Sea Marine Research Articles

Streptomyces sp. K47 alkaline proteases: partial purification and analysis by zymography

Microbial enzymes are used as organic catalysts in different industrial processes. In this study, we aimed to produce and investigate alkaline proteases from a novel actinobacterium strain isolated from a Black Sea marine sediment. The optimal production conditions for Streptomyces sp. K47 alkaline proteases was 4-days incubation at 28ºC in a salt-free medium buffered with 50 mM Tris-HCl buffer (pH 9.0) and containing glucose (1.0%, w v-1) and yeast extract (0.5%, w v-1). The enzyme solution was partially purified using (NH4)2SO4 precipitation (40-70%). After desalting, it was purified 1-84 fold with a recovery of 19.42%. Zymogram analyses revealed the presence of more than one protease enzyme. The enzyme solution exhibited maximum activity at pH 9.0 and 37ºC, remaining stable after a 2-hour incubation at all tested conditions. Streptomyces sp. K47 has the potential to be used in industrial processes because of its ability to produce multiple protease enzymes displaying stability in a broad pH and temperature range.

Barents Sea plankton production and controlling factors in a fluctuating climate

AbstractThe Barents Sea and its marine ecosystem is exposed to many different processes related to the seasonal light variability, formation and melting of sea-ice, wind-induced mixing, and exchange of heat and nutrients with neighbouring ocean regions. A global model for the RCP4.5 scenario was downscaled, evaluated, and combined with a biophysical model to study how future variability and trends in temperature, sea-ice concentration, light, and wind-induced mixing potentially affect the lower trophic levels in the Barents Sea marine ecosystem. During the integration period (2010–2070), only a modest change in climate variables and biological production was found, compared to the inter-annual and decadal variability. The most prominent change was projected for the mid-2040s with a sudden decrease in biological production, largely controlled by covarying changes in heat inflow, wind, and sea-ice extent. The northernmost parts exhibited increased access to light during the productive season due to decreased sea-ice extent, leading to increased primary and secondary production in periods of low sea-ice concentrations. In the southern parts, variable access to nutrients as a function of wind-induced mixing and mixed layer depth were found to be the most dominating factors controlling variability in primary and secondary production.

Antiproliferative Illudalane Sesquiterpenes from the Marine Sediment Ascomycete Aspergillus oryzae.

The new asperorlactone (1), along with the known illudalane sesquiterpene echinolactone D (2), two known pyrones, 4-(hydroxymethyl)-5-hydroxy-2H-pyran-2-one (3) and its acetate 4, and 4-hydroxybenzaldehyde (5), were isolated from a culture of Aspergillus oryzae, collected from Red Sea marine sediments. The structure of asperorlactone (1) was elucidated by HR-ESIMS, 1D, and 2D NMR, and a comparison between experimental and DFT calculated electronic circular dichroism (ECD) spectra. This is the first report of illudalane sesquiterpenoids from Aspergillus fungi and, more in general, from ascomycetes. Asperorlactone (1) exhibited antiproliferative activity against human lung, liver, and breast carcinoma cell lines, with IC50 values < 100 µM. All the isolated compounds were also evaluated for their toxicity using the zebrafish embryo model.

Inequitable protection of multidimensional biogeochemical regions in the Mediterranean Sea

