is one of many factors, such as temperature, that are interlinked and must be considered together when studying environmental bacteria.     

A Molecular Biomarker System for Assessing the Health of Coral (Montastraea faveolata) During Heat Stress

Using a novel molecular biomaker system (MBS), we assessed the physiological status of coral (Montastraea faveolata) challenged by heat stress by assaying specific cellular and molecular parameters. This technology is particularly relevant for corals because heat stress is thought to be an essential component of coral bleaching. This phenomenon is widely believed to be responsible for coral mortality worldwide, particularly during 1997–1998. Specific parameters of coral cellular physiology were assayed using the MBS that are indicative of a nonstressed or stressed condition. The MBS distinguished the separate and combined effects of heat and light on the 2 coral symbionts, a scleractinian coral and a dinoflagellate algae (zooxanthellae). This technology aids in the accurate diagnosis of coral condition because each parameter is physiologically well understood. Finally, the MBS technology is relatively inexpensive, easy to implement, and precise, and it can be quickly adapted to a high-throughout robotic system for mass sample analysis. Accepted May 25, 2000.     

Enzymatic Activity, Bacterial Distribution, and Organic Matter Composition in Sediments of the Ross Sea (Antarctica)

Enzymatic activities of aminopeptidase and β-glucosidase were investigated in Antarctic Ross Sea sediments at two sites (sites B and C, 567 and 439 m deep, respectively). The sites differed in trophic conditions related to organic matter (OM) composition and bacterial distribution. Carbohydrate concentrations at site B were about double those at site C, while protein and lipid levels were 10 times higher. Proteins were mainly found in a soluble fraction (>90%). Chloropigment content was generally low and phaeopigments were almost absent, indicating the presence of reduced inputs of primary organic matter. ATP concentrations (as a measure of the living microbial biomass) were significantly higher at site B. By contrast, benthic bacterial densities at site C were about double those at site B. Bacterial parameters do not appear to be “bottom-up controlled” by the amount of available food but rather “top-down controlled” by meiofauna predatory pressure, which was significantly higher at site B. Aminopeptidase and β-glucosidase extracellular enzyme activities (EEA) in Antarctic sediments appear to be high and comparable to those reported for temperate or Arctic sediments and characterized by low aminopeptidase/β-glucosidase ratios (about 10). Activity profiles showed decreasing patterns with increasing sediment depth, indicating vertical shifts in both availability and nutritional quality of degradable OM. Vertical profiles of aminopeptidase activity were related to a decrease in protein concentration and/or to an increase in the insoluble refractory proteinaceous fraction. The highest aminopeptidase activity rates were observed at site C, characterized by much lower protein concentrations. Differences in EEA between sites do not seem to be explained by differences in the in situ temperature (−1.6 and −0.8°C at sites B and C, respectively). Aminopeptidase activity profiles are consistent with the bacterial biomass and frequency of dividing cells. Enzyme substrate affinity was generally dependent upon substrate concentrations. EEA, normalized to bacterial numbers, indicated specific activities comparable to those reported for equally deep sediments at temperate latitudes. Vertical patterns of specific enzymatic activity appeared to be controlled by chloroplastic pigment concentrations that accumulate in the deeper sediment layers. The overall conclusion from the analysis of EEA in Antarctic sediments is that enzyme-dependent transformations of OM proceed at rates similar to those measured in temperate environments. Protein carbon potentially liberated by aminopeptidase activities (12.597 to 26.190 mg of C m⁻² day⁻¹) indicates that the whole protein pool could be mobilized within 1.3 to 17 h. Carbohydrate carbon mobilization (773 to 2,552 mg of C m⁻² day⁻¹) is sufficient to turn over the carbohydrate pool within 16 to 20 h. Such rates are 6 to 45 times higher than fluxes of particulate organic proteins and carbohydrates, indicating an “uncoupled hydrolysis” by the Antarctic benthic assemblages, in which bacteria appear to be able to rapidly exploit episodic OM pulses.