The 24 hour recovery kinetics from n starvation in Phaeodactylum tricornutum and Emiliania huxleyi

The adsorption and absorption kinetics of BDE-47 by Chlorella sp. and the role of extracellular polymer substances influenced by environmental factors

Microalgae act as the entrance of polybrominated diphenyl ethers (PBDEs) from abiotic to biotic environments, which controlled the environmental fate of PBDEs in aquatic environments. Combing with typical coastal environmental characteristics including extracellular polymer substances (EPS) enrichment, light limitation and nitrogen starvation, the changes of adsorption and absorption kinetics of BDE-47 by Chlorella sp. and the role of EPS therein were investigated. The results quantified the adsorption and absorption kinetics of BDE-47 by Chlorella sp. cells and fitted it by the Lagergren pseudo first order model. Furthermore, we found the adsorption and absorption kinetics could be changed by the above mentioned environmental factors. To be specific, the total BDE-47 adsorption amounts per microalgal cell were increased as the increase of ambient EPS (proteins or carbohydrates), attributing to the increase of soluble (SL)-EPS contents; increased total BDE-47 adsorption amounts but decreased absorption rates were found under light limitation and nitrogen starvation, which were attributed to increased bound (B)-EPS contents and protein/carbohydrates (P/C) ratios therein, respectively. Therefore, our study elucidated the adsorption and absorption kinetics of PBDEs by microalgae could be influenced by ambient environmental changes, clarified the roles of SL-EPS, B-EPS contents and P/C ratios, providing a solid basis for evaluating the environmental fate of PBDEs in the marine environments.

Toxicity of 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) on the green microalgae Chlorella sp. and the role of cellular oxidative stress

Polybrominated diphenyl ethers (PBDEs) are toxic to marine organisms including the major primary producer phytoplankton, while the toxic mechanisms haven't yet been fully clarified. Therefore, we comprehensively studied the toxic mechanisms of BDE-47 on the marine chlorophyte Chlorella sp., with a focus on the role of cellular oxidative stress. The results indicate that BDE-47 stress resulted in the inhibition of population growth as well as cell death and programmed cell death. The antioxidant system was activated in both low and high BDE-47 treatments, but only microalgal cells in the high BDE-47 treatment showed cellular oxidative stress. By adding ROS inhibitor, the relief of photosynthetic inhibition, Ca²⁺ overproduction and cell death was found. Therefore, we conclude that photosynthetic damage, cell death and cellular oxidative stress were the major mechanisms of BDE-47 toxicity to Chlorella sp., and that cellular oxidative stress played an important role in mediating the other mechanisms.

Nitrogen Limitation Decreases the Repair Capacity and Enhances Photoinhibition of Photosystem II in a Diatom

Macronutrient limitation and increased solar exposure coincide with ocean warming-enhanced stratification, with consequences for phytoplankton within the upper mixing layer. In this study, we grew a diatom, Thalassiosira punctigera, under nitrogen-limited and replete conditions for more than 14 generations and investigated both the biochemical composition of treated cells and their photochemical responses to high light and UV exposure. The photosynthetic pigment and the particulate organic nitrogen (PON) content significantly decreased in the low nitrate grown cells, with drastic decline of the absorption of UV absorbing compounds. Under an acute exposure to high light or UV radiation, we observed a significant decline in the photochemical yield along with an increase of nonphotosynthetic quenching (NPQ), with the former lowered and the latter raised in the low-nitrogen grown cells. The results reveal a decreased repair rate and enhanced photoinhibition of the diatom under nitrogen limitation when exposed to increased levels of light and UV radiation, suggesting a higher vulnerability of the diatom phytoplankton under influences of oceanic global change.

The toxic mechanisms of BDE-47 to the marine diatom Thalassiosira pseudonana-a study based on multiple physiological processes

Polybrominated diphenyl ethers (PBDEs), a series of highly persistent organic pollutants (POPs), are ubiquitous in marine ecosystems. As key primary producers, microalgae are of great importance on evaluating the environmental outcome of PBDEs pollution. In this study, the toxic mechanisms of BDE-47 on the marine diatom Thalassiosira pseudonana were evaluated by measuring multiple physiological processes. Three concentrations of BDE-47 (25, 15 and 5 μg L^-1) were used along with two controls (blank: no BDE-47 or DMSO; negative control: only DMSO). Experiments lasted 144 h (6 days), in which the actual BDE-47 concentrations, cell densities, nutrient (nitrate and phosphate) uptake, pigment compositions, photosynthetic physiology, cell morphology and cellular contents (organic carbon and nitrogen) were measured at 12-48 h intervals. The toxic mechanisms of BDE-47 on T. pseudonana cells were evaluated by measuring multiple physiological processes including photosynthesis, nutrient uptake, cellular material synthesis and cell cycle progressions. The cell divisions of T. pseudonana were severely inhibited by the stress of BDE-47, but the photosynthetic parameters were much less declined and recovered earlier than the cell divisions in the same BDE-47 treatments. The unsuppressed uptake rates of nutrients, increased cell volume and cellular contents indicated the cellular material synthesis proceeded normally. Finally, we found that the cell cycle was arrested in G2/M phase under the stress of BDE-47, we thus concluded that the inhibition of cell divisions by BDE-47 was not due to the lack of energy or cellular materials, where the cell cycle arrest happened; this might be the most important toxicological outcome.