In many cases, protected areas are placed opportunistically and do not fully capture the representation of ecological components for achieving biodiversity conservation goals. In marine systems, where threats and biodiversity are inherently multidimensional and include vertical depth gradients, the designation of protected areas can be more challenging than in the terrestrial realm. Here, we aimed to evaluate the representation equality of biogeochemical regions at different depths in the Mediterranean Sea as a surrogate for biodiversity. We conducted a gap-analysis evaluating the current extent and coverage of protected areas in the Mediterranean Sea biodiversity hotspot to achieve an equitable representation and conservation strategy for the basin. We used biogeochemical regions across different depths, which allow more complex 3D marine spatial planning that includes spatial complexity of marine systems. A total of 60 biogeochemical regions distributed across three bathomes (spaces determined by depth; i.e. epipelagic, mesopelagic, and bathypelagic zones) and the seafloor were assessed. We found that biogeochemical regions in the Mediterranean Sea were not equally protected across bathomes, and that global conservation targets were not met. The bathypelagic bathome had the lowest representation in the Mediterranean Sea marine protected area (MPA) network, as well as the lowest protection equality. The sub-basins with highest protection included the Algero-Provençal basin and the Tyrrhenian Sea. These two sub-basins were the only ones that passed the Aichi protection target of 10%, while all others were lower than the target. The Ionian Sea was identified as the most highly diverse sub-basin regarding biogeochemical regions but with the least protection and with low conservation equality, making this sub-basin a high priority for conservation. Amongst countries, non-European Mediterranean countries had lower values of protection and conservation equality metrics. In general, the bathypelagic bathome was found as the least protected, the most fragmented, and with the lowest protection equality, thus its protection should be enhanced through all the Mediterranean Sea. Decision makers in the Mediterranean can apply the gap-analysis approach presented here as a useful starting point for improving protection equality in the region, incorporating these metrics in the evaluation of new proposed protected areas and sites for conservation.

Factors influencing return rate and marine residence duration in sea trout populations in Central Norway.

Brown trout (Salmo trutta) display extensive plasticity in marine migratory behaviours, with marine migrations considered to be an adaptive strategy which enables sea trout to maximize growth and reproductive potential. However, marine migrations are not without associated costs, including threats posed by ever-increasing salmon lice (Lepeophtheirus salmonis) infestations. In the present study, we used passive integrated transponder technology to characterize variability in sea trout migration behaviour amongst three catchments situated in a region of intensive salmon farming in central Norway. Specifically, we investigate how lice infestation, out-migration date and body size alter sea trout return rate and marine residence duration during the first out-migration to sea from each catchment. Distinct catchment-specific differences in sea trout out-migration size and the number of cohorts were observed, but larger body size did not guarantee the successful return of migrating trout. The marine residence duration of individuals that successfully returned to freshwater was positively correlated with lice infestation risk, suggesting for these individuals the lethal infestation threshold had not been reached. Our results also suggest that sea trout populations from lotic-dominated catchments are potentially at greater risk from size-related threats to their survival encountered during their marine migrations than sea trout from lentic-dominated catchments. The variability in sea trout migratory behaviour amongst catchments observed here emphasizes the challenges fisheries managers face when deciding the best actions to take to protect the anadromous portion of brown trout populations.

Increasing the cost-effectiveness of nutrient reduction targets using different spatial scales

In this paper, we investigate the potential gains in cost-effectiveness from changing the spatial scale at which nutrient reduction targets are set for the Baltic Sea, with particular focus on nutrient loadings from agriculture. The costs of achieving loading reductions are compared across five levels of spatial scale, namely the entire Baltic Sea; the marine basin level; the country level; the watershed level; and the grid square level. A novel highly-disaggregated model, which represents decreases in agricultural profits, changes in root zone N concentrations and transport to the Baltic Sea is used. The model includes 14 Baltic Sea marine basins, 14 countries, 117 watersheds and 19,023 10-by-10 km grid squares. The main result which emerges is that there is a large variation in the total cost of the program depending on the spatial scale of targeting: for example, for a 40% reduction in loads, the costs of a Baltic Sea-wide target is nearly three times lower than targets set at the smallest level of spatial scale (grid square). These results have important implications for both domestic and international policy design for achieving water quality improvements where non-point pollution is a key stressor of water quality.

