Shellfish-algal systems as important components of fisheries carbon sinks: Their contribution and response to climate change

Mariculture increases microbially-driven carbon metabolism and sequestration in coastal ecosystems

Mariculture has expanded significantly in recent decades due to rising seafood demand and its contribution to ocean carbon sequestration. While the mechanisms of carbon sequestration in mariculture are well-established, the roles of microorganisms in sedimentary carbon sequestration have rarely been explored. How microorganisms mediate organic carbon metabolism and their effects on coastal carbon pools remain unclear. Here we tested the carbon fraction and contents, as well as extracellular hydrolase activities in macroalgae culture area, fish or abalone culture area, and control area without mariculture. We profiled microbial community composition and carbon metabolism characteristics in sediments through 16S rRNA gene amplicon sequencing and metagenomics. Our findings revealed that macroalgae culture areas exhibited a significantly greater potential for carbon sequestration than the control area, the concentration of TOC in seawater and the contents of SOC, DOC, and ROC in sediments were significantly (p < 0.05) increased by 18.93 %, 6.98 %, 33.98 %, and 18.30 % respectively. These results can be attributed to decreased activities of extracellular hydrolase and a lower abundance of carbon-degrading genes. Moreover, metabolic profiling identified taxa from families such as Alteromonadaceae, Pseudomonadaceae, Rhodobacteraceae, Enterobacteriaceae, and Flavobacteriaceae, which are highly metabolically flexible in utilizing a wide range of organic and inorganic energy sources, playing crucial roles in carbon formation. Their respiratory metabolism, such as sulfate reduction, thiosulfate oxidation, and denitrification as well as secondary metabolism products could also affect the formation and persistence of sedimentary carbon pools. Specifically, increased total nitrogen (TN) and nitrate-nitrogen (NO3-) could potentially enhance microbial degradation of organic carbon, decreasing carbon stock within coastal sediments. This study enhanced our understanding of microbial regulation of the organic carbon pool in the mariculture ecosystem.

The hidden acceleration pump uncovers the role of shellfish in oceanic carbon sequestration

Whether shellfish mariculture should be included in the blue carbon profile as a strategy to combat climate change has been controversial. It is highly demanding not only to provide calibration quantitation, but also to provide an ecosystem-based mechanism. In this study, we chose mussel farms as a case study to evaluate their contributions to carbon sinks and their responses to sedimentary carbon fixation and sequestration. First, we quantified the air-sea CO2 flux in the mussel aquacultural zone and observed a weak carbon sink (-0.15 ± 0.07 mmol·m-2·d-1) during spring. Next, by analyzing the carbon composition in the sediment, we recorded a noticeable and unexpected increase in the sedimentary recalcitrant carbon (RC) content in the mussel farming case. To address this surprising sedimentary phenomenon, a long-term indoor experimental test was conducted to distinguish the consequences of mussel engagement with sedimentary RC. Our observational data support the idea that mussel engagement promotes accumulation of RC in sediments by 2.5-fold. Furthermore, the relative intensity of carboxylic-rich alicyclic molecule (CRAM)-like compounds (recalcitrant dissolved organic matter (RDOM)) increased by 451.4 % in the mussel-engaged sedimentary dissolved organic matter (DOM) in comparison to the initial state. Mussel engagement had a significantly positive effect on the abundance of sedimentary carbon-fixing genes. Therefore, we definitively conclude that mussel farming is blue carbon positive and propose a new alternative theory that mussel farming areas may have high carbon sequestration potential via an ecologically integrated "3 M" (microalgae-mussel-microbiota) consortium. The "mussel pump" accelerates carbon sequestration and enhances climate-related ecosystem services.

