Evaluation of the sinks and sources of atmospheric CO2 by artificial upwelling

The microbial carbon pump and climate change

The ocean has been a regulator of climate change throughout the history of Earth. One key mechanism is the mediation of the carbon reservoir by refractory dissolved organic carbon (RDOC), which can either be stored in the water column for centuries or released back into the atmosphere as CO2 depending on the conditions. The RDOC is produced through a myriad of microbial metabolic and ecological processes known as the microbial carbon pump (MCP). Here, we review recent research advances in processes related to the MCP, including the distribution patterns and molecular composition of RDOC, links between the complexity of RDOC compounds and microbial diversity, MCP-driven carbon cycles across time and space, and responses of the MCP to a changing climate. We identify knowledge gaps and future research directions in the role of the MCP, particularly as a key component in integrated approaches combining the mechanisms of the biological and abiotic carbon pumps for ocean negative carbon emissions.

Unveiling the potential for artificial upwelling in algae derived carbon sink and nutrient mitigation

Mariculture algae may present a crucial part of ocean-based solutions for climate change, with the ability to sequester carbon and remove nutrients. However, the expansion of mariculture algae faces multiple challenges. Here, we measure the changes in algae derived carbon sinks and nitrogen (N) and phosphorus (P) removal between 2010 and 2020 in Shandong Province, China. We further identify the key driving factors, namely area, algal species proportion, and yield, that influence the changes. The results show that algae derived carbon sinks and nutrient removal growth rates in Shandong Province have slowed significantly since 2014, mainly due to area limitations, laver-oriented species change, and unstable yields. Artificial upwelling (AU) has the potential to enhance the yield and subsequently offset the loss of carbon sinks and nutrient removal caused by negative driving factors. Scenario analysis indicates that a complete deployment of AU by 2030 will offset up to a 44.52 % decrease in the mariculture algae area, or a 72.57 % increase in the laver share of the algal species combination compared to 2020. Similar conclusions are reached regarding the role of AU in N and P removal. This study also identifies ancillary challenges such as low energy efficiency and high costs faced by applying AU.

Potential risks of CO2 removal project based on carbonate pump to marine ecosystem

The development of marine carbon sequestration project has an important potential for carbon neutralization in the short-term (several decades). Marine carbon sequestration technology is based on biological and carbonate pumps to increase particulate organic carbon and authigenic insoluble carbonates to the deep sea or seafloor, aiming to achieve permanent carbon sequestration. Particularly, chemical carbon sequestration technology based on carbonate pump is proposed and considered to achieve short-term marine carbon sequestration in recent years. This technology mainly includes alkaline mineral addition and combining CO₃^2- to insoluble carbonates to improve marine carbon fixation capacity. Potential marine ecosystem risks of chemical CO₂ removal method should be considered before being a feasible technology. We reviewed the potential effects of marine chemical carbon sequestration project on marine organisms. Marine chemical carbon sequestration had two main effects on marine organisms: released chemicals effect, and particle effect. Released chemicals in mineral weathering directly affected phytoplankton and bacteria community. Particles formed during carbon sequestration process mainly affected filter feeding organisms. The toxic effects of particles on aquatic organisms increased with decreasing sizes and increasing concentrations of particle. Algae and crustaceans were the most sensitive groups exposed to metal nanoparticles (nm-μm) in seawaters, thus could be used as target species to evaluate ecological risk of small particles generated in chemical carbon sequestration project. Embryos or larva of filter feeding organisms were more sensitive to large clay and metal microparticles (μm‑mm) than adults, thus could be used as sensitive groups to establish safety concentration of large particles. The relatively inert metal nanoparticles and microparticles had higher safety concentrations than active ones. These particle concentration thresholds could be as a reference to design concentrations and initial sizes of applied minerals in marine chemical carbon sequestration project. This will ensure that the ecological risk is minimized when carbon fixation efficiency is maximized.

Eco-engineering approaches for ocean negative carbon emission

The goal of achieving carbon neutrality in the next 30-40 years is approaching worldwide consensus and requires coordinated efforts to combat the increasing threat of climate change. Two main sets of actions have been proposed to address this grand goal. One is to reduce anthropogenic CO₂ emissions to the atmosphere, and the other is to increase carbon sinks or negative emissions, i.e., removing CO₂ from the atmosphere. Here we advocate eco-engineering approaches for ocean negative carbon emission (ONCE), aiming to enhance carbon sinks in the marine environment. An international program is being established to promote coordinated efforts in developing ONCE-relevant strategies and methodologies, taking into consideration ecological/biogeochemical processes and mechanisms related to different forms of carbon (inorganic/organic, biotic/abiotic, particulate/dissolved) for sequestration. We focus on marine ecosystem-based approaches and pay special attention to mechanisms that require transformative research, including those elucidating interactions between the biological pump (BP), the microbial carbon pump (MCP), and microbially induced carbonate precipitation (MICP). Eutrophic estuaries, hypoxic and anoxic waters, coral reef ecosystems, as well as aquaculture areas are particularly considered in the context of efforts to increase their capacity as carbon sinks. ONCE approaches are thus expected to be beneficial for both carbon sequestration and alleviation of environmental stresses.

