Oligotrophic Conditions Research Articles

Oligotrophic characteristics and genomic analysis of the Rhodoligotrophos defluvii lm1T isolated from activated sludge

Oligotrophic conditions have important role in the evolution of life and also the study of ecological status and ecological functions of oligotrophic bacteria in such ecosystems. In this study, the oligotrophic characteristics of the experimental strain Rhodoligotrophos defluvii lm1T (hereinafter referred to as lm1T) isolated from activated sludge were investigated. The results indicated that lm1T was able to grow normally in Luria-Bertani (LB) and diluted LB solid agar plates, demonstrating it to be a facultative oligotrophic bacterium. Research about carbon source and genome analysis revealed that lm1T lacks 6-phosphofructokinase in the glycolysis process, resulting in the inability to convert glucose-6-phosphate to fructose-6-phosphate, which prevented lm1T from utilizing glucose. And in line with that, lm1T can utilize 1,6-diphosphofructose, acetate, propionate, and butyrate to meet its growth requirements. lm1T possesses a wide range of nitrogen sources utilization, including NaNO3, urea, NH4Cl, (NH4)2SO4, peptone, and yeast extract, enabling its adaptation to various complex environments. Additionally, comparative genomic analysis of lm1T and the model strain Rhodoligotrophos appendicifer 120-1T revealed that lm1T had more genes related to degradation metabolism, which aided its adaptation to complex environments.

Photoprotection by photoinhibitory and PSII-reaction centre quenching controls growth of Ulva rigida (Chlorophyta) and is a pre-requisite for green tide formation

Main ConclusionThe combined photoinhibitory and PSII-reaction centre quenching against light stress is an important mechanism that allows the green macroalga Ulva rigida to proliferate and form green tides in coastal ecosystems.Eutrophication of coastal ecosystems often stimulates massive and uncontrolled growth of green macroalgae, causing serious ecological problems. These green tides are frequently exposed to light intensities that can reduce their growth via the production of reactive oxygen species (ROS). To understand the physiological and biochemical mechanisms leading to the formation and maintenance of green tides, the interaction between inorganic nitrogen (Ni) and light was studied. In a bi-factorial physiological experiment simulating eutrophication under different light levels, the bloom-forming green macroalga Ulva rigida was exposed to a combination of ecologically relevant nitrate concentrations (3.8–44.7 µM) and light intensities (50–1100 µmol photons m−2 s−1) over three days. Although artificial eutrophication (≥ 21.7 µM) stimulated nitrate reductase activity, which regulated both nitrate uptake and vacuolar storage by a feedback mechanism, nitrogen assimilation remained constant. Growth was solely controlled by the light intensity because U. rigida was Ni-replete under oligotrophic conditions (3.8 µM), which requires an effective photoprotective mechanism. Fast declining Fv/Fm and non-photochemical quenching (NPQ) under excess light indicate that the combined photoinhibitory and PSII-reaction centre quenching avoided ROS production effectively. Thus, these mechanisms seem to be key to maintaining high photosynthetic activities and growth rates without producing ROS. Nevertheless, these photoprotective mechanisms allowed U. rigida to thrive under the contrasting experimental conditions with high daily growth rates (12–20%). This study helps understand the physiological mechanisms facilitating the formation and persistence of ecologically problematic green tides in coastal areas.

Long-term study of phytoplankton dynamics in a supply reservoir reveals signs of trophic state shift linked to changes in hydrodynamics associated with flow management and extreme events

