Diatom Growth Research Articles

Bacterial diketopiperazines stimulate diatom growth and lipid accumulation

Diatoms are photosynthetic microalgae that fix a significant fraction of the world's carbon. Because of their photosynthetic efficiency and high-lipid content, diatoms are priority candidates for biofuel production. Here, we report that sporulating Bacillus thuringiensis and other members of the Bacillus cereus group, when in co-culture with the marine diatom Phaeodactylum tricornutum, significantly increase diatom cell count. Bioassay-guided purification of the mother cell lysate of B. thuringiensis led to the identification of two diketopiperazines (DKPs) that stimulate both P. tricornutum growth and increase its lipid content. These findings may be exploited to enhance P. tricornutum growth and microalgae-based biofuel production. As increasing numbers of DKPs are isolated from marine microbes, the work gives potential clues to bacterial-produced growth factors for marine microalgae.

Effects of Nitrogen Sources on the Growth and Biochemical Composition of Diatom (Amphora coffeaeformis) Used for Shrimp Larviculture

Amphora coffeaeformis is a benthic diatom and serves as a rich nutritional feed for various aquaculture industries. The objective of this experiment was to compare the effects of different nitrogen sources on the growth and biochemical composition of the diatom (A. coffeaeformis) culture. Sodium nitrate in Guillard’s f medium, used as a control treatment, was compared with sodium nitrite and ammonium sulfate. The results in the 3rd batch of culture showed that all treatments led to the highest growth on Day 6. The highest cell dry weight occurred in the ammonium sulfate treatment with significance (0.78±0.06 mg/mL). The cultures in each treatment were harvested at the exponential phase (Day 3) for biochemical composition analysis. A. coffeaeformis in the ammonium sulfate treatment significantly accumulated the highest protein content, at 44.82 % by dry weight, while the highest lipid content at 29.78 % by dry weight was significantly found in the sodium nitrate treatment. The cell size and structure showed no significant differences among the treatments. The experiment demonstrated the effects of different nitrogen sources on the growth and biochemical composition of A. coffeaeformis. After that, we selected A. coffeaeformis that was cultured with the nitrate and ammonia nitrogen sources for feeding to shrimp larvae. This experiment was conducted to evaluate the effects of A. coffeaeformis on the growth, survival rate, salinity stress test, and ammonia stress test for shrimp. The experiment was designed with four treatments, as follows: 1) the control feed with flake winner (no water exchange for 28 days), 2) Am-NO3 supplement with A. coffeaeformis cultured using nitrates (no water exchange for 28 days), 3) Am-NH3 supplement with A. coffeaeformis cultured using ammonia (no water exchange for 28 days), and 4) the positive control feed with flake winner (daily water exchange at 20 %). The results showed that the growth of shrimp fed with the diet supplemented with A. coffeaeformis cultured using nitrate and ammonia were not significantly different from the negative and positive controls (P > 0.05). However, the survival rate in the negative control was significantly lower than the other groups (P < 0.05). In this study, the stress test for white shrimp postlarvae utilized 2 methods: the ammonia or salinity stress tests. The survival rate of the shrimp after the ammonia stress test was not statistically and significantly different (P > 0.05). However, the results from the salinity stress test indicated that the highest survival rate was observed in the shrimp fed with the diet supplemented with A. coffeaeformis cultured using nitrate. A. coffeaeformis cultured with the sodium nitrate treatment had a positive effect on shrimp tolerance to salinity changes.

