Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean

Experimental degradation of Ulva prolifera in sand sediments: Implications for dissolved carbon and alkalinity export from green-tide macroalgae buried in sandy intertidal zones

The degradation of massive Ulva prolifera (U. prolifera), generated during blooms of the macroalgae (i.e., green tides), exerts multi-faceted impacts on biogeochemical cycling in coastal seawater and sediments. The kinetics of U. prolifera degradation in permeable sediments and subsequent export of degradation products remain poorly understood. In this study, we conducted a 30-d experimental investigation of Ulva prolifera degradation in sandy sediments under conditions of periodic water exchange (WE) to characterize the degradation kinetics and quantify the export of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total alkalinity (TA). The results demonstrated that U. prolifera degradation in the sandy sediments followed first-order kinetics, independent of WE frequency. Compared to control systems without WE, increasing WE frequency accelerated degradation rates by a factor of 2.5-3.6, resulting in 1.8-6.8-fold increases in the export of DOC, DIC, and TA. DOC derived from U. prolifera degradation was predominantly composed of labile protein-like components, though its lability gradually declined as degradation progressed. The production of DIC and TA was predominantly through anaerobic degradation of U. prolifera coupled to sulfate reduction and/or denitrification. The resulting DIC presented primarily as carbonate alkalinity, which may exert legacy effects on marine carbon sequestration potential. Our findings suggest that fine particulate organic matter (FPOM) represents a significant byproduct of U. prolifera degradation under dynamic conditions. The export and subsequent degradation of FPOM may substantially influence dissolved carbon dynamics in coastal ecosystems.

Herbivore grazing enhances macroalgal organic carbon release and alters their carbon sequestration fate in the ocean

Herbivore grazing on macroalgae promotes the release of macroalgal organic carbons into seawater and potentially impacts their bioavailability. However, the influence of herbivores on the fate of macroalgal organic carbon remains unclear, hindering a comprehensive and in-depth understanding of the role of macroalgae in ocean carbon cycle. Here, we cocultured suspended herbivore (Apohyale sp.) and benthic herbivore (Nereis diversicolor) with macroalgae (Ulva prolifera) in the laboratory, and found that the two grazers promote the release of macroalgal organic carbon through different pathways. Apohyale sp. Can simultaneously increase the release of different forms of organic carbon by feeding on U. prolifera thalli, including dissolved organic carbon (DOC), particluate organic carbon (POC), and algal organic detritus; while N. diversicolor demonstrated a preference for ingesting algal detritus and POC, thereby reducing the detrital carbon but greatly promoting their conversion to DOC. The amount of organic carbon released per day after predation by Apohyale sp. is much higher (7.2 vs 0.5 mg C d-1) than by N. diversicolor. Meanwhile, through long-term microbial degradation experiments, we found that herbivores significantly alter the fate of macroalgae organic carbon. Although the proportions of stable carbon (recalcitrant DOC and recalcitrant POC) in different forms of macroalgal organic carbon varied after predation, the absolute amount of their residuals in seawater were 2-3 times higher than those not ingested by herbivores. Our results highlight that herbivores play a pivotal role in promoting carbon flow in marine food webs and have a significant impact on macroalgal carbon sequestration.

Seawater warming rather than acidification profoundly affects coastal geochemical cycling mediated by marine microbiome

The most concerning consequences of climate change include ocean acidification and warming, which can affect microbial communities and thus the biogeochemical cycling they mediate. Therefore, it is urgent to study the impact of ocean acidification and warming on microbial communities. In the current study, metagenomics was utilized to reveal how the structure and function of marine microorganisms respond to ocean warming and acidification. In terms of community structure, Non-metric Multidimensional Scaling analysis visualized the similarity or difference between the control and the warming or acidification treatments, but the inter-group differences were not significant. In terms of gene functionality, warming treatments showed greater effects on microbial communities than acidification. After treatment with warming, the relative abundance of genes associated with denitrification increased, suggesting that ocean nitrogen loss can increase with increased temperature. Conversely, acidification treatments apparently inhibited denitrification. Warming treatment also greatly affected sulfur-related microorganisms, increasing the relative abundance of certain sulfate-reducing prokaryote, and enriched microbial carbon-fixation pathways. These results provide information on the response strategies of coastal microorganisms in the changing marine environments.