Bering Sea marine heatwaves: Patterns, trends and connections with the Arctic

The conterminous marine system of the Bering Sea serves as an important connection between the Pacific and the Arctic. Surface water temperatures of the Northern Pacific have been rising over the past decades with associated changes in extremes: marine heatwaves (MHWs). This study aims to explore the spatiotemporal evolution characteristics and occurrence mechanisms of MHWs in the Bering Sea. Our findings reveal that MHW metrics are above average in most parts of the Bering Sea, with the number of days being more than 50 a year. Frequencies of MHWs are relatively high in the western sectors while durations and intensities are high in the eastern and southern sectors of the Bering Sea. Increasing trends in the MHW days are noticed almost everywhere while similar increases in MHW intensities are found in the northern Bering Sea. In addition, Chukchi Sea ice concentrations show a negative correlation with heatwave frequencies and days while the Arctic Oscillation has no significant connection. Positive correlations are observed between Chukchi sea temperatures and Alaskan air temperatures, implying influences on the MHW frequencies and days. While the annual trends in the MHW frequencies and days peak over several periods, the latest decade (2010–2019) has seen the highest of both. Our findings suggest that the spatiotemporal distribution of MHW metrics is connected with underlying physical processes in the Bering Sea and neighbouring climatic patterns such as the Pacific teleconnections, sea ice extent, air temperature, and its location within the Arctic.

Strategic Environmental Assessment in marine spatial planning of the North Sea and the Baltic Sea – An implementation tool for an ecosystem-based approach?

With the enactment of the European Maritime Spatial Planning Directive many coastal Member States are confronted with the challenge of applying Strategic Environmental Assessments (SEAs) in the context of setting up marine spatial plans for the first time. In addition, they are obliged to apply an ecosystem-based approach (EBA) in their new marine/maritime spatial planning (MSP) frameworks. This study examines how the emerging practice of applying SEAs in MSP contributes to the implementation of an EBA. Five case studies from the North Sea and the Baltic Sea marine regions are investigated. The assessment framework is based on the EBA key elements that members of the Baltic Marine Environment Protection Commission and Visions and Strategies around the Baltic Sea agreed upon. The findings of this study underline that SEAs in MSP are potentially a valuable implementation tool for an EBA. However, so far planning reality lacks behind this potential. Analysis shows that not yet all criteria for EBA key elements are addressed in the case studies. In particular, ‘alternative development’, ‘precaution’ and ‘adaptation’ require more specific actions. Key elements were often addressed in a general manner, not reaching the stage of explicit measures. It is necessary to refine vague concepts, to operationalise principles and to advance in knowledge and comparable and transparent methodologies to meet the demands of an EBA. Recommendations are given how to close the gaps from an EBA perspective.

Macrolides: From Toxins to Therapeutics.

Macrolides are a diverse class of hydrophobic compounds characterized by a macrocyclic lactone ring and distinguished by variable side chains/groups. Some of the most well characterized macrolides are toxins produced by marine bacteria, sea sponges, and other species. Many marine macrolide toxins act as biomimetic molecules to natural actin-binding proteins, affecting actin polymerization, while other toxins act on different cytoskeletal components. The disruption of natural cytoskeletal processes affects cell motility and cytokinesis, and can result in cellular death. While many macrolides are toxic in nature, others have been shown to display therapeutic properties. Indeed, some of the most well known antibiotic compounds, including erythromycin, are macrolides. In addition to antibiotic properties, macrolides have been shown to display antiviral, antiparasitic, antifungal, and immunosuppressive actions. Here, we review each functional class of macrolides for their common structures, mechanisms of action, pharmacology, and human cellular targets.

Molecular Docking and Dynamics Simulation Study of Hyrtioserectus Isolated Scalarane Sesterterpenes as Potential SARS-CoV-2 Dual Target Inhibitors.