Impact of Increased Nutrients and Lowered pH on Photosynthesis and Growth of Three Marine Phytoplankton Communities From the Coastal South West Atlantic (Patagonia, Argentina)

Effect of global change variables on the structure and photosynthesis of phytoplankton communities was evaluated in three different sites of the Patagonian coast of Argentina: enclosed bay (Puerto Madryn, PM), estuarine (Playa Unión, PU), and open waters (Isla Escondida, IE). We exposed samples to two contrasting scenarios: Present (nutrients at in situ levels) vs. Future (with lowered pH and higher nutrients inputs), and determined growth and photosynthetic responses after 2 days of acclimation. Under the Future condition phytoplankton growth was higher in the estuarine site compared to those in PM and IE. This effect was the most pronounced on large diatoms. While the increase of photosynthetic activity was not always observed in the Future scenario, the lower photosynthetic electron requirement for carbon fixation (Φe,C = ETR/PmB) in this scenario compared to the Present, suggests a more effective energy utilization. Long-term experiments were also conducted to assess the responses along a 4 days acclimation period in PU. Diatoms benefited from the Future conditions and had significantly higher growth rates than in the Present. In addition, Φe,C was lower after the acclimation period in the Future scenario, compared to the Present. Our results suggest that the availability, frequency and amount of nutrients play a key role when evaluating the effects of global change on natural phytoplankton communities. The observed changes in diatom growth under the Future scenario in PU and IE and photosynthesis may have implications in the local trophodynamics by bottom up control.

Spatiotemporal dynamics of succession and growth limitation of phytoplankton for nutrients and light in a large shallow lake

Understanding the limiting factors of phytoplankton growth and competition is crucial for the restoration of aquatic ecosystems. However, the role and synergistic effect of co-varying environmental conditions, such as nutrients and light on the succession of phytoplankton community remains unclear. In this study, a hydrodynamic-ecological modeling approach was developed to explore phytoplankton growth and succession under co-varying environmental conditions (nutrients, total suspended solids (TSS) and variable N:P ratios) in a large shallow lake called Lake Chagan, in Northeast China. A phytoplankton bloom model was nested in the ecological modeling approach. In contrast to the traditonal ecological modeling, competition between phytoplankton species in our study was modeled at both the species/functional group and phenotype levels. Six phytoplankton functional groups, namely diatoms, green algae, Anabaena, Microcystis, Aphanizomenon and Oscillatoria and each of them with three limitation types (i.e., light-limitation, nitrogen-limitation and phosphorus-limitation) were included in the bloom model. Our results demonstrated that the average biomass proportion of the three limitation types (light-limitation, nitrogen-limitation and phosphorus-limitation) in the six phytoplankton function groups accounted for approximately 50%, 37% and 23% of the total phytoplankton biomass, respectively. TSS suppressed the growth of diatoms and green algae, but favored the dominance of cyanobacteria in Lake Chagan, especially in the turbid water phase (TSS ≥ 60 mg/L). In addition, it was reported that the potential of either N-fixing or non-N-fixing cyanobacterial blooming along the gradients of N:P ratios could exist under the influence of the co-environmental factors in the lake. The proportion of non-N-fixing cyanobacteria (i.e., Microcystis and Oscillatoria) exceeded the proportion of N-fixing cyanobacteria (i.e., Anabaena and Aphanizomenon) when the N:P ratios exceeded 20. Non-N-fixing cyanobacteria would become dominant at higher TSS concentrations and lower light intensities in the turbid water. N-fixing cyanobacteria favored lower N:P ratios and higher light intensities in the clearwater phase (where TSS ≤ 60 mg/L). To sustain a good ecological status in the lake, our results suggest that nutrient and TSS levels in the lake should be maintained at or below the thresholds (TN ≤ 1.5 mg/L; TP ≤ 0.1 mg/L; N:P ratios between 15 and 20; and TSS ≤ 60 mg/L). These findings can help improve water quality management practices to restore aquatic ecosystems.

