Coccolithophores and diatoms resilient to ocean alkalinity enhancement: A glimpse of hope?

"Assessment of potential eutrophication in coastal waters of Gran Canaria: Impact on plankton community under CO2 depletion"

Population growth in coastal tourist areas is leading to enhanced waste production, raising concerns about potential nutrient release increases and the resulting impact on marine ecosystems through eutrophication. Knowledge of the specific impacts of eutrophication on plankton communities in many of these regions is limited, highlighting the need for further research and appropriate environmental management strategies. To help address these gaps, we conducted a 30-day mesocosm study in the coastal waters of Gran Canaria, Canary Islands, a major European tourist destination, and the third most densely populated autonomous community in Spain. With the aim of assessing the effects of nutrient input on biomass, primary production (PP) and recycling processes by phytoplankton, zooplankton, and bacterioplankton, we simulated three nutrient discharge intensities (Low, Medium, and High), with daily additions of 0.1, 1, and 10 μmol L-1 of nitrate, respectively, along with phosphate and silicate. We observed that PP, chlorophyll a (Chl-a), and biomass increased linearly with nutrient input, except in the High treatment, where CO2 depletion (<1.0 μmol L-1) and an alkalinity increase (>2500 μmol L-1) resulted in reduced PP. Despite limitations in nitrogen (Control, Low, and Medium) or carbon (High) availability across treatments, which led to stabilized or decreased PP rates and dissolved organic carbon (DOC) concentrations, bacterial degradation remained active in all treatments. This microbial activity resulted in an accumulation of recalcitrant chromophoric dissolved organic matter (CDOM), indicating the resilience of carbon recycling processes under varying nutrient conditions. Furthermore, a clear succession was evident in all enriched treatments, transitioning from an oligotrophic condition dominated by pico- and nanophytoplankton to a eutrophic state primarily composed of diatoms. However, under CO2 depletion, diatoms experienced a decline in the High treatment, leading to the proliferation of potentially mixotrophic dinoflagellates. Microzooplankton was less sensitive than mesozooplankton to the decrease in prey availability and high pH caused by CO2 depletion. Interestingly, the Medium treatment showed high efficiency in terms of PP, despite reaching CO2 levels near of 1.0 μmol L-1 by the end of the experiment. PP rates increased from 10 to 100 μg C·L-1·d-1 during the first week and remained stable as diatoms predominated throughout the study period. These findings provide valuable insights into the responses of plankton communities to varying nutrient inputs and emphasize the importance of considering the effects of DIC depletion, along with changes in total alkalinity, in eutrophication scenarios as well as in ocean alkalinity enhancement experiments aimed at reducing carbon dioxide emissions.

Bio-Informed Porous Mineral-Based Composites

Certain biominerals, such as sea sponges and echinoderm skeletons, display a fascinating combination of mechanical properties and adaptability due to the well-defined structures spanning various length scales. These materials often possess high density normalized mechanical properties because they contain well-defined pores. The density-normalized mechanical properties of synthetic minerals are often inferior because the pores are stochastically distributed, resulting in an inhomogeneous stress distribution. The mechanical properties of synthetic materials are limited by the degree of structural and compositional control currently available fabrication methods offer. In the first part of this review, examples of structural elements nature uses to impart exceptional density normalized Young's moduli to its porous biominerals are showcased. The second part highlights recent advancements in the fabrication of bio-informed mineral-based composites possessing pores with diameters that span a wide range of length scales. The influence of the processing of mineral-based composites on their structures and mechanical properties is summarized. Thereby, it is aimed at encouraging further research directed to the sustainable, energy-efficient fabrication of synthetic lightweight yet stiff mineral-based composites.

Plankton food web structure and productivity under ocean alkalinity enhancement

Ocean alkalinity enhancement (OAE) is a nature-based technology for CO2 removal and storage, but little is known about its environmental safety. We tested a CO2-equilibrated OAE deployment in a close-to-natural community using in situ mesocosms in the oligotrophic subtropical North Atlantic and assessed metazoan zooplankton to inform about food web stability, structure, and production. In addition, a literature review complemented experimental results by summarizing physiological responses of marine animals to decreasing proton concentrations, or increased pH. The food web studied proved resistant, and zooplankton physiologically tolerant, to the OAE tested. We observed short-term effects of OAE on zooplankton reproduction and productivity, which were likely trophically mediated. Yet, these did not affect zooplankton populations or their nutritional value as food for fish. Our study demonstrates an environmentally safe OAE application, but also stresses the risks of more intense OAE options, and the vulnerabilities of other marine ecosystems.