Simple SummaryThe ongoing coronavirus disease-2019 (COVID-19) pandemic shows unprecedented challenges for the worldwide healthcare system. Despite the large clinical data concerning several therapeutic interventions and drug repurposing, results are still either preliminary or lacking adequate-clinical efficiency. Herein, different pharmacoinformatics approaches have been adopted such as molecular docking, ADME properties prediction and all-atom MD simulation to investigate several marine-derived scalarane derivatives as lead candidates against two of the major COVID-19 targets; main protease and Nsp15 endoribonuclease. The presented study clearly illustrates the fitness of the proposed scalarane molecules as promising clinical candidates for further development and future in-vitro/in-vivo studies against SARS-CoV-2.Presently, the world is under the toll of pandemic coronavirus disease-2019 (COVID-19) outbreak caused by SARS-CoV-2. Lack of effective and safe therapeutics has stressed the scientific community for developing novel therapeutics capable of alleviating and stopping this pandemic. Within the presented study, molecular docking, ADME properties and all-atom molecular dynamic (MD) simulation, along with two standard antiviral agents (lopinavir and benzopurpurin-4B), were applied to investigate 15 scalaranes sesterterpenes natural compounds, purified from the Red Sea marine sponge Hyrtios erectus, as potential COVID-19 dual-target inhibitors. Following multi-step docking within COVID-19 main protease and Nsp15 endoribonuclease cavities, nine promising drug-like compounds exhibited higher docking scores as well as better interactions with the target’s crucial residues than those of reference ligands. Compounds 2, 6, 11, and 15, were predicted to simultaneously subdue the activity of the two COVID-19 targets. Dynamics behavior of the best-docked molecules, compounds 15 and 6, within COVID-19 target pockets showed substantial stability of ligand-protein complexes as presented via several MD simulation parameters. Furthermore, calculated free-binding energies from MD simulation illustrated significant ligand’s binding affinity towards respective target pockets. All provided findings supported the utility of scalarane-based sesterterpenes, particularly compounds 15 and 6, as promising lead candidates guiding the development of effective therapeutics against SARS-CoV-2.

Comprehensive Overview on the Chemistry and Biological Activities of Selected Alkaloid Producing Marine-Derived Fungi as a Valuable Reservoir of Drug Entities.

Marine-associated fungal strains act as a valuable reservoir of bioactive diverse secondary metabolites including alkaloids which are highly popular by their biological activities. This review highlighted the chemistry and biology of alkaloids isolated from twenty-six fungal genera associated with marine organisms and marine sea sediments. The selected fungi are from different marine sources without focusing on mangroves. The studied fungal genera comprises Acrostalagmus, Arthrinium, Chaetomium, Cladosporium, Coniothyrium, Curvularia, Dichotomomyces, Eurotium, Eutypella, Exophiala, Fusarium, Hypocrea, Microsphaeropsis, Microsporum, Neosartorya, Nigrospora, Paecilomyces, Penicillium, Pleosporales, Pseudallescheria, Scedosporium, Scopulariopsis, Stagonosporopsis, Thielavia, Westerdykella, and Xylariaceae. Around 347 alkaloid metabolites were isolated and identified via chromatographic and spectroscopic techniques comprising 1D and 2D NMR (one and two dimensional nuclear magnetic resonance) which were further confirmed using HR-MS (high resolution mass spectrometry) and Mosher reactions for additional ascertaining of the stereochemistry. About 150 alkaloids showed considerable effect with respect to the tested activities. Most of the reported bioactive alkaloids showed considerable biological activities mainly cytotoxic followed by antibacterial, antifungal, antiviral, antioxidant; however, a few showed anti-inflammatory and antifouling activities. However, the rest of the compounds showed weak or no activity toward the tested biological activities and required further investigations for additional biological activities. Thus, alkaloids isolated from marine-associated fungi can afford an endless source of new drug entities that could serve as leads for drug discovery combating many human ailments.

Early spring subglacial discharge plumes fuel under-ice primary production at a Svalbard tidewater glacier