Kuroshio intrusion drives the Trichodesmium assemblage and shapes the phytoplankton community during spring in the East China Sea

To understand the influence of Kuroshio intrusion on the phytoplankton community, a field investigation was conducted in spring 2017 in the East China Sea (ECS), and 130 seawater samples were collected and analyzed. Trichodesmium comprised the highest cell abundance contributing about 66% of the total phytoplankton followed by diatoms (17%) and dinoflagellates (16%). The dominance of the Kuroshio Waters (KW) and the Taiwan Warm Currents (TWC) were higher than the Coastal Waters (CW). The vertical distribution of physicochemical parameters depicted the intrusion of KW at the bottom layer, but it failed to reach the surface as strong upwelling was not initiated. Therefore, the dissolved inorganic phosphate (DIP) concentrations and P/N ratios were the lowest in the CW and the upper water layers, which limited the diatom growth in this area. Besides, the dinoflagellates cell abundance was also lower except in the surface and CW, though they comprised the maximum richness of species among the phytoplankton community. However, the unique characteristics such as diazotrophy and gas vacuoles of Trichodesmium made the situation advantageous, and they comprised the maximum cell abundance in this area especially in KW and the TWC. Temperature, DIP and P/N ratios appeared to be the major environmental drivers for Trichodesmium proliferation in the ECS during the study period.

Improvement of water quality using a circulation device equipped with oxidation carriers and light emitting diodes in eutrophic pond mesocosms

To enhance water quality in eutrophic urban ponds, the effects of a portable water circulation device, equipped with biological oxidation carriers and light-emitting diodes (LEDs), on dissolved oxygen (DO), water transparency, total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and plankton community in laboratory and field mesocosms were examined. In the laboratory, water quality was monitored in vessels containing water and mud collected from an urban pond, which were subjected to different treatments (control, device without LEDs, and device with LEDs) for 28 days. In the treatments using the device (with and without LEDs), DO increased by aeration, whereas TP decreased by mitigation of mud anoxia. Reductions in TN and the presence of phyto- and zooplankton were observed in those treatments with LEDs, in which periphyton growth was stimulated in the carriers. Under field conditions, water quality of the pond was monitored for 40 days in enclosures. Clear increases in DO and transparency and reductions in TN, TP, and COD were detected in the treatments using the device. Reductions in cyanobacteria and increases in diatoms, green algae, and zooplankton were observed in the treatments with LEDs. Thus, the circulation device effectively improved oxygen condition of the bottom and nutrient flux between sediment and overlying water. In addition, these results suggest that the COD removal and growth of diatoms and green algae by LEDs in carriers play roles on developing periphyton at the bottom and suitable phytoplankton community in the water column.

Growth Rate-dependent Cell Death of Diatoms due to Viral Infection and Their Subsequent Coexistence in a Semi-continuous Culture System.

Viral infections are a major factor in diatom cell death. However, the effects of viruses on diatom dynamics remain unclear. Based on laboratory studies, it is hypothesized that virus-induced diatom mortality is dependent on the diatom growth rate. The present study aimed to elucidate the relationship between the diatom growth rate and virus-induced mortality using model systems of the marine planktonic diatom, Chaetoceros tenuissimus and its infectious viruses. We also examined the fate of diatom populations in a semi-continuous dilution culture system, in which host growth rates were controlled at 0.69, 2.08, and 3.47 day–1. Diatom populations gradually decreased following the viral inoculation of each culture system, and virus-induced mortality inversely correlated with the diatom growth rate. Furthermore, the viral burst size was slightly higher in lower growth rate cultures. These results suggested that the host physiological status related to the growth rate affected viral infection and proliferation. Diatom populations were not completely lysed or washed out in any of the dilution systems; they showed steady growth in the presence of infectious viruses. This may be partially explained by defective interference particles from viruses and cell debris. The present results indicate that diatoms in dilution environments maintain their populations, even under viral pressure. Moreover, diatom populations with a low growth rate may partially sustain higher growth populations through nutrient recycling following virus-induced cell death. The results of the present study provide insights into diatom dynamics in natural environments in the presence of infectious viruses.