This study analyses over a decade (2009–2022) of monitoring data to understand the impact of hydrological characteristics on water quality and phytoplankton dynamics in Prospect Reservoir, a critical water supply for Greater Sydney, Australia, known for its excellent water quality. Water quality and phytoplankton dynamics were related to hydrodynamics, linked to flow management and the water quality of inflows. Phytoplankton biovolume increased after a prolonged drawdown and subsequent refill event, mainly driven by dinoflagellates, and corresponded to increases in total phosphorus and water temperature. The hydrological period following the 2019/2020 summer bushfires (post-bushfire) that impacted connected reservoirs, was marked by increased flow activity and nutrient loading, leading to significant shifts in the phytoplankton community. Functional group classification and ordination analysis indicated a transition from taxa typically dominant in oligotrophic conditions to meso‑eutrophic. This transition correlated with elevated nutrient levels and chlorophyll-a (Chl-a), and reduced Secchi depth and dissolved oxygen, providing evidence of eutrophication. Q index indicated good water quality post-bushfire, contrasting with a eutrophic status assessment using Chl-a. Our findings highlight the importance of analysing long-term datasets encompassing varied hydroclimatological conditions for a deeper understanding of reservoir behaviour. A comprehensive approach to water quality assessment is recommended, combining functional group classification, Q index and Chl-a measurements for effective reservoir health assessment. This research provides novel insights into the effects of disturbances such as bushfires, on water quality and phytoplankton dynamics in an underrepresented geographic region, offering valuable knowledge for managing water resources amidst growing climate variability.

Thinning enhances forest soil C storage by shifting the soil toward an oligotrophic condition

Forests are significant carbon reservoirs, with approximately one-third of this carbon stored in the soil. Forest thinning, a prevalent management technique, is designed to enhance timber production, preserve biodiversity, and maintain ecosystem functions. Through its influence on biotic and abiotic factors, thinning can profoundly alter soil carbon storage. Yet, the full implications of thinning on forest soil carbon reservoirs and the mechanisms underpinning these changes remain elusive. In this study, we undertook a two-year monitoring initiative, tracking changes in soil extracellular enzyme activities (EEAs), microbial communities, and other abiotic parameters across four thinning intensities within a temperate pine forest. Our results show a marked increase in soil carbon stock following thinning. However, thinning also led to decreased dissolved organic carbon (DOC) content and a reduced DOC to soil organic carbon (SOC) ratio, pointing toward a decline in soil carbon lability. Additionally, fourier transform infrared spectroscopy (FTIR) analysis revealed an augmented relative abundance of aromatic compounds after thinning. There was also a pronounced increase in absolute EEAs (per gram of dry soil) post-thinning, implying nutrient limitations for soil microbes. Concurrently, the composition of bacterial and fungal communities shifted toward oligotrophic dominance post thinning. Specific EEAs (per gram of soil organic matter) exhibit a significant reduction following thinning, indicating a deceleration in organic matter decomposition rates. In essence, our findings reveal that thinning transitions soil toward an oligotrophic state, dampening organic matter decomposition, and thus bolstering the soil carbon storage potential of forest. This study provides enhanced insights into the nuanced relationship between thinning practices and forest soil carbon dynamics, serving as a robust foundation for enlightened forest management strategies.

Distribution and biogeochemical perspectives of nutrients in the Eastern Equatorial Indian Ocean