Abstract. Subglacial upwelling of nutrient-rich bottom water is known to sustain elevated summer primary production in tidewater-glacier-influenced fjord systems. However, the importance of subglacial upwelling during the early spring season has not been considered yet. We hypothesized that subglacial discharge under sea ice is present in early spring and that its flux is sufficient to increase phytoplankton primary productivity. We evaluated the effects of the submarine discharge on primary production in a seasonally fast-ice covered Svalbard fjord (Billefjorden) influenced by a tidewater outlet glacier in April and May 2019. We found clear evidence for subglacial discharge and upwelling. Although the estimated bottom-water entrainment factor (1.6) and total fluxes were lower than in summer studies, we still observed substantial impact on the fjord ecosystem and primary production at this time of the year. The subglacial discharge leads to a salinity-stratified surface water layer and sea ice formation with low bulk salinity and permeability. The combination of the stratified surface layer, a 2-fold higher under-ice irradiance due to thinner snow cover, and higher N and Si concentrations at the glacier front supported phytoplankton primary production 2 orders of magnitude higher (42.6 mg C m−2 d−1) compared to a marine reference site at the fast-ice edge. Reciprocal transplant experiments showed that nutrient supply increased phytoplankton primary production by approximately 30 %. The brackish-water sea ice at the glacier front with its low bulk salinity contained a reduced brine volume, limiting the inhabitable brine channel space and nutrient exchange with the underlying seawater compared to full marine sea ice. Microbial and algal communities were substantially different in subglacial-influenced water and sea ice compared to the marine reference site, sharing taxa with the subglacial outflow water. We suggest that with climate change, the retreat of tidewater glaciers in early spring could lead to decreased under-ice phytoplankton primary production. In contrast, sea ice algae production and biomass may become increasingly important, unless sea ice disappears first, in which case spring phytoplankton primary production may increase.

Particulate Amines in the Background Atmosphere of the Yangtze River Delta, China: Concentration, Size Distribution, and Sources

Amines are important for new particle formation and subsequent growth in the atmosphere. Consequently, the processes involved are receiving more attention in recent years. Here, we conduct a field observation in order to investigate the atmospheric particulate amines at a background site in the Yangtze River Delta (YRD) during the summer of 2018. Four amines in PM2.5, i.e., methylamine (MA), dimethylamine (DMA), diethylamine (DEA), and trimethylamine (TMA), were collected, twice daily and analyzed. During the campaign, our measurements found the concentrations of MA, DMA, DEA, and TMA of 15.0 ± 15.0, 6.3 ± 6.9, 20.4 ± 30.1, and 4.0 ± 5.9 ng m−3, respectively, and the four amines correlated well with each other. The concentration of amines appear to be independent of whether they were collected during the day or night. Both MA and DMA exhibited a bimodal size distribution that had peaks at 0.67 and 1.1 µm, suggesting amines preferably distribute on submicron particles. Boundary layer height (BLH), relative humidity, and pH of aerosols were found have a negative relationship with amines, while aerosol liquid water content (ALWC) was found to have a positive relationship with amines. The PMF (positive matrix factorization) source apportionment results showed that the main source of amines in Chongming Island was of anthropogenic origin such as industrial and biomass emission, followed by marine sources including sea salt and marine biogenic sources. Given that the YRD region is still suffering from complex atmospheric pollution and that the knowledge on aerosol amines is still limited, more field studies are in urgent need for a better understanding of the pollution characteristics of amines.

Effect of Individual and Combined Addition of Marine Fish Collagen and Sea Pineapple Shell Extract on Dough Rheology and Rice Bread Quality

ABSTRACT Marine fish collagen (MFC) and sea pineapple shell extract (SPSE) were used as functional supplements to manufacture fermented rice bread (FRB). The addition of MFC (0.5%), SPSE (0.5%), and the mixture of MFC (0.25%) and SPSE (0.25%) significantly decreased the storage (G’) and loss modulus (G”) of the rice dough and increased its values of phase shift tangent (tan δ = G”/G’) with increasing angular frequency. The addition of SPSE significantly decreased the brightness but increased the yellowness and redness of control FRB. On the first day, the FRB with SPSE had the highest hardness, while the mixture of MFC and SPSE had the highest springiness and cohesiveness. The hardness of all samples increased, but the springiness and cohesiveness decreased during storage. Based on sensory evaluation, the sequence of acceptance for overall are MFC, control, MFC+SPSPE, and SPSE. Moreover, MFC had the highest scores for texture, taste, and appearance; control had the highest score for flavor; and SPSE and MFC+SPSE had the highest scores for color. Therefore, the MFC or the mixture of MFC and SPSE is a good functional ingredient for FRB, which improved FRB’s quality properties.