Atmospheric Processing of Iron-Bearing Mineral Dust Aerosol and Its Effect on Growth of a Marine Diatom, Cyclotella meneghiniana

Iron (Fe) is a growth-limiting micronutrient for phytoplankton in major areas of oceans and deposited wind-blown desert dust is a primary Fe source to these regions. Simulated atmospheric processing of four mineral dust proxies and two natural dust samples followed by subsequent growth studies of the marine planktic diatom Cyclotella meneghiniana in artificial sea-water (ASW) demonstrated higher growth response to ilmenite (FeTiO3) and hematite (α-Fe2O3) mixed with TiO2 than hematite alone. The processed dust treatment enhanced diatom growth owing to dissolved Fe (DFe) content. The fresh dust-treated cultures demonstrated growth enhancements without adding such dissolved Fe. These significant growth enhancements and dissolved Fe measurements indicated that diatoms acquire Fe from solid particles. When diatoms were physically separated from mineral dust particles, the growth responses become smaller. The post-mineralogy analysis of mineral dust proxies added to ASW showed a diatom-induced increased formation of goethite, where the amount of goethite formed correlated with observed enhanced growth. The current work suggests that ocean primary productivity may not only depend on dissolved Fe but also on suspended solid Fe particles and their mineralogy. Further, the diatom C. meneghiniana benefits more from mineral dust particles in direct contact with cells than from physically impeded particles, suggesting the possibility for alternate Fe-acquisition mechanism/s.

Анализ состава фитопланктонных микроводорослей различных акваторий Черного моря вблизи полуострова Крым

Preliminary studies of cultivated phytoplankton microalgae were carried out in water samples collected during four expeditions at oceanographic stations of the coastal waters of the northwestern and northeastern parts of the Black Sea in different seasons of 2019-2020. Some features of the development of the algological component in conditions of changing oceanological characteristics of the environment have been established. In spring, the most active growth of diatoms was revealed in the area from Balaklava to the water area of the Sevastopol Bay.

Toxicity Going Nano: Ionic Versus Engineered Cu Nanoparticles Impacts on the Physiological Fitness of the Model Diatom Phaeodactylum tricornutum

Increasing input of Metal Engineered Nano Particles (MeENPs) in marine ecosystems has raised concerns about their potential toxicity on phytoplankton. Given the lack of knowledge on MeENPs impact on these important primary producers, the effects of Copper Oxide (CuO) ENPs on growth, physiology, pigment profiles, fatty acid (FA) metabolism, and oxidative stress were investigated in the model diatom Pheodactylum tricornutum, to provide suitable biomarkers of CuO ENP exposure versus its ionic counterpart. Diatom growth was inhibited by CuO ENPs but not Ionic Cu, suggesting CuO ENP cytotoxicity. Pulse Modulated Amplitude (PAM) phenotyping evidenced a decrease in the electron transport energy flux, pointing to a reduction in chemical energy generation following CuO ENPs exposure, as well as an increase in the content of the non-functional Cu-substituted chlorophyll a (CuChl a). A significant decrease in eicosapentaenoic acid (C20:5) associated with a significant rise in thylakoid membranes FAs reflected the activation of counteractive measures to photosynthetic impairment. Significant increase in the omega 6/omega 3 ratio, underline expectable negative repercussions to marine food webs. Increased thiobarbituric acid reactive substances reflected heightened oxidative stress by CuO ENP. Enhanced Glutathione Reductase and Ascorbate Peroxidase activity were also more evident for CuO ENPs than ionic Cu. Overall, observed molecular changes highlighted a battery of possible suitable biomarkers to efficiently determine the harmful effects of CuO ENPs. The results suggest that the occurrence and contamination of these new forms of metal contaminants can impose added stress to the marine diatom community, which could have significant impacts on marine ecosystems, namely through a reduction of the primary productivity, oxygen production and omega 6 production, all essential to sustain heterotrophic marine life.