The seasonal reversal of ocean circulation associated with seasonal change in the direction of prevailing winds and the occurrence of several anomalous events in the Eastern Equatorial Indian Ocean (EEIO) make this region dynamic and complex in terms of its biogeochemical characteristics. Two multidisciplinary cruises were conducted to measure nutrients and associated physicochemical parameters across the water column (up to 1000 m) of the EEIO during boreal summer and winter monsoons to understand the distribution of nutrients and their spatio-temporal variability from a biogeochemical perspective. The seasonality in the thermohaline structure of the region is indistinct except for surface salinity drop during summer monsoon due to more precipitation on-site and in adjoining areas. Low concentrations of chlorophyll at the surface and in the deep chlorophyll maxima represent the oligotrophic nature of this region. Surface water was found nutrient-depleted (0.03–0.4 µM Nitrate, 0.02–0.13 µM Phosphate). The maxima of vertical profiles of nitrate and phosphate were recorded at a shallower depth (150–200 m) when compared to its maxima in usual oceanic conditions, but a silicate maximum was recorded in deeper water. In the surface and upper mixed layer paucity of nutrients resulted in low N:P and N:Si ratios. Therefore, nitrogen limitation is evident. The overall ratio of N:P yielded a mean value of 15.33 and matched with the representative literature value for the Indian Ocean. The minimum oxygen values (<50 µM) in the deep water (150–200 m) indicated a hypoxic condition. No signature of denitrification and a moderate nitrate deficit were observed in deep waters. The negative values of Nitrate anomaly (N-tracer) at 50–100 m depth were attributed to a Watermass influenced by denitrification. The prevailing oligotrophic condition caused limited synthesis of organic matter and subsequently little decomposition in deep water. The maxima in the apparent oxygen utilization (AOU) profile are confined to 150 to 200 m depth and represent the most active zone for regeneration that is limited to shallow depth. Regenerated nutrients reached maxima at shallower depth and primarily control material cycling in this region. Supply of nitrate to the surface water based on the preformed values of prevailing water mass was primarily by Bay of Bengal water. According to the findings of this study, preformed nitrate concentrations between 100 and 200 metres below the surface were found very low, indicating that Indonesian Through Flow (ITF) has little impact on the distribution of nutrients in this area.

HPLC validation as a management tool in artificial water storage ponds

Pigment analysis through high-performance liquid chromatography (HPLC) offers a faster and more replicable study of the phytoplankton community structure than traditional taxonomy, but its application to small-sized freshwater ecosystems is infrequent. We carried out a taxonomy-based validation for the application of HPLC to the management of two artificial water storage ponds. We also investigated the error and variability sources that affected the relationships between calculated biovolumes of phytoplankton groups and associated pigment concentrations. For both qualitative and quantitative approaches, agreement was obtained between techniques that consistently identified biomass peaks and structural trophic differences between the ponds. Considering the most relevant pigment to biovolume ratios for total biomass, and partial biomass proxies of green algae and fucoxanthin containing algae, a decreasing tendency was obtained as total and partial biomass increased in all the studied models. Bidirectional variability affecting these relationships scaled under high biomass conditions and suggested a better suitability of HPLC application under oligotrophic conditions, which is in line with previous studies. Additionally, pigment data was able to identify mean stored volume as the main environmental factor that drove trophic differences between the ponds. We conclude that HPLC is accurate not just for monitoring purposes, but to carry out ecosystem-level assessments in artificial ponds. Therefore, we endorse the use of HPLC as part of the monitoring networks in small-sized freshwater ecosystems.

Lake Water Quality Assessment Through GIS based Interpolation Method: A Case Study of Beris Dam, Kedah, Malaysia

Preliminary research on physicochemical and water quality in the Beris dam shows that the water catchment area has the best standards and quality during the observation of the wet season and dry season mainly in the northern states of peninsular Malaysia. The analysis of water quality assessment using the DOE-WQI and Carlson Trophic State Index (CTSI) resulted that during the dry season (February), the average DOE-WQI reading is 88 or Class II; while during the wet season, the average DOE-WQI reading recorded 87.1 (October). Although the CTSI score was 34.2 - 27.0 during wet season and dry season, both seasons were classified as oligotrophic conditions. Some physicochemical parameters also show a high correlation between them and indicates minor water quality issues. The use of more efficient interpolation techniques in water quality studies also reinforces the evaluation of water quality. As a reference point for future water research, GIS interpolation application methods can be more effectively and efficiently used with Inverse Distance Weighted (IDW), Ordinary Kriging and Spline techniques. HIGHLIGHTS Investigation of physicochemical and water quality in lake water in Beris Dam Assessment by using water quality index by DOE-WQI and Carlson Trophic State Index (CTSI) to acknowledge water quality and trophic condition. Analyzing physicochemical data into mathematical method to compare between physicochemical data and correlate between them. Application of GIS based interpolation methods to analyze and referencing physicochemical data into spatial interpolation maps. Comparing appropriate spatial interpolation techniques to analyze physicochemical observation data and index. GRAPHICAL ABSTRACT

Methane-dependent complete denitrification by a single Methylomirabilis bacterium.