Seasonal effects of fish, seaweed and abalone cultures on dissolved organic matter and carbon sequestration potential in Sansha Bay, China

Coastal bays serve as undeniable dissolved organic matter (DOM) reactors and the role of prevalent mariculture in DOM cycling deserves investigation. This study, based on four seasonal field samplings and a laboratory incubation experiment, examined the source and seasonal dynamics of DOM and fluorescent dissolved organic matter (FDOM) in the seawater of fish (Larimichthys crocea, LC), seaweed (Gracilaria lemaneiformis, GL) and abalone (Haliotis sp., HA) culturing zones in Sansha Bay, China. Using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC), three fluorescent components were identified, i.e. protein-like C1, protein-like C2, and humic-like C3. Our results showed that mariculture activities dominated the DOM pool by seasonal generating abundant DOM with lower aromaticity and humification degrees. Accounting for 40-95 % of total fluorescent components, C1 (Ex/Em = 300/340 nm) was regarded the same as D1 (Ex/Em = 300/335 nm) identified in a 180-day degradation experiments of G. lemaneiformis detritus, indicating that the cultured seaweed modulated DOM through the seasonal production of C1. In addition, the incubation experiment revealed that 0.7 % of the total carbon content of seaweed detritus could be preserved as recalcitrant dissolved organic carbon (RDOC). However, fish culture appeared to contribute to liable DOC and protein-like C2, exerting a substantial impact on DOM during winter but making a negligible contribution to carbon sequestration, while abalone culture might promote the potential export and sequestration of seaweed-derived carbon to the ocean. Our results highlight the influences of mariculture activities, especially seaweed culture, in shaping DOM pool in coastal bays. These findings can provide reference for future studies on the carbon accounting of mariculture.

The mucosal immunity in crustaceans: Inferences from other species

Crustaceans such as shrimps and crabs, hold significant ecological significance and substantial economic value within marine ecosystems. However, their susceptibility to disease outbreaks and pathogenic infections has posed major challenges to production in recent decades. As invertebrate, crustaceans primarily rely on their innate immune system for defense, lacking the adaptive immune system found in vertebrates. Mucosal immunity, acting as the frontline defense against a myriad of pathogenic microorganisms, is a crucial aspect of their immune repertoire. This review synthesizes insights from comparative immunology, highlighting parallels between mucosal immunity in vertebrates and innate immune mechanisms in invertebrates. Despite lacking classical adaptive immunity, invertebrates, including crustaceans, exhibit immune memory and rely on inherent "innate immunity factors" to combat invading pathogens. Drawing on parallels from mammalian and piscine systems, this paper meticulously explores the complex role of mucosal immunity in regulating immune responses in crustaceans. Through the extrapolation from well-studied models like mammals and fish, this review infers the potential mechanisms of mucosal immunity in crustaceans and provides insights for research on mucosal immunity in crustaceans.

Ocean acidification alters shellfish-algae nutritional value and delivery

The ecological effects of climate change and ocean acidification (OA) have been extensively studied. Various microalgae are ecologically important in the overall pelagic food web as key contributors to oceanic primary productivity. Additionally, no organism exists in isolation in a complex environment, and shifts in food quality may lead to indirect OA effects on consumers. This study aims to investigate the potential effects of OA on algal trophic composition and subsequent bivalve growth. Here, the growth and nutrient fractions of Chlorella sp., Phaeodactylum tricornutum and Chaetocetos muelleri were used to synthesize and assess the impact of OA on primary productivity. Total protein content, total phenolic compounds, and amino acid (AA) and fatty acid (FA) content were evaluated as nutritional indicators. The results demonstrated that the three microalgae responded positively to OA in the future environment, significantly enhancing growth performance and nutritional value as a food source. Additionally, certain macromolecular fractions found in consumers are closely linked to their dietary sources, such as phenylalanine, C14:0, C16:0, C16:1, C20:1n9, C18:0, and C18:3n. Our findings illustrate that OA affects a wide range of crucial primary producers in the oceans, which can disrupt nutrient delivery and have profound impacts on the entire marine ecosystem and human food health.