Sustainability challenges in developing marine-based adventure tourism in Ambon

There has been growing demands for nature-based tourism involving diverse activities. Marine-based adventure tourism is one of it, which is based on a unique combination of marine biodiversity, scenic beauty, and sea activities such as sea kayaking and diving. The consequences of rapid growth destroy the natural resources because in many cases, any type of tourism industry only focusing on economic development. Sustainable marine-based adventure tourism should balance with many local factors, especially the residents’ socioeconomic activities. The unplanned marine-based adventure tourism development can gradually destroy its natural environmental resources on which it depends, the marine ecosystem. In this paper, the authors describe the possible effects of marine-based-adventure tourism development on natural resources along with solid waste pollution challenges and finally, how the residents are involved in overcoming these sustainability challenges.

Sea Surface Heights and Marine Gravity Determined from SARAL/AltiKa Ka-band Altimeter Over South China Sea

SARAL/AltiKA (SRL) is the first altimetry satellite with a Ka-band altimeter. To validate the advantages of the Ka-band altimeter over traditional Ku-band altimeters in marine geodetic applications, a comprehensive analysis is carried out over the South China Sea (SCS) (0–30° N, 105–125° E) from three aspects, namely the influence of load on waveforms, the precision of sea surface heights (SSHs), and the precision of altimeter-derived marine gravity field. Coastal waveforms of SRL, Jason-2, and CryoSat-2 are separately compared with corresponding ocean-type waveforms. The radius of coastal influence on SSHs of SRL/exact repeat mission (SRL/ERM) is the smallest, being about 3 km. Crossover discrepancies, global mean sea surface models, and tide gauge data are used to assess the precision of altimetric SSHs. Compared with the SSH precision of Ku-band Jason-2/ERM, the SSH precision of Ka-band SRL/ERM is 4.6% higher over the SCS and 10% higher in offshore areas. Gridded gravity anomalies are derived from measurements of SRL/drifting phase (SRL/DP) and CryoSat-2 through the inverse Vening-Meinesz formula, respectively. According to the assessment by shipborne gravity data and global marine gravity models, the precision of SRL/DP-derived gravity is higher than that of CryoSat-2-derived gravity over the SCS, especially in offshore areas. In some cycles, ground tracks of SRL/ERM have large drifting of more than 10 km from nominal tracks. The results show that the Ka-band altimeter of SRL has better precision in SSHs and marine gravity recovery than the Ku-band altimeter over the SCS, particularly in offshore areas.

Trend correlations for coastal eutrophication and its main local and whole-sea drivers – Application to the Baltic Sea

Coastal eutrophication is a major environmental issue worldwide. In the Baltic Sea, eutrophication affects both the coastal waters and the open sea. Various policy frameworks aim to hinder its progress but eutrophication-relevant water quality variables, such as chlorophyll-a concentrations, still exhibit opposite temporal trends in various Baltic Sea marine and coastal waters. In this study, we investigate the temporal-trend linkages of measured water quality variables and their various anthropogenic, climatic and hydrospheric drivers over the period 1990–2020 with focus on the Swedish coastal waters and related marine basins in the Baltic Sea.We find that it is necessary to distinguish more and less isolated coastal waters, based on their water exchanges with the open sea, to capture different coastal eutrophication dynamics. In less isolated coastal waters, eutrophication is primarily related to nitrogen concentrations, while it is more related to phosphorus concentrations in more isolated coastal waters. In the open sea, trends in eutrophication conditions correlate best with trends in climatic and hydrospheric drivers, like wind speed and water salinity, respectively. In the coastal waters, driver signals are more mixed, with considerable influences from anthropogenic land-based nutrient loads and sea-ice cover duration. Summer chlorophyll-a concentration in the open sea stands out as a main change driver of summer chlorophyll-a concentration in less isolated coastal waters. Overall, coastal waters are a melting pot of driver influences over various scales, from local land-based drivers to large-scale total catchment and open sea conditions. The latter in turn depend on long-term integration of pathway-dependent influences from the various coastal parts of the Baltic Sea and their land-based nutrient load drivers, combined with overarching climate conditions and internal feedback loops. As such, our results challenge any unidirectional local source-to-sea paradigm and emphasize a need for concerted local land-catchment and whole-sea measures for robust coastal eutrophication management.