Response of photosynthesis and the antioxidant defense system of two microalgal species (Alexandrium minutum and Dunaliella salina) to the toxicity of BDE-47

Polybrominated diphenyl ethers (PBDEs), a persistent organic pollutant are ubiquitous in aquatic ecosystems, which are causing serious environmental concerns. In this study, we chose BDE-47 as a representative PBDEs, to investigate its toxic effects on two microalgal species and the response of their antioxidant system. The results indicated Alexandrium minutum (a dinoflagellate) was more sensitive to BDE-47 than Dunaliella salina (a chlorophyte), as determined by growth rates, cellular structure and photosynthetic parameters. Cellular reactive oxygen species (ROS) levels were significantly elevated under the exposure of BDE-47 in both species, corresponding to an increase of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) activities, while glutathione peroxidase (GPX) activities decreased in D. salina and increased in A. minutum. The different enzymes responses between the two species indicated different mechanisms in their antioxidant system, and we deduced that A. minutum might have a higher efficiency for scavenging H₂O₂ than D. salina.

Effect of CO2-induced seawater acidification on growth, photosynthesis and inorganic carbon acquisition of the harmful bloom-forming marine microalga, Karenia mikimotoi

Karenia mikimotoi is a widespread, toxic and non-calcifying dinoflagellate, which can release and produce ichthyotoxins and hemolytic toxins affecting the food web within the area of its bloom. Shifts in the physiological characteristics of K. mikimotoi due to CO2-induced seawater acidification could alter the occurrence, severity and impacts of harmful algal blooms (HABs). Here, we investigated the effects of elevated pCO2 on the physiology of K. mikimotoi. Using semi-continuous cultures under controlled laboratory conditions, growth, photosynthesis and inorganic carbon acquisition were determined over 4-6 week incubations at ambient (390ppmv) and elevated pCO2 levels (1000 ppmv and 2000 ppmv). pH-drift and inhibitor-experiments suggested that K. mikimotoi was capable of acquiring HCO3-, and that the utilization of HCO3- was predominantly mediated by anion-exchange proteins, but that HCO3- dehydration catalyzed by external carbonic anhydrase (CAext) only played a minor role in K. mikimotoi. Even though down-regulated CO2 concentrating mechanisms (CCMs) and enhanced gross photosynthetic O2 evolution were observed under 1000 ppmv CO2 conditions, the saved energy did not stimulate growth of K. mikimotoi under 1000 ppmv CO2, probably due to the increased dark respiration. However, significantly higher growth and photosynthesis [in terms of photosynthetic oxygen evolution, effective quantum Yield (Yield), photosynthetic efficiency (α), light saturation point (Ek) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity] were observed under 2000 ppmv CO2 conditions. Furthermore, elevated pCO2 increased the photo-inhibition rate of photosystem II (β) and non-photochemical quenching (NPQ) at high light. We suggest that the energy saved through the down-regulation of CCMs might lead to the additional light stress and photo-damage. Therefore, the response of this species to elevated CO2 conditions will be determined by more than regulation and efficiency of CCMs.

Effect of various nitrogen conditions on population growth, temporary cysts and cellular biochemical compositions of Karenia mikimotoi

The harmful algal bloom (HAB)-forming dinoflagellate Karenia mikimotoi was exposed to different nitrogen (N) conditions, in order to study the population growth, temporary cyst production and cellular biochemical compositions in laboratory. The results indicated the population growth of K. mikimotoi was inhibited by different levels of N starvation but showed similar fast recovery after the resupplement of N, and temporary cysts were induced in the period of N starvation. K. mikimotoi grew well in inorganic (NO3-, NO2- and NH4+) and organic (urea) nitrogen sources, but the growth parameters (K, Tp, r) showed differences when simulated by Logistic model regressions. When the cellular organic compounds were measured simultaneously, K. mikimotoi cultured in urea produced more short-chained fatty acids while K. mikimotoi cultured in NH4+ produced more non-fatty acids compounds, indicating the potential change of toxins production cultured by various N sources. We concluded that K. mikimotoi could adapt to fluctuating N environments typical of coastal environments including total N concentration (deficiency or recovery) and relative compositions (different N sources).

Application of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater treatment

Ammonium ([Formula: see text]-N)-rich wastewater, a main cause for eutrophication, can serve as a promising medium for fast microalgae cultivation with efficient [Formula: see text]-N removal. To achieve this goal, a well-controlled three-stage treatment process was developed. Two trophic modes (mixotrophy and heterotrophy) in Stage 1 and Stage 2, with two nitrogen availability conditions (N sufficient and N deprived) in Stage 2, and different [Formula: see text]-N concentrations in Stage 3 were compared to investigate the effects of nitrogen sufficiency conversion on indigenous strain UMN266 for [Formula: see text]-N removal. Results showed that mixotrophic cultures in the first two stages with N deprivation in Stage 2 was the optimum treatment strategy, and higher [Formula: see text]-N concentration in Stage 3 facilitated both microalgal growth and [Formula: see text]-N removal, with average and maximum biomass productivity of 55.3 and 161.0 mg L(-1) d(-1), and corresponding removal rates of 4.2 and 15.0 mg L(-1) d(-1), respectively, superior to previously published results. Observations of intracellular compositions confirmed the optimum treatment strategy, discovering excellent starch accumulating property of strain UMN266 as well. Combination of bioethanol production with the proposed three-stage process using various real wastewater streams at corresponding stages was suggested for future application.