Methane-dependent nitrate and nitrite removal in anoxic environments is thought to rely on syntrophy between ANME-2d archaea and bacteria in the genus 'Candidatus Methylomirabilis'. Here we enriched and purified a single Methylomirabilis from paddy soil fed with nitrate and methane, which is capable of coupling methane oxidation to nitrate reduction via nitrite to dinitrogen independently. Isotope labelling showed that this bacterium we name 'Ca. Methylomirabilis sinica' stoichiometrically performed methane-dependent complete nitrate reduction to dinitrogen gas. Multi-omics analyses collectively demonstrated that 'M. sinica' actively expressed a well-established pathway for this process, especially including nitrate reductase Nap. Furthermore, 'M. sinica' exhibited a higher nitrate affinity than most denitrifiers, implying its competitive fitness under oligotrophic nitrogen-limited conditions. Our findings revise the paradigm of methane-dependent denitrification performed by two organisms, and the widespread presence of 'M. sinica' in public databases suggests that the coupling of methane oxidation and complete denitrification in single cells substantially contributes to global methane and nitrogen budgets.

Characterization of phytoplankton functional types in the western Bay of Bengal: HPLC- and CHEMTAX-based approach.

This study investigates phytoplankton functional group variations in the western Bay of Bengal (WBoB) during the Spring Intermonsoon. Samples were collected from four cross-shore transects: Mahanadi (MN), Vamsadhara (VD), Godavari (GD), and Krishna (KS). East India Coastal Current and warm gyre influenced the southern transects (KS, GD), VD was experiencing moderate upwelling and MN was characterized by low salinity and oligotrophic conditions due to freshwater input. In response to hydrography, phytoplankton biomass and functional types differed within and between the transects. Chlorophyll-a (Chl-a) was spatially high in VD and low in MN. The subsurface Chlorophyll-a maxima (SSCM) was prominent and shallow in the MN and VD, compared to the southern transects. Total diagnostic pigments concentration was high in VD, followed by GD, KS and MN. Phytoplankton functional groups and each groups contribution to Chl-a was calculated through CHEmical Taxonomy (CHEMTAX). Diatoms and cyanophytes were the dominant functional types in the surface layers. Progressive shift from diatoms in the nearshore region to cyanophytes in the offshore was observed. The low saline and low-nutrient conditions were conducible for the growth of cyanophytes, while nutrient-rich optimum light layer of SSCM and upper layer of VD were favorable for diatoms. Cryptophytes contribution to Chl-a was higher in southern transects compared to the north. Prymnesiophytes and prasinophytes were high in the subsurface and deep layers could be due to their adaptions to light and nutrients. The present study highlights the significance of physical processes associated hydrography in structuring the phytoplankton functional types.

Living benthic foraminifera from Almirante Câmara and Grussaí canyons and adjacent slope areas (Campos Basin, Southwest Atlantic): Response to trophic and hydrodynamic conditions

Living (rose Bengal stained) benthic foraminiferal faunas, grain size, geochemical parameters (total organic matter, chlorophyll-a, pheophytin-a, molecular markers, and vertical flux of particulate organic matter) were investigated in 14 stations in two submarine canyons Almirante Câmara and Grussaí considered mature and immature, respectively, and on the adjacent continental slope, from 400 to 1300 m water depth, in the southwestern Atlantic Ocean. Samples were retrieved during two campaigns, in the austral winter 2008 and the austral summer 2009. The density, richness, and species composition of the benthic foraminiferal fauna decrease with depth in the canyons and along the adjacent slope transects, reflecting the availability and quality of sedimentary organic matter. Both canyons act as traps of labile organic matter, mainly towards the head of the Almirante Câmara canyon, promoting high specimen density, species richness, and the occurrence of species indicative of pulses of phytodetritus (e.g., Bolivina pacifica, Nonionella opima, and Nonionella stella). Towards the deeper regions of Almirante Câmara canyon and in the adjacent slope area, the lower abundance, diversity, and richness reflect more oligotrophic conditions and an adverse environment in terms of easily metabolizable organic matter. This multiproxy study demonstrates that the canyons in the southeastern Brazilian continental margin play an important role in accumulating organic matter.

