Contrasting impact of elevated atmospheric CO2 on nitrogen cycle in eutrophic water with or without Eichhornia crassipes (Mart.) Solms

Elevated CO2 exacerbates the risk of methylmercury exposure in consuming aquatic products: Evidence from a complex paddy wetland ecosystem

Elevated CO2 levels and methylmercury (MeHg) pollution are important environmental issues faced across the globe. However, the impact of elevated CO2 on MeHg production and its biological utilization remains to be fully understood, particularly in realistic complex systems with biotic interactions. Here, a complete paddy wetland microcosm, namely, the rice-fish-snail co-culture system, was constructed to investigate the impacts of elevated CO2 (600 ppm) on MeHg formation, bioaccumulation, and possible health risks, in multiple environmental and biological media. The results revealed that elevated CO2 significantly increased MeHg concentrations in the overlying water, periphyton, snails and fish, by 135.5%, 66.9%, 45.5%, and 52.1%, respectively. A high MeHg concentration in periphyton, the main diet of snails and fish, was the key factor influencing the enhanced MeHg in aquatic products. Furthermore, elevated CO2 alleviated the carbon limitation in the overlying water and proliferated green algae, with subsequent changes in physico-chemical properties and nutrient concentrations in the overlying water. More algal-derived organic matter promoted an enriched abundance of Archaea-hgcA and Deltaproteobacteria-hgcA genes. This consequently increased the MeHg in the overlying water and food chain. However, MeHg concentrations in rice and soil did not increase under elevated CO2, nor did hgcA gene abundance in soil. The results reveal that elevated CO2 exacerbated the risk of MeHg intake from aquatic products in paddy wetland, indicating an intensified MeHg threat under future elevated CO2 levels.

The long overlooked microalgal nitrous oxide emission: Characteristics, mechanisms, and influencing factors in microalgae-based wastewater treatment scenarios

Microalgae-based wastewater treatment is particularly advantageous in simultaneous CO₂ sequestration and nutrients recovery, and has received increasing recognition and attention in the global context of synergistic pollutants and carbon reduction. However, the fact that microalgae themselves can generate the potent greenhouse gas nitrous oxide (N₂O) has been long overlooked, most previous research mainly regarded microalgae as labile organic carbon source or oxygenic approach that interfere bacterial nitrification-denitrification and the concomitant N₂O production. This study, therefore, summarized the amount and rate of N₂O emission in microalgae-based systems, interpreted in-depth the multiple pathways that lead to NO formation as the key precursor of N₂O, and the pathways that transform NO into N₂O. Reduction of nitrite could take place in either the cytoplasm or the mitochondria to form NO by a series of enzymes, while the NO could be enzymatically reduced to N₂O at the chloroplasts or the mitochondria respectively under light and dark conditions. The influences of abiotic factors on microalgal N₂O emission were analyzed, including nitrogen types and concentrations that directly affect the nitrogen transformation routes, illumination and oxygen conditions that regulate the enzymatic activities related to N₂O generation, and other factors that indirectly interfere N₂O emission via NO regulation. The uncertainty of microalgae-based N₂O emission in wastewater treatment scenarios were emphasized, which would be particularly impacted by the complex competition between microalgae and ammonia oxidizing bacteria or nitrite oxidizing bacteria over ammonium or inorganic carbon source. Future studies should put more efforts in improving the compatibility of N₂O emission results expressions, and adopting consistent NO detection methods for N₂O emission prediction. This review will provide much valuable information on the characteristics and mechanisms of microalgal N₂O emission, and arouse more attention to the non-negligible N₂O emission that may impair overall greenhouse gas reduction efficiency in microalgae-based wastewater treatment systems.

Effects of anthropogenic nitrogen additions and elevated CO2 on microbial community, carbon and nitrogen content in a replicated wetland

Anthropogenic deposition of nitrogen (N) and elevated CO₂ (_eaCO₂) are expected to increase continuously and rapidly in the near future and influence global carbon cycling. These parameters affect the ecosystem by regulating the microbial community and contribute to soil organic matter decomposition. The study was performed to understand the effects of N additions (4 and 6mgl^-1) and _eaCO₂ (700 ppm) on carbon (C)/nitrogen (N) content in the soil, microbial community, and plant biomass (Alternanthera philoxeroides species). The results showed that when the atmospheric CO₂ concentration was raised, the total organic carbon (TOC) in the soil statistically increased (P < 0.05) by 4% and 3% under low and high N additions respectively, while the inorganic carbon content also increased by 1% and 3% (P > 0.05) under the same conditions. The increase in the soil TOC content was a result of the movement of carbon from water to the soil due to the presence of vascular tissues of plants in the water. The redundancy analysis (RDA) results revealed that the presence of plant species was responsible for the carbon content increment in the soil. The plant biomass content increased by 30.96% (P = 0.081) and 31.36%, (P = 0.002) under low and high N addition respectively due to the increment in atmospheric CO₂. The nitrogen content in the plant species decreased (p > 0.05) by 8.62% and 6.25% at low and high N addition respectively when atmospheric CO₂ was raised. This suggests that soil microbes competed with the plants for inorganic nitrogen in the soil and the microbes used up the inorganic nitrogen before it got to the plants. The gram-positive bacteria and fungi population decreased under high N addition and _eaCO₂ while gram-negative bacteria increased, suggesting that N additions and _eaCO₂ affected the microbial function and correlated with the nitrogen reduction in the soil. The results from this study serve as a guide to researchers and stakeholders in making policies with regard to the constant increasing CO₂ concentration in the atmosphere.