Lipid biomarkers indicate the dynamics of particulate organic carbon and its carbon sequestration effects during the degradation of Ulva prolifera

Millions of tons of Ulva prolifera sink to the seafloor and gradually degrade after green tide occurred annually in the Yellow Sea, releasing substantial amounts of particulate organic carbon (POC) into marine environments. However, monitoring the dynamics of macroalgae-derived POC and its carbon sequestration effects is challenging due to severe environmental disturbances. Here, we conducted a long-term simulated degradation experiment with U. prolifera in the laboratory. During degradation, 86-90 % of U. prolifera-derived POC was readily degraded by microorganisms, while 10-14 % was stabilized in seawater as bio-recalcitrant POC. Microbial community structure underwent significant succession, driving the degradation of U. prolifera and the release and transformation of POC. 28-isofucosterol and POC concentrations changed concurrently and showed a significant positive correlation throughout the degradation. Hence, we propose that lipid biomarkers, i.e. 28-isofucosterol, can be used to track the release of U. prolifera-derived POC and to potentially reveal its carbon sequestration in marine environments.

Regulation of seawater dissolved carbon pools by environmental changes in Ulva prolifera originating sites: A new perspective on the contribution of U. prolifera to the seawater carbon sink function

The Ulva prolifera bloom is considered one of the most serious ecological disasters in the Yellow Sea in the past decade, forming a carbon sink in its source area within a short period but becoming a carbon source at its destination. To explore the effects of different environmental changes on seawater dissolved carbon pools faced by living U. prolifera in its originating area, U. prolifera were cultured in three sets with different light intensity (54, 108, and 162 μmol m-2 s-1), temperature (12, 20, and 28 °C) and nitrate concentration gradients (25, 50, and 100 μmol L-1). The results showed that moderate light (108 μmol m-2 s-1), temperature (20 °C), and continuous addition of exogenous nitrate significantly enhanced the absorption of dissolved inorganic carbon (DIC) in seawater by U. prolifera and most promoted its growth. Under the most suitable environment, the changes in the seawater carbonate system were mainly dominated by biological production and denitrification, with less influence from aerobic respiration. Facing different environmental changes, U. prolifera continuously changed its carbon fixation mode according to tissue δ13C results, with the changes in the concentrations of various components of DIC in seawater, especially the fluctuation of HCO3- and CO2 concentrations. Enhanced light intensity of 108 μmol m-2 s-1 could shift the carbon fixation pathway of U. prolifera towards the C4 pathway compared to temperature and nitrate stimulation. Environmental conditions at the origin determined the amount of dissolved carbon fixed by U. prolifera. Therefore, more attention should be paid to the changes in marine environmental conditions at the origin of U. prolifera, providing a basis for scientific management of U. prolifera.

Response to the CO2 concentrating mechanisms and transcriptional time series analysis of Ulva prolifera under inorganic carbon limitation

Ulva prolifera is a dominant species in green tides and has been affecting marine ecosystem for many years. Due to the low availability of CO2 in the environment, U. prolifera utilizes the CO2 concentrating mechanisms (CCMs) to increase intracellular inorganic carbon concentration. However, the transcriptional response mechanism and temporal changes of U. prolifera CCMs based on transcriptomics have not been thoroughly described. Therefore, we induced U. prolifera CCMs in a low CO2 environment to explore the dynamic regulation of CCMs expression under inadequate inorganic carbon supply. The results showed that inorganic carbon limitation increased the inorganic carbon affinity of U. prolifera, upregulating CCMs. The first 24 h of inorganic carbon environmental changes were the most active period for U. prolifera's expression regulation. U. prolifera gradually achieved a new steady state by regulating metabolic processes such as nucleic acids, energy, and ethylene-activated signaling pathways. In the carbon fixation system of U. prolifera, there are characteristics of both biophysical and biochemical CCMs. After 24 h of inorganic carbon limitation, the biophysical CCMs becomes more effective under conditions of inorganic carbon depletion. This study aids in exploring the CCMs of U. prolifera and their evolution in response to environmental changes.

Sea surface carbon dioxide during early summer at the Tuandao nearshore time series site

To investigate air-sea CO2 flux at the Qingdao nearshore site and its temporal variations, a high-resolution continuous observation of surface carbon dioxide partial pressure (pCO2) was carried out at Zhongyuan Pier near Tuandao from May 25 to July 8, 2019. It was observed that during this period, surface pCO2 varied between ∼490 and ∼690 μatm, mainly associated with sea surface temperature. Surface pCO2 also displayed substantial diurnal variations, with an average amplitude of 64 ± 21 μatm, largely dominated by biological activities. During the observational period, this site acted as a source of atmospheric CO2, releasing 361 mmol CO2 m-2. The notable diurnal variations in air-sea CO2 flux, such as the observed average amplitude of 10.9 mmol m-2 d-1 in this study, pose a challenge for accurately estimating the air-sea CO2 flux in coastal regions without high-resolution observations.