Active microbiota persist in dry permafrost and active layer from Elephant Head, Antarctica.

Dry permafrost is a challenging environment for microbial life due to cold, dry, and often oligotrophic conditions. In 2016, Elephant Head, Antarctica, was confirmed as the second site on Earth to contain dry permafrost. It is geographically distinct from the McMurdo Dry Valleys where dry permafrost has been studied previously. Here, we present the first study of the microbial activity, diversity, and functional potential of Elephant Head dry permafrost. Microbial activity was measured using radiorespiration assays with radiolabeled acetate as a carbon source at 5, 0, and -5°C. Low, but detectable, rates of microbial activity were measured in some samples at 0 and -5°C. This is distinct from previous studies of McMurdo Dry Valley dry permafrost which concluded that dry permafrost represents a cold-arid limit to life on the planet. The isolation of cold-adapted organisms from these soils, including one capable of subzero growth, further supports that the Elephant Head dry active layer and dry permafrost harbor viable microbial life, which may be active in situ. Metagenomic, 16S rRNA gene, and internal transcribed spacer and amplicon sequencing identified similar microbial communities to other Antarctic and cold environments. The Elephant Head microbial community appears to be adapted for survival in cold, dry, and oligotrophic conditions based on the presence of cold adaptation and stress response genes in the metagenomes. Together, our results show that dry permafrost environments do not exclude active microbial life at subzero temperatures, suggesting that the cold, dry soils of Mars may also not be as inhospitable as previously thought.

"Candidatus Siderophilus nitratireducens": a putative nap-dependent nitrate-reducing iron oxidizer within the new order Siderophiliales.

Nitrate leaching from agricultural soils is increasingly found in groundwater, a primary source of drinking water worldwide. This nitrate influx can potentially stimulate the biological oxidation of iron in anoxic groundwater reservoirs. Nitrate-dependent iron-oxidizing (NDFO) bacteria have been extensively studied in laboratory settings, yet their ecophysiology in natural environments remains largely unknown. To this end, we established a pilot-scale filter on nitrate-rich groundwater to elucidate the structure and metabolism of nitrate-reducing iron-oxidizing microbiomes under oligotrophic conditions mimicking natural groundwaters. The enriched community stoichiometrically removed iron and nitrate consistently with the NDFO metabolism. Genome-resolved metagenomics revealed the underlying metabolic network between the dominant iron-dependent denitrifying autotrophs and the less abundant organoheterotrophs. The most abundant genome belonged to a new Candidate order, named Siderophiliales. This new species, "Candidatus Siderophilus nitratireducens," carries genes central genes to iron oxidation (cytochrome c cyc2), carbon fixation (rbc), and for the sole periplasmic nitrate reductase (nap). Using thermodynamics, we demonstrate that iron oxidation coupled to nap based dissimilatory reduction of nitrate to nitrite is energetically favorable under realistic Fe3+/Fe2+ and NO3-/NO2- concentration ratios. Ultimately, by bridging the gap between laboratory investigations and nitrate real-world conditions, this study provides insights into the intricate interplay between nitrate and iron in groundwater ecosystems, and expands our understanding of NDFOs taxonomic diversity and ecological role.