Testing the reliable proxies to understand the mid-Holocene climate variability records from Chandratal lake, Western Himalayas

Chandratal lake (32°28′30.65″ N 77°37′1.42″ E) located at the junction of Indian Summer Monsoon and Westerlies in the Northwestern Himalaya give an opportunity to reconstruct climate variability over the last ~6300 cal yr BP. Here, we used multi-proxy (Amino acids, Biogenic silica, grain size and total organic carbon) to link climate change and organic burial during the mid-Holocene period. The proxy records captured the changes in monsoon patterns, linking with Holocene climate optimum (HCO) during the ~6344- 5821 cal BP. The sediment profile begins with a wetter and moist climate mostly from the terrestrial environment inputs, corresponding to the warm Holocene Climate Optimum (HCO) period (~6344 - 5821 cal yr BP). A stable condition of the geochemical proxies during ~5821-3780 cal yr BP reveals a shifting of a moisture source, catchment stability and cold/dry climate in correspondence with reduced precipitation.The period from ~3780 to 2129 cal yr BP, follows a gradual increase of organic matter deposition and supported with an elevated atomic C/N ratio of 13.1 (mean value). Further association of organic matter with the coarser grain particles, suggesting a wetter climate from increasing runoff which is in correlation with the reinforcement of the precipitation during this time interval. From ~2129 to 696 cal yr BP, reduced precipitation/dry condition was observed with a shift in the autochthonous production of organic matter (C/N ratio of 8.91). Increase in BSi content suggests an ameliorated climate at this interval; however, the lower organic matter content (mean, 1.79%) and shift in the correlation of organic matter from coarse grains to finer grain size, suggesting little contributions from the terrestrial activities favouring the dry environment and abundant diatoms growth. The paleoenvironmental variations illustrate in the study is comparable to other findings recorded in the Himalayan region.

Comparative Proteomic Analysis Reveals New Insights Into the Common and Specific Metabolic Regulation of the Diatom Skeletonema dohrnii to the Silicate and Temperature Availability.

Silicate (Si) and temperature are essential drivers for diatom growth and development in the ocean. Response of diatoms to these particular stress has been investigated; however, their common and specific responses to regulate intracellular development and growth are not known. Here, we investigated the combination of physiological characteristics and comparative proteomics of the diatom Skeletonema dohrnii grown in silicate- and temperature-limited conditions. Results show that cell carbon and lipid quotas were higher at lower-temperature cells, whereas cellular phosphate was higher in cells grown with lower Si. In silicate-limited cells, nitrate transporters were downregulated and resulted in lower nitrate assimilation, whereas the phosphate transporters and its assimilation were reduced in lower-temperature conditions. In photosynthesis, lower silicate caused impact in the linear electron flow and NADPH production, whereas cycling electron transport and ATP production were affected by the lower temperature. Concerning cell cycle, imbalances in the translation process were observed in lower-silicate cells, whereas impact in the transcription mechanism was observed in lower-temperature cells. However, proteins associated with carbon fixation and photorespiration were downregulated in both stress conditions, while the carbohydrate and lipid synthesis proteins were upregulated. Our results showed new insights into the common and specific responses on the proteome and physiology of S. dohrnii to silicate and temperature limitation, providing particular nutrient (Si)- and temperature-dependent mechanisms in diatoms.

Environmentally relevant concentrations and sizes of microplastic do not impede marine diatom growth

The current knowledge about the ecological effects of microplastic (MP) remains limited, and to-date ecotoxicity tests often utilize standard microplastic with one or two distinct size classes and expose the organisms to unrealistically high MP concentrations. We exposed the marine diatom Phaeodactylum tricornutum to microplastic particles of a mimicked realistic size frequency distribution complemented with serial experiments with distinct size classes. To do so, we exposed this diatom to a concentration series of different sized polyethylene (PE) microbeads (sizes: 10–106 µm; 1.25 ×102–1.25 ×107 particles/L) in a 72-h growth inhibition test. No effect on the growth of P. tricornutum by virgin PE microbeads up to 1.25 × 107 particles/L (or 499 mg/L), indicating environmentally relevant concentrations and sizes of MP does not alter the growth of marine diatoms. Results of smaller sized MPs (10–20 µm) did not differ from those obtained with larger MPs (90–106 µm) and mix sized MPs (10–106 µm), i.e. no impact on the microalgae growth. As a pioneer work, our results contribute with high quality dose-response data to an improved risk assessment of microplastic under realistic present and future marine MP pollution.