A novel method for numerical simulation of the interaction between level ice and marine structures

Finite Element Method (FEM) and Smoothed Particle Hydrodynamics (SPH) method are effective methods to study the interaction between marine structures and sea ice. However, both FEM and SPH methods have their own shortcomings in numerical simulations. There are mesh distortions and disappearance in FEM, and tensile instability, difficulty in applying boundary conditions, and low computational efficiency in SPH. Thus, to make up those problems, it is essential to develop a new numerical method. In the present study, the FEM–SPH adaptive method was applied in the numerical simulation of icebreaking, which could convert finite elements into SPH particles based on given conditions. Numerical models of cone and icebreaker interactions with level ice were established. Then, numerical simulation results were compared with empirical formula and model test results. In addition, the effects of cone angle and ice strength were analyzed. It was demonstrated that this algorithm could accurately predict the icebreaking resistance, which is the most concerned parameter in practice. Simultaneously, it could effectively simulate the accumulation process of ice rubbles. Thus, the FEM–SPH adaptive method is considered to be an effective way to simulate the interaction between marine structures and sea ice, and has great potential in the numerical simulation of icebreaking.

Underwater images contrast enhancement and its challenges: a survey

Exploration of the deep sea and ocean in the marine industry has continued to gain interest in recent years. To get the detailed imaging of deep sea layers, marine vessels and robots are fitted with advanced imaging technologies. There are certain factors like water properties and impurities that affect the quality of the photographs captured by the underwater imaging devices. As sea water absorbs colors, so processing of sea imaging data becomes more challenging. Water light attenuation is a phenomenon that is caused by the absorbance and scattering factors. Certain studies showed that the existence of certain intrinsic shortcomings are attributed to the appearance of objects and ambient noise in underwater images. As a result, it is difficult in a real-time system to distinguish objects from their surroundings in these images. We measures the algorithms performance with respect to various aspects, effect of the hardware and software parts for underwater images and critical review of different underwater image enhancement algorithms. First, we describe some well-known techniques of spatial and frequency domains. Then, we list the existing quantitative measurements which are required to measure the quality of the enhanced image. Finally, the performance of various up-to-date existing methods is compared based on the outcomes of standard quantitative measurements, and factors such as requirements/suitability, and technical aspects, are included. Furthermore, a variety of image databases used for image contrast enhancement is discussed in detail. This study expands the scope for other researchers to understand the important characteristics of different underwater image contrast enhancement methods, and also provides future research directions.

Bioluminescence of ctenophores near the boundary of oxygen-depleted waters at the redoxcline of the Black Sea.

Vertical distribution of ctenophores near the boundary of oxygen-depleted waters of the Black Sea redoxcline was studied by use of video observations with real-time water sampling, horizontal MultiNet towing, and soundings using bathyphotometers with simultaneous vertical plankton net sampling. The results of the study showed for the first time that the daytime accumulation of ctenophores above the upper boundary of the suboxic zone changes the biophysical properties of the medium, causing an increase in the daytime intensity of bioluminescence near the redoxcline. The dynamics of this glow is in antiphase to that in the surface layers, where it is associated with the bioluminescence of phytoplankton. Therefore, in the deep-sea areas, two types of bioluminescence peaks differ in the light generation sources: the nighttime glow of phytoplankton in surface layers and the daytime glow of zooplankton in layers of oxygen-depleted waters at the redoxcline. The discovery of this new phenomenon allows the use of bioluminescent methods for the rapid assessment of the depth of the daytime zooplankton layers for the subsequent hauls of plankton nets. This significantly expands the possibilities of studying the structure and functioning of the pelagic ecosystem of the Black Sea and other marine basins with a redoxcline.