Nutrient inputs shape ecosystem functioning gradients along the pristine, upper Neretva River, Bosnia and Herzegovina

Ecosystem functions are the backbone of ecosystem services that rivers provide to human societies. Ecosystem functioning emerges from the interaction between biological communities and their environment. As environmental conditions in rivers change along their longitudinal continuum, so does functioning. Sometimes, these changes do not follow smooth gradients but rather great discontinuities. This can be the case in calcareous, karstic rivers due to the sudden massive inputs of groundwater along the landscape, a typical phenomenon for Balkan rivers. Despite their high geodiversity and their great ecological value, Balkan rivers remain understudied. Here, we investigated how ecosystem functions and their diversity (estimated as multifunctionality) change along the continuum of the karstic, free-flowing Neretva River in Bosnia and Herzegovina. For this purpose, we measured a subset of fundamental ecosystem functions (ecosystem gross primary production, biofilm net primary production and enzymatic activities, organic matter decomposition) in 11 river reaches from the Neretva headwaters to river sections upstream of the Jablanica reservoir. We found different functions reached their maximum in different sections of the Neretva depending on nutrient inputs. While organic matter decomposition was highest in headwaters due to the input of nutrients from riparian vegetation, biofilm enzymatic activity expressed highest values at middle sections due to groundwater inputs of NH4+-N. Primary production was highest at the most downstream sections due to the accumulation of NO3--N and PO43--P within the catchment area. As a result, average multifunctionality peaked at sites with the highest nutrient concentration across the Neretva river continuum, indicating a stronger influence of nutrient inputs than network position. The pristine conditions of the Neretva result in oligotrophic conditions along its upper course. Our results emphasize the great sensitivity of ecosystem functioning in the Neretva to nutrient inputs and environmental discontinuities, either natural or human-made. Potential major, long-term impacts in the area might alter existing environmental gradients and thus ecosystem functioning in rivers at local and regional scale.

Assessment of Physicochemical Parameters by Remote Sensing of Bacalar Lagoon, Yucatán Peninsula, Mexico

Remote sensing is an invaluable research tool for the analysis of marine and terrestrial water bodies. However, it has some technical limitations in waters with oligotrophic conditions or close to them due to the low spectral response of some water parameters to the signal from the sensors to be used. In this work, we use remote sensing to evaluate a set of water quality parameters (dissolved oxygen, total dissolved solids, oxidation–reduction potential, electrical conductivity, salinity, and turbidity) in the Bacalar Lagoon, located in the Mexican Caribbean, which has experienced in recent years a dramatic change from its natural oligotrophic condition to mesotrophic and eutrophic due to anthropogenic contamination. This was accomplished through the correlation and linear regression analysis between reflectance images processed from Landsat 8 and Sentinel 2, with in situ measurements for each physicochemical parameter considered, and the development of statistical models to predict their values in places where only the reflectance values were available. The results of this work indicate the feasibility of using remote sensing to monitor electrical conductivity, salinity, turbidity, and total dissolved solids since their predicted values agree with those reported at various sites within this lagoon.

Phosphate-related genomic islands as drivers of environmental adaptation in the streamlined marine alphaproteobacterial HIMB59.

These results shed light on the evolutionary strategies of microbes with streamlined genomes to adapt and survive in the oligotrophic conditions that dominate the surface waters of the global ocean. At the individual level, these microbes have been subjected to evolutionary constraints that have led to a more efficient use of nutrients, removing non-essential genes named as "streamlining theory." However, at the population level, they conserve a highly diverse gene pool in flexible genomic islands resulting in polyclonal populations on the same genomic background as an evolutionary response to environmental pressures. Localization of these islands at equivalent positions in the genome facilitates horizontal transfer between clonal lineages. This high level of environmental genomic heterogeneity could explain their cosmopolitan distribution. In the case of the order HIMB59 within the class Alphaproteobacteria, two factors exert evolutionary pressure and determine this intraspecific diversity: phages and the concentration of P in the environment.