Heading towards carbon neutrality: how do marine carbon sinks serve as important handle for promoting marine ecological civilization construction?

As an efficient long-term carbon sink, marine carbon sinks and the associated carbon sink effects, technology, accounting and trading market construction warrant investigation across various disciplines. However, information on the interrelationships and their development over time with respect to the research conducted in China is limited, affecting the ability to drive research directions and optimize continued advancement in this field. Therefore, in this study, we aimed to understand the current situation of marine carbon sink research in China to promote a deeper level of scientific development based on the research literature related to marine blue carbon sinks in the core databases of the China National Knowledge Internet (CNKI) and Web of Science (WOS). We used bibliometric tools in the Citespace software to quantitatively compare and analyse the main characteristics of marine blue carbon sink research including publication volume, time, journals, authors and institutions. We also explored the popular research topics, frontier areas, and theme evolution trends through keyword clustering and emergent and co-occurring knowledge maps. The key recommended research directions for ocean carbon sinks are: (1) to promote the unified carbon sink market research of land and sea integration through multidisciplinary and cross-disciplinary research; (2) to achieve new breakthroughs in ocean carbon sinks with the support of coastal wetlands and seawater offshore aquaculture environments; (3) to explore the protection provided by ocean carbon sinks with a comprehensive eco-compensation mechanism; (4) to improve the application of marine carbon sinks by taking the theory and technological innovation research related to marine carbon sinks as the guide. Ultimately, our work helps characterise the current situation of marine carbon sink research, promote the research in this field to a deeper level of development and provide reference for subsequent scholars to carry out related research.

A review of applied research on low-carbon urban design: based on scientific knowledge mapping

The construction of low-carbon cities is an essential component of sustainable urban development. However, there is a lack of a comprehensive low-carbon city design and evaluation system that incorporates "carbon sink accounting-remote sensing monitoring-numerical modelling-design and application" in an all-around linkage, multi-scale coupling, and localized effects. This paper utilizes the Citespace tool to evaluate low-carbon city design applications by analyzing literature in the Web of Science (WOS) core collection database. The results reveal that low-carbon cities undergo four stages: "measurement-implementation-regulation - management." The research themes are divided into three core clustering evolutionary pathways: "extension of carbon sink functions," "spatialisation of carbon sink systems," and "full-cycle, full-dimensional decarbonisation." Applications include "Utility studies of multi-scale carbon sink assessments," "Correlation analysis of carbon sink influencing factors," "Predictive characterisation of multiple planning scenarios," and "Spatial planning applications of urban sink enhancement." Future low-carbon city construction should incorporate intelligent algorithm technology in real-time to provide a strong design basis for multi-scale urban spatial design with the features of "high-precision accounting, full-cycle assessment and low-energy concept."

Macroalgae culture-induced carbon sink in a large cultivation area of China

Macroalgae culture-induced carbon sink in sediments has been little investigated. Here, total organic carbon (TOC), total nitrogen (TN), and δ13C were examined in sediments in a cultivation field of macroalgae (kelp and Gracilariopsis lemaneiformis) in Sansha Bay, Southeast China. Both proxies of C/N (TOC to TN ratio) and δ13C indicated a multisource of TOC. Based on a three-endmember model, macroalgae-derived TOC (TOCma) accounted for < 35% of the total TOC, averaging 16 ± 9% (mean ± SD). On average, terrestrial and phytoplankton-derived TOC showed much higher percentages of 24 ± 17% and 60 ± 20%, respectively (t-test, p < 0.02). A preliminary estimate suggested that TOCma represents a carbon sink of 8.2 × 103 tons per year, corresponding to about 22% of the sink associated with phytoplankton and macroalgae and 8 ± 6% of the macroalgae carbon production in Sansha Bay. Considering its production magnitude, the macroalgae-induced carbon sink seems to be insignificant, on a national or global scale, to phytoplankton, though it should be taken into account given the small cultivation area.