Recent climate-driven ecological changes in tropical montane lakes of Rwenzori Mountains National Park, central Africa

Rwenzori Mountains National Park, which straddles the border between the Democratic Republic of Congo and Uganda, has experienced rapid glacier loss since the beginning of the twentieth century, yet there has been little investigation of aquatic biodiversity change in the park. This study presents a paleolimnological analysis from Lake Mahoma (2990 m asl), which is situated in the bamboo-forest transition zone. Diatom and organic geochemistry data from a 39-cm-long sediment core with a basal age of c. 1715 CE were compared with new analyses of previously published data from Lakes Bujuku (3891 m asl) and Lower Kitandara (3989 m asl), in the alpine zone. Comparisons were made to determine if aquatic ecosystem changes exhibited similar inter-lake patterns over the past ~ 150 years of climate warming and glacial recession, or if only local change was apparent. The diatom flora of Lake Mahoma is acidophilous, dominated by Aulacoseira ikapoënsis since at least the mid eighteenth century. In recent decades, the obligate nitrogen-heterotroph Nitzschia palea increased in importance, concurrent with declining δ15Norg values. We suggest that these late twentieth century changes were linked to regional warming and increased thermal stratification of Lake Mahoma. Regional comparisons of the Rwenzori lakes were done using existing organic geochemistry records (total organic carbon, C/N and δ13Corg) and through diatom compositional turnover analyses, and categorisation of species into one of four diatom growth morphology traits, or guilds: tychoplanktonic, high-profile, low-profile and motile. Over the past 150 years, all three lakes showed unidirectional, compositional diatom turnover, indicating that deterministic processes had affected diatom communities. Declining turnover at each site is broadly mirrored by an increase in tychoplanktonic taxa, along with concomitant declines in high-profile diatoms at Lake Mahoma, and low-profile diatoms at Lake Bujuku, and at least for the past 60 years, at Lower Kitandara. The interplay between diatom guilds at all sites is mainly a consequence of competition for available resources. Sediment organic carbon at all sites comes from both autochthonous and allochthonous sources, the relative abundances of which are influenced by the time elapsed since lakes had glaciers in their catchment.

Potential antifouling properties of copper loaded zeolites on fouling diatoms

Biofouling is a phenomenon affecting all the infrastructures immerged in marine and freshwater environments. This colonization leads to a deterioration of the surfaces implied and affects the performances of the different structures as the decrease of ship speed or the increase of fuel consumption. Polemics around harmful antifouling coatings being more and more concerning, an alternative to toxic compounds is here proposed. The effect of three different zeolites, pure or copper loaded, is evaluated on two fouling diatoms, Cylindrotheca closterium and Amphora sp. These organisms display different sensitivities to copper usually used in antifouling paint. Finally, it has been shown that pure zeolites had stimulating effect on microalgae growth while copper loaded ones seemed efficient to inhibit their development. However, natural zeolite appeared less effective than the two synthetic after 7 days incubation. These differences seemed to be due to the bonds created by copper in the materials and to the leaching of metallic ions. Finally, the constant release of active agent in the medium by the minerals appeared to be the key for an interesting application in antifouling systems. • Loading of different natural and synthetic zeolites with copper at 5 wt%. • Preservation of typical zeolite's structure. • Bioassays on Cylindrotheca closterium and Amphora sp. • Pure zeolite enhanced both diatoms growth. • Copper doped materials are efficient on both diatoms.