Biogenic silica accumulation and diatom assemblage variations through the Eocene-Oligocene Transition: A Southern Indian Ocean versus South Atlantic perspective

It is widely interpreted that there is a significant link between climate cooling at the Eocene-Oligocene Transition (EOT; ∼34 Ma) and subsequent diatom proliferation in the world's oceans during the mid to late Cenozoic. Yet, our understanding of biogenic silica flux through the EOT interval is based on limited data from a few sites, and there are many complications in making a meaningful comparison based on biogenic silica concentration data generated using different techniques. Here, we present new biogenic opal flux and diatom assemblage records across EOT from Ocean Drilling Program Site 748 (Southern Kerguelen Plateau, southern Indian Ocean), in addition to new biogenic opal flux records from the South Atlantic (Deep Sea Drilling Project Site 511, Falkland Plateau and Ocean Drilling Program Site 1090, Agulhas Ridge). Observed opal flux shifts and variability at Site 748 are consistent with published data from nearby Site 744; both sites show considerable shifts in biogenic opal accumulation rates corresponding to shifts in published benthic oxygen isotope records through the EOT. In contrast, our new opal flux record for Site 511, derived from biogenic opal concentration measurements via spectrophotometry, differs from the published record derived from insoluble residues, whereas new data generated for Site 1090 are generally consistent with previously published flux reconstructions. The South Atlantic biogenic opal flux records, however, are dissimilar from one another, and both are dissimilar from the Southern Indian Ocean records. Also, the taxonomic composition of the diatom assemblages from Sites 511, 748 and 1090 display considerable differences, with hemiauloids and rhizosolenids – generally inferred as indicative of oligotrophic conditions – being the dominant diatoms at the Southern Kerguelen Plateau (Site 748). Published records show that hemiauloid taxa are absent from the earliest Oligocene interval at Site 1090, which is widely seen as a record of deposition from nutrient-rich waters sustaining abundant diatom assemblages. These differences in diatom assemblages testify to regional differences in nutrient concentrations. In particular, the timing of biogenic opal flux pulses between Sites 1090 and 748 imply a shift in the locus of opal deposition to areas further south in the Southern Ocean across the EOT, likely related to proto-ACC development and strengthening of frontal boundaries. Thus, combined geochemical and micropaleontological evidence points to a regionally variable rather than a global, unified opal flux response to climate cooling through the Eocene-Oligocene Transition.

Vertical distribution of nitrogen sources for new and regenerated production in the western Bay of Bengal

The study investigated the new and regenerated production using 15N tracers in the western Bay of Bengal, with observations were made in a vertical water column and sunlight gradients under stratified oligotrophic conditions, which are the major changes that distinguish the hydrobiological parameters associated with the deep chlorophyll maximum (DCM). In this study, new and regenerated production were measured in at different zones such as the surface, the DCM region and the euphotic depth (below 1% of light intensity) during the high stratification period. The results showed that new and regenerated productivity at the surface was low, while higher light intensity decreased productivity at the surface and productivity in the DCM layer increased significantly. Productivity at different location within regions is primarily controlled by the supply of nitrogen nutrients from deep within the region. In contrast, productivity at a given location with the available nutrient pool, at different depths is highly influenced by light availability. The study region exhibited potential for moderate export production during spring inter-monsoon, as is the case in other parts of the Indian Ocean. The f-ratio was estimated to be 0.2, which is a very low value in deep water and plays an important role in the Earth’s carbon cycle, as carbon transported to the deep ocean is absorbed from the atmosphere, and the ocean thus acts as a carbon reservoir. The results also found that the availability of nitrogen nutrients is capable of exporting a significant part of the total production to the deep ocean when conditions are favorable. Our results suggest that the Bay of Bengal may be equally important in its efficiency to export productivity to the deep sea and the DCM layer. Statistical analysis revealed a positive correlation between nitrogen productivity and chlorophyll-a, dissolved inorganic nitrogen and total nitrogen: phosphorous ratio.