Effects of rising atmospheric CO2 levels on physiological response of cyanobacteria and cyanobacterial bloom development: A review

Increasing atmospheric CO₂ concentration negatively impacts aquatic ecosystems and may exacerbate the problem of undesirable cyanobacterial bloom development in freshwater ecosystems. Elevated levels of atmospheric CO₂ may increase the levels of dissolved CO₂ in freshwater systems, via air-water exchanges, enhancing primary production in the water and catchments. Although high CO₂ levels improve cyanobacterial growth and increase cyanobacterial biomass, the impacts on their internal physiological processes can be more complex. Here, we have reviewed previous studies to evaluate the physiological responses of cyanobacteria to high concentrations of CO₂. In response to high CO₂ concentrations, the pressures of inorganic carbon absorption are reduced, and carbon concentration mechanisms are downregulated, affecting the intracellular metabolic processes and competitiveness of the cyanobacteria. Nitrogen and phosphorus metabolism and light utilization are closely related to CO₂ assimilation, and these processes are likely to be affected by resource and energy reallocation when CO₂ levels are high. Additionally, the responses of diazotrophic and toxic cyanobacteria to elevated CO₂ levels were specifically reviewed. The responses of diazotrophic cyanobacteria to elevated CO₂ concentrations were found to be inconsistent, probably because of differences in other factors in experimental designs. Toxic cyanobacteria tended to be superior to non-toxic strains at low levels of CO₂; however, the specific effects of microcystin on the regulation require further investigation. Furthermore, the effects of increasing CO₂ levels on cyanobacterial competitiveness in phytoplankton communities and nutrient cycling in aquatic ecosystems were reviewed. High CO₂ concentrations may make cyanobacteria less competitive relative to other algal taxa; however, due to the complexity of natural systems and the specificity of algal species, the dominant positions of the cyanobacteria do not seems to be changed. To better understand cyanobacterial responses to elevated CO₂ levels and help control cyanobacterial bloom developments, this review has identified key areas for future research.

Regrowth strategies of Leymus chinensis in response to different grazing intensities

In temperate grassland ecosystems, grazing can affect plant growth by foraging, trampling, and excretion. The ability of dominant plant species to regrow after grazing is critical, since it allows the regeneration of photosynthetic tissues to support growth. We conducted a field experiment to evaluate the effects of different grazing intensities (control, light, medium, and heavy) on the physiological and biochemical responses of Leymus chinensis and the carbon (C) sources utilized during regrowth. Light grazing promoted regrowth and photoassimilate storage of L. chinensis, by increasing the net photosynthetic rate (P_n ), photosynthetic quenching, light interception, sugar accumulation, sucrose synthase activities, and fructose supply from stems. At medium grazing intensity, L. chinensis had low P_n , light interception, and sugar accumulation, but higher expression of a sucrose transporter gene (LcSUT1) and water-use efficiency, which reflected a tendency to store C in belowground to promote survival. This strategy was associated with regulation by abscisic acid (ABA), jasmonate, and salicylic acid (SA) signaling. However, L. chinensis tolerated heavy grazing by increased ABA and jasmonate-induced promotion of C assimilation and osmotic adjustment, combined with photoprotection against photo-oxidation, suggesting a strategy based on regrowth. In addition, stems were the main C source organs and energy supply rather than roots. Simultaneously, SA represented a weaker defense than ABA and jasmonate. Therefore, L. chinensis adopted different strategies for regrowth under different grazing intensities, and light grazing promoted regrowth the most. Our results demonstrate the regulation of C reserves utilization by phytohormones, and this regulation provides an explanation for recent results about grazing responses.

Interactions between elevated CO2 levels and floating aquatic plants on the alteration of bacterial function in carbon assimilation and decomposition in eutrophic waters

Elevated atmospheric CO₂ concentration (eCO₂) may have different effects on the bacterial community with regard to C assimilation and decomposition in eutrophic waters compared to that in fresh waters with intermediate levels of nutrients and oceans. Aquatic plant growth under eCO₂ could further modify microbial activities associated with the C cycle in eutrophic waters. Therefore, there is an urgent need to further study how eCO₂ and its interactions with the growth of aquatic plants affect the composition and function of the bacterial community involved in mediating the C cycle in eutrophic waters. Accordingly, we designed a microcosm experiment to investigate the effects of ambient and high CO₂ concentrations on bacterial community composition and function in eutrophic waters with and without the growth of Eichhornia crassipes (Mart.) Solms. The results from 16S rRNA gene sequencing, function prediction, and q-PCR showed that eCO₂ significantly increased the abundance of bacterial and functional genes involved in CO₂ assimilation (photosynthetic bacteria; cbbL IA & IC, cbbL ID, cbbM, pufM) and C decomposition (Acidimicrobiia, Thermoleophilia, Gaiellales; ChiA), illustrating the functional enrichment with photoautotrophy, hydrocarbon degradation, cellulolysis, and aromatic hydrocarbon degradation. However, eCO₂ decreased the abundance of some chemoautotrophic bacteria, including nitrifying bacteria (Nitrospirae, Nitrosomonadaceae). In contrast, the cultivation of E. crassipes decreased the abundance of photosynthetic bacteria but increased the abundance of bacteria involved in complex C decomposition associated with root exudates and degradation, e.g. Fibrobacteres, Sphingobacteriales, Sphingomonadales, and Rhizobiales. eCO₂ and growth of E. crassipes had opposite effects on algal density in eutrophic waters, creating interactive effects that further decreased the diversity of the bacterial community and abundance of some CO₂-assimilating bacteria with nitrifying characteristics (Nitrosomonadaceae) and some C-degrading bacteria (Fibrobacteres) with denitrifying properties (Flavobacteriaceae, Sphingomonadaceae, and Gemmobacter). Therefore, the interactions between aquatic plants and the bacterial community in eutrophic waters under eCO₂ would be beneficial to the environment and help alleviate the greenhouse effect.