Contrasting species-specific stress response to environmental pH determines the fate of coccolithophores in future oceans

Molecular mechanisms driving species-specific environmental sensitivity in coccolithophores are unclear but crucial in understanding species selection and adaptation to environmental change. This study examined proteomic and physiological changes in three species under varying pH conditions. We showed that changing pH drives intracellular oxidative stress and changes membrane potential. Upregulation in antioxidant, DNA repair and cell cycle-related protein-groups indicated oxidative damage across high (pH 8.8) and low pH (pH 7.6) compared to control pH (pH 8.2), and correlated with reduced growth rates. Upregulation of mitochondrial proteins suggested higher metabolite demand for restoring cellular homeostasis under pH-induced stress. Photosynthetic rates generally correlated with CO2 availability, driving higher net carbon fixation rates at low pH. The intracellular pH-buffering capacity of the coastal Chrysotila carterae and high metabolic adaptability in the bloom-forming Gephyrocapsa huxleyi will likely facilitate their adaptation to ocean acidification or artificial ocean alkalinisation. However, the pH sensitivity of the ancient open-ocean Coccolithus braarudii will possibly result in reduced growth and shrinking of its ecological niche.

Resilience of Phytoplankton and Microzooplankton Communities under Ocean Alkalinity Enhancement in the Oligotrophic Ocean

Ocean alkalinity enhancement (OAE) is currently discussed as a potential negative emission technology to sequester atmospheric carbon dioxide in seawater. Yet, its potential risks or cobenefits for marine ecosystems are still mostly unknown, thus hampering its evaluation for large-scale application. Here, we assessed the impacts OAE may have on plankton communities, focusing on phytoplankton and microzooplankton. In a mesocosm study in the oligotrophic subtropical North Atlantic, we investigated the response of a natural plankton community to CO2-equilibrated OAE across a gradient from ambient alkalinity (2400 μmol kg-1) to double (4800 μmol kg-1). Abundance and biomass of phytoplankton and microzooplankton were insensitive to OAE across all size classes (pico, nano and micro), nutritional modes (autotrophic, mixotrophic and heterotrophic) and taxonomic groups (cyanobacteria, diatoms, haptophytes, dinoflagellates, and ciliates). Consequently, plankton communities under OAE maintained their natural chlorophyll a levels, size structure, taxonomic composition and biodiversity. These findings suggest a high tolerance of phytoplankton and microzooplankton to CO2-equilibrated OAE in the oligotrophic ocean. However, alternative application schemes involving more drastic perturbations in water chemistry and nutrient-rich ecosystems require further investigation. Nevertheless, our study on idealized OAE will help develop an environmentally safe operating space for this climate change mitigation solution.

Species differences in carbon drawdown during marine phytoplankton growth

Ocean alkalinity enhancement (OAE) has been proposed as a mitigation method for negative carbon emission. Its effects on marine phytoplankton communities would depend on species differences in tolerance to high pH, which results from phytoplankton photosynthetic drawdown of dissolved inorganic carbon (DIC). In this study, 20 marine phytoplankton species were grown in sealed batch cultures and DIC, pH and chlorophyll a (Chl-a) were measured at the peaks of biomass. These results revealed a wide range of species differences. The drawdown DIC (ΔDIC) vs. increases in pH (ΔpH) graph resembled a Michaelis-Menten curve: significantly linear for ΔDIC < ~1000 μM and starting to plateau at ΔDIC > 1000 μM. This indicated that two mechanisms were operating: CO2 limitation at ΔpH < 1.41 and biologically-mediated precipitation-CO2 released carbon uptake at ΔpH > 1.41. These findings suggest that the potential effects of OAE on the phytoplankton communities would depend on the species differences in oceans.

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.