Isolation and characterization of bacteria from the gut of a mesopelagic copepod Cephalophanes reflugens (Copepoda: Calanoida)

Abstract The mesopelagic calanoid copepod Cephalophanes reflugens is characterized by its large eyes with semi-parabolic reflectors. It is hypothesized that the eyes aid in finding crustacean carcasses as food in poor deep-sea light and oligotrophic conditions. Here, the crustacean carcasses-associated luminous, chitin-degrading bacteria may play a vital role in detecting and digesting foods by the copepod. In this study, we collected adult female C. reflugens (n = 9) from the mesopelagic layers off southwestern Japan in the Pacific Ocean and obtained a total of 47 bacterial isolates from the gut contents. None of the isolates were luminescent, but almost half of the isolates were chitinolytic. Based on the 16S rRNA gene sequences, these chitinolytic bacteria were identified as members of the genus Vibrio, Photobacterium and Pseudoalteromonas. In culture experiments at 15°C, most of the chitinolytic isolates efficiently utilized colloidal chitin as the sole carbon source and degraded the carapace strip of shrimp (Litopenaeus vannamei). Some isolates showed these chitinoclastic activities even at low temperatures (&amp;lt;5°C), equivalent to that of the deep-sea waters. The present results imply that chitin-degrading bacteria significantly contribute to chitin digestion in the gut of C. reflugens, whereas the involvement of luminous bacteria in the feeding mechanism remains unidentified.

Carbon limitation and aluminium toxicity prevents dominance of Crassula helmsii on weakly buffered soils

Invasions of the aquatic weed Crassula helmsii pose a serious threat to native vegetation of soft water lakes, which increases the need for understanding the factors that regulate the success or failure of C. helmsii. Creating favorable conditions for native species seems promising to control C. helmsii, as previous research indicated that C. helmsii is a poor (root) competitor. We studied the development of C. helmsii growing together with the native Littorella uniflora (biomass, cover, tissue nutrient composition) at two different calcareous poor soil types (organic/acid, mineral/buffered) and under two different water tables (not inundated, submerged) in a greenhouse experiment. We found that when growing under submerged conditions, C. helmsii coexisted with L. uniflora and other native species without becoming dominant due to carbon limitation in the water layer. In contrast, we found that C. helmsii can easily become dominant over L. uniflora when growing on desiccated buffered fen soils with moderate nutrient availability. On the acidic/organic soils, C. helmsii development was poor and die-off was observed under both water level treatments, probably induced due to aluminum toxicity under a low acidity. These results indicate that creating oligotrophic and carbon-poor conditions are required for controlling C. helmsii. Restoration measures can preferably be taken before the onset of the rainy season and/or be followed by maintenance of a temporal artificial high water table to stimulate carbon limitation. Liming more or less organic shores could alleviate aluminum toxicity under acidic conditions which can potentially stimulate the development of C. helmsii.

Responses of a polycyclic aromatic hydrocarbon-degrading bacterium, Paraburkholderia fungorum JT-M8, to Cd (II) under P-limited oligotrophic conditions

For the bioremediation of mixed-contamination sites, studies on polycyclic aromatic hydrocarbon (PAH) degradation or Cd (II) tolerance in bacteria are commonly implemented in nutrient-rich media. In contrast, in the field, inocula usually encounter harsh oligotrophic habitats. In this study, the environmental strain Paraburkholderia fungorum JT-M8 was used to explore the overlooked Cd (II) defense mechanism during PAH dissipation under P-limited oligotrophic condition. The results showed that the growth and PAH degradation ability of JT-M8 under Cd (II) stress were correlated with phosphate contents and exhibited self-regulating properties. Phosphates mainly affected the Cd (II) content in solution, while the cellular distribution of Cd (II) depended on Cd (II) levels; Cd (II) was mainly located in the cytoplasm when exposed to less Cd (II), and vice versa. The unique Cd (II) detoxification pathways could be classified into three aspects: (i) Cd (II) ionic equilibrium and dose–response effects regulated by environmental matrices (phosphate contents); (ii) bacterial physiological self-regulation, e.g., cell surface-binding, protein secretion and active transport systems; and (iii) specific adaptive responses (flagellum aggregation). This study emphasizes the importance of considering culture conditions when assessing the metal tolerance and provides new insight into the bacterial detoxification process of complex PAH-Cd (II) pollutants.