Effect of diatom abundance and biogenic silica availability on the population growth of tintinnid ciliates at Suruga Bay

The effects of diatom abundance and biogenic silica (BSi) deposition on the population growth of tintinnid ciliates were investigated using incubations of sub-surface waters (10 m depth) of Suruga Bay, Japan in April and November 2018. Seawater for incubation was pre-filtered using a 200-µm mesh to remove mesozooplankton grazers. Initial concentrations of dissolved inorganic nitrogen were similar in both seasons, but silicic acid and phosphate were higher in April than in November. This allowed us to observe the growth of planktonic communities under varied silicic acid availability and explore the effects of BSi accumulation and food availability on the growth of tintinnids. Phytoplankton blooms dominated by diatoms formed in both, April and November incubations, but their biomass was higher in April than in November, demonstrating the effects of nutrients, particularly silicic acid, on diatom growth. The BSi concentration increased in both incubations but was higher in April than in November, confirming the effects of diatom abundance and size on BSi deposition. Tintinnid biomass also increased in both incubations but was higher in April than in November, showing that the biomass of tintinnid was affected by the BSi concentration. Species composition of diatoms affected the species composition of tintinnids by providing selected material for the construction of their lorica. Moreover, the results showed that the food supply mainly provided by pico-nanoflagellates via microbial loop and detritus may limit tintinnids’ population growth in natural planktonic ecosystems and this demonstrates tintinnids’ role as top predators in the microbial loop

Comparison of phytoplankton community structure in two tropical estuaries of East Coast of India

Phytoplankton community of two tropical river estuaries of the North-east coast of India was different due to dissolved nutrients concentration in those estuarine waters. The first study site was the Saptamukhi river estuary which is located in the Indian Sundarban (inside mangrove forest) and the second study site was Mahanadi estuary located in Orissa coast (adjacent to industrial and coastal fishing zone). The Saptamukhi estuary was mostly devoid of any anthropogenic influence and here the nutrient source was auto-generated. In Mahanadi estuarine water, the source of the major nutrients was industrial effluent and fishing waste. The Saptamukhi estuarine water had less dissolved phosphorus concentration but huge dissolved nitrogen available for phytoplankton uptake. A sufficient amount of dissolved silicate encouraged the diatom growth over other phytoplankton groups here. Due to the availability of the high amount of dissolved phosphorus and the low amount of dissolved nitrogen in the Mahanadi estuary the phytoplankton community was a mixed population of Bacillariophyceae, Cyanophyceae, Chlorophyceae and Dinophyceae and seasonal Chlorophycean bloom observed during the study period. In both the estuarine water primary productivity was high but the community respiration was higher and the estuaries were heterotrophic. Monsoonal runoff from land considerably changed the community in both estuarine water. Dissolved silicate concentration in both the estuarine water was sufficient for Diatom growth. Dissolve inorganic nitrogen and dissolved inorganic phosphate ratio played a major role for the community change of phytoplankton in two estuarine waters.

A margin of safety approach for the assessment of environmentally realistic chemical mixtures in the marine environment based on combined passive sampling and ecotoxicity testing

Organisms in the marine environment are being exposed to an increasing variety of chemicals. This research presents an effect-based monitoring method for the derivation of a margin of safety for environmentally realistic chemical mixtures. The method is based on a combination of passive sampling and ecotoxicity testing. First, passive sampling was performed using H2O-philic divinylbenzene Speedisks during 3 sampling campaigns between 2016 and 2018 at 4 sampling locations in the Belgian part of the North Sea. Next, we exposed the marine diatom Phaeodactylum tricornutum to Speedisk extracts that were reconstituted in HPLC-grade water and defined the MoS of each sample as the highest no-observed effect concentration, expressed as relative enrichment factor (REF). A REF was defined by comparing the concentrations of 89 personal care products, pesticides and pharmaceuticals in the biotest medium with those measured in water grab samples to relate exposure concentrations in the tests to environmental concentrations. Across eight marine samples, diatom growth inhibition was observed at REF ≥ 3.2 and margins of safety were found between REF 1.1–11.0. In addition, we found that reconstitution of extracts in HPLC-water was suitable to overcome the solvent-related challenges in biotesting that are usually associated with passive sampler extract spiking, whilst it still allowed REFs up to 44 in the biotest medium to be achieved. This method, however, likely covers mainly the polar fraction of environmentally realistic chemical mixtures and less the non-polar fraction. Nevertheless, for 5 out of 8 samples, the Margin of Safety (MoS) was found to be lower than 10, which represents the typically lowest possible assessment factor applied to no effects ecotoxicological data in conventional environmental risk assessments, suggesting ecological risks for these samples.