Current and Future Potential of Shellfish and Algae Mariculture Carbon Sinks in China

Shellfish and algae mariculture make up an important part of the marine fishery carbon sink. Carbon sink research is necessary to ensure China achieves its goal of carbon neutrality. This study used the material quality assessment method to estimate the carbon sink capacity of shellfish and algae. Product value, carbon storage value, and oxygen release value were used to calculate the economic value of shellfish and algae carbon sequestration. The results showed that the annual average shellfish and algae carbon sink in China was 1.10 million tons from 2003 to 2019, of which shellfish accounted for 91.63%, wherein Crassostreagigas, Ruditapesphilippinarum, and Chlamysfarreri were the main contributors. The annual average economic value of China’s shellfish and algae carbon sequestration was USD 71,303.56 million, and the product value was the main contributor, accounting for 99.11%. The carbon sink conversion ratios of shellfish and algae were 8.37% and 5.20%, respectively, thus making shellfish the aquaculture species with the strongest carbon sink capacity and the greatest carbon sink potential. The estimated growth rate in the shellfish and algae removable carbon sink was 33,900 tons/year in China, but this trend was uncertain. The capacity for carbon sequestration and exchange by aquaculture can be improved by expanding breeding space, promoting multi-level comprehensive breeding modes, and marine artificial upwelling projects.

Energy Management and Operational Planning of an Ecological Engineering for Carbon Sequestration in Coastal Mariculture Environments in China

China is now accelerating the development of an ecological engineering for carbon sequestration in coastal mariculture environments to cope with climate change. Artificial upwelling as the ecological engineering can mix surface water with bottom water and bring rich nutrients to the euphotic zone, enhance seaweed growth in the oligotrophic sea area, and then increase coastal carbon sequestration. However, one of the major obstacles of the artificial upwelling is the high energy consumption. This study focused on the development of energy management technology for air-lift artificial upwelling by optimizing air injection rate. The fundamental principle underlying this technology is that the mode and intensity of air injection are adjusted from the feedback of information on velocity variation in tidal currents, illumination, and temperature of the surface layer. A series of equations to control air injection was derived based on seaweed growth and solar power generation. Although this finding was originally developed for the air-lift artificial upwelling, it also can be used in other areas of engineering, such as water delivery, aeration, and oxygenation. The simulations show that using a variable air injection rate can lift more nitrogen nutrients of 28.2 mol than using a fixed air injection rate of 26.6 mol, mostly with the same energy cost. Using this control algorithm, the changed temperature and dissolved oxygen profiles prove the effective upwelling in the experiments and the average weights of kelp are 33.1 g in the experimental group and 10.1 g in the control group. The ecological engineering was successfully increasing crop yield for carbon sequestration in coastal mariculture environments.

Submarine Groundwater Discharge helps making nearshore waters heterotrophic

Submarine groundwater discharge (SGD) is the submarine seepage of all fluids from coastal sediments into the overlying coastal seas. It has been well documented that the SGD may contribute a great deal of allochthonous nutrients to the coastlines. It is, however, less known how much carbon enters the ocean via the SGD. Nutrients (NO₃, NO₂, NH₄, PO₄, SiO₂), alkalinity and dissolved inorganic carbon (DIC) in the submarine groundwater were measured at 20 locations around Taiwan for the first time. The total N/P/Si yields from the SGD in Taiwan are respectively 3.28 ± 2.3 × 10⁴, 2.6 ± 1.8 × 10² and 1.89 ± 1.33 × 10⁴ mol/km²/a, compared with 9.5 ± 6.7 × 10⁵ mol/km²/a for alkalinity and 8.8 ± 6.2 × 10⁵ mol/km²/a for DIC. To compare with literature data, yields for the major estuary across the Taiwan Strait (Jiulong River) are comparable except for P which is extremely low. Primary production supported by these nutrient outflows is insufficient to compensate the DIC supplied by the SGD. As a result, the SGD helps making the coastal waters in Taiwan and Jiulong River heterotrophic.