Effect of Hg2+ on the microphysical and chemical properties of oil-producing Nannochloropsis sp

Light intensity can significantly regulate cadmium transformation into CdS nanoparticles in microalgae (Dunaliella salina and Phaeodactylum tricornutum)

Light is a critical factor influencing algal growth and contributes to the uptake of metal elements by algae. However, the impact of light on the bioavailability and transformation of heavy metals requires further exploration, particularly in the context of bioremediation efforts. This study explores how varying light intensities (1000, 2000, and 3000 lux) influence the ability of these algae to absorb Cd, distribute it within cells, and transform Cd (II) into CdS NPs. By using ICP-MS, it was found that increasing the light intensity to 2000 lux could increase the Cd uptake capacity of Dunaliella salina and Phaeodactylum tricornutum by 28 % and 14 %, respectively. Changes in the percentage of Cd (II) in each component (medium, intracellular, and adsorption on cell surface) with the different light intensities supported the interpretation that the increase in Cd uptake by algal cells was a result of increased cellular adsorption and accumulation. Further analyses by HRTEM-EDS and SEC-ICP-MS showed that increasing light intensity not only influenced the size of CdS NPs but also significantly enhanced the algae's efficiency in transforming Cd(II) into CdS NPs. It is found that the transformation ratio of CdS NPs by D. salina and P. tricornutum increased to 16 % and 52 % respectively, after 10 days of Cd exposure under 2000 lux light intensity. These findings underscore the significance of light intensity as an environmental factor in the uptake and transformation of Cd by algae, with profound implications for its application in bioremediation.

Transformation of Mercuric Ions to Mercury Nanoparticles in Diatom Chaetoceros curvisetus

Particulate HgS play crucial roles in the mercury (Hg) cycle. Approximately 20-90% of dissolved Hg can be transformed into particulate HgS by algae. However, detailed knowledge regarding these particles, including sizes and distribution, remains unknown. The present study explored the formation, distribution, and excretion of mercury nanoparticles (HgNPs) in diatom Chaetoceros curvisetus. The results demonstrated that HgNPs (HgS nanoparticles, 29.6-66.2 nm) formed intracellularly upon exposure to 5.0-100.0 μg L-1 Hg(II), accounting for 12-27% of the total Hg. HgNP concentrations significantly increased with increasing intracellular Hg(II) concentrations, while their sizes remained unaffected. HgNPs formed intracellularly and partly accumulated inside the cells (7-11%). Subsequently, the sizes of intracellular HgNPs gradually decreased to facilitate expulsion, 21-50% of which were excreted. These suggested the vital roles of HgNPs in comprehending marine Hg fate. Their unique physicochemical properties and bioavailability would influence Hg biotransformation in the ocean. Additionally, both intracellular and extracellular HgNPs contributed to Hg settling with cells, ultimately leading to Hg burial in sediments. Overall, these findings further deepened our understanding of Hg biotransformation and posed challenges in accurately estimating marine Hg flux and Hg burial.

Investigation of toxic effect of mercury on Microcystis aeruginosa: Correlation between intracellular mercury content at single cells level and algae physiological responses

Single-cell studies can help to understand individual differences and obtain atypical cellular characteristics in view of cellular heterogeneity. Herein, the accumulation of mercury (Hg) in single algae cells was studied by droplet chip-time resolved inductively coupled plasma mass spectrometry analytical system, and the relation of Hg accumulation to the physiological responses of algae cell was explored. When low concentrations of Hg²⁺ (5-20 μg/L) were used in the exposure experiment, the content of Hg in single cells increased in first 2 h, then decreased with further increase of exposure time to 96 h, probably due to the growth dilution effect of the algae. When exposed to 30 μg/L Hg²⁺, the uptake of Hg by individual cells increased over time, which was associated with increased cell membrane permeability. The exposure to Hg²⁺ (5-30 μg/L) inhibited the growth of algae in a concentration-dependent manner and serious growth inhibition occurred under the exposure concentration of 30 μg/L. While the exposure concentration was lower than 20 μg/L, algal cells exhibited a recover tendency due to the self-protection mechanism of algal cells. Bivariate results showed that intracellular Hg accumulation was significantly negatively correlated with cells growth in terms of OD₆₈₀, photosynthetic pigments, F_v/F_m and PI_abs. On the contrast, reactive oxygen species content, superoxide dismutase activity, and cell membrane permeability were significantly positively correlated with the accumulation of intracellular Hg. These results are helpful to further understand the toxic effect of Hg on algae.