Evidence of Interdomain Ammonium Cross-Feeding From Methylamine- and Glycine Betaine-Degrading Rhodobacteraceae to Diatoms as a Widespread Interaction in the Marine Phycosphere.

Dissolved organic nitrogen (DON) compounds such as methylamines (MAs) and glycine betaine (GBT) occur at detectable concentrations in marine habitats and are also produced and released by microalgae. For many marine bacteria, these DON compounds can serve as carbon, energy, and nitrogen sources, but microalgae usually cannot metabolize them. Interestingly though, it was previously shown that Donghicola sp. strain KarMa—a member of the marine Rhodobacteraceae—can cross-feed ammonium such that the ammonium it produces upon degrading monomethylamine (MMA) then serves as nitrogen source for the diatom Phaeodactylum tricornutum; thus, these organisms form a mutual metabolic interaction under photoautotrophic conditions. In the present study, we investigated whether this interaction plays a broader role in bacteria–diatom interactions in general. Results showed that cross-feeding between strain KarMa and P. tricornutum was also possible with di- and trimethylamine as well as with GBT. Further, cross-feeding of strain KarMa was also observed in cocultures with the diatoms Amphora coffeaeformis and Thalassiosira pseudonana with MMA as the sole nitrogen source. Regarding cross-feeding involving other Rhodobacteraceae strains, the in silico analysis of MA and GBT degradation pathways indicated that algae-associated Rhodobacteraceae-type strains likely interact with P. tricornutum in a similar manner as the strain KarMa does. For these types of strains (such as Celeribacter halophilus, Roseobacter denitrificans, Roseovarius indicus, Ruegeria pomeroyi, and Sulfitobacter noctilucicola), ammonium cross-feeding after methylamine degradation showed species-specific patterns, whereas bacterial GBT degradation always led to diatom growth. Overall, the degradation of DON compounds by the Rhodobacteraceae family and the subsequent cross-feeding of ammonium may represent a widespread, organism-specific, and regulated metabolic interaction for establishing and stabilizing associations with photoautotrophic diatoms in the oceans.

Under-ice mesocosms reveal the primacy of light but the importance of zooplankton in winter phytoplankton dynamics.

Factors that regulate planktonic communities under lake ice may be vastly different from those during the open‐water season. Expected changes in light availability, ice cover, and snowfall associated with climate change have accelerated the need to understand food web processes under ice. We hypothesized that light limitation (bottom‐up control) outweighs zooplankton grazing (top‐down control) influence on phytoplankton biovolume and community structure under ice in a north temperate lake. Using in situ under‐ice mesocosm experiments, we found that light had stronger effects on phytoplankton abundance than zooplankton, as expected. Specifically, low light limited growth of diatoms, cryptophytes, and chrysophytes. Zooplankton, however, also significantly affected some individual phytoplankton groups by decreasing diatoms and cryptophytes, in contrast to the common assumption that zooplankton grazing has negligible effects under ice. Ammonium and soluble reactive phosphorus (SRP) were lowest in high light treatments presumably through uptake by phytoplankton, whereas ammonium and SRP were highest in high zooplankton treatments, likely a result of zooplankton excretion. In situ experimental studies are commonly applied to understand food web dynamics in open‐water conditions, but are extremely rare under ice. Our results suggest that changes in the light environment under ice have significant, rapid effects on phytoplankton growth and community structure and that zooplankton may play a more active role in winter food webs than previously thought. Changes in snow and ice dynamics associated with climate change may alter the light environment in ice‐covered systems and significantly influence community structure.