Response of dissolved inorganic carbon dynamics to simulated tidal hydrological processes in coastal wetlands

To clarify the impacts of tidal hydrological process shifts caused by sea level rise on the blue carbon cycle, a typical coastal wetland in Jiaozhou Bay was selected for this study. The soils of Suaeda salsa (SS) and Phragmites australis (PA) wetlands were collected to simulate the effects of three types of tidal hydrological processes (Neap tide group, NT; Middle tide group, MT; Spring tide group, ST) on the soil-water dissolved inorganic carbon (DIC) dynamic. The results showed that the concentration of water dissolved inorganic carbon (WDIC) increased rapidly (115% higher) at early stage (days 0-4) under the influence of the tidal hydrological processes. Significant differences were found in WDIC concentration during different tidal hydrological processes (P < 0.05), which were expressed as MT (52.7 ± 13.3 mg L-1) > ST (52.5 ± 12.9 mg L-1) > NT (48.4 ± 10.1 mg L-1). After experiencing the tidal hydrological processes, the soil DIC content showed a net accumulation (55.1 ± 1.29 mg L-1vs. 46.7 ± 1.76 mg L-1, P < 0.001), whereas the soil inorganic carbon (SIC) decreased (2.73 ± 1.64 mg L-1vs. 4.61 ± 1.71 mg L-1), which may be attributed to the dissolution of SIC caused by the uptake of CO2 to form DIC. The accumulation of soil DIC was directly related to the SIC (λ = 1.03, P < 0.01), and indirectly related to soil nutrients (SOC substrate, λ = -0.003) and microbes (microbial biomass, λ = -0.10), and was mainly dominated by abiotic processes (abiotic: 58.1 ± 1.8% to 82.7 ± 2.46% vs. biotic: 17.4 ± 2.46% to 41.9 ± 1.76%). The increase of tidal frequency generally inhibited the accumulation of soil DIC content and promoted the output of WDIC. However, the response of soil DIC in different wetland types to tidal frequency was divergent, which was mainly regulated by the trade-off between soil nutrients and SIC content. Taken together, tidal hydrological processes and their frequency changes reshaped DIC dynamics, promoted the dissolution of SIC and the potential uptake of CO2. These findings enhance the comprehension of the inorganic carbon cycle within coastal wetlands, particularly amidst the backdrop of climate change and the rising sea levels.

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.

Effective capping of dissolved sulfide generated in Ulva prolifera-rich marine sediments by iron-rich red soil

Bloom-induced macroalgal enrichment on the seafloor can substantially facilitate dissolved sulfide (DS) production through sulfate reduction. The reaction of DS with sedimentary reactive iron (Fe) is the main mechanism of DS consumption, which however usually could not effectively prevent DS accumulation caused by pulsed macroalgal enrichment. Here we used incubations to investigate the performance of Fe-rich red soil for buffering of DS produced from macroalgae (Ulva prolifera)-enriched sediment. Based on our results, a combination of red soil additions (6.8 kg/m2) before and immediately after pulsed macroalgal deposition (455 g/m2) can effectively cap DS within the red soil layer. The effective DS buffering is mainly due to ample Fe-oxide surface sites available for reaction with DS. Only a small loss (4 %) of buffering capacity after 18-d incubation suggests that the red soil is capable of prolonged DS buffering in macroalgae-enriched sediments.

Spatial distribution characteristics and interaction effects of DOM and microbial communities in kelp cultivation areas

The influence of macroalgae cultivation on aquaculture carbon sinks is significant, with microbial carbon (C) pumps contributing to a stable inert dissolved carbon pool in this context. Concurrently, dissolved organic matter (DOM) exchange at the marine sediment-water interface profoundly affects global ecosystem element cycling. However, the interactions between DOM and bacterial communities at the sediment-water interface in kelp cultivation areas, especially regarding microbial function prediction, have not been fully explored. This study analyzed the DOM characteristics, environmental factors, and bacterial community structure in the Tahewan kelp--Saccharina japonica cultivated area and compared them with those in non-cultivated areas. The results indicated significantly higher dissolved organic carbon (DOC) concentrations in the kelp culture area, particularly in surface seawater and overlying water. The dominant bacterial phyla in both regions included Pseudomonadota, Actinomycetota, and Bacteroidota in both regions, while Desulfobacterota was more prevalent in the sediment environment of the cultivated region. Parallel factor analysis (EEM-PARAFAC) was used to identify DOM components, among which component C2 (a microbial humic-like substance DOM) was highly resistant to microbial degradation. We infer that C2 has similar properties to recalcitrant dissolved organic matter (RDOM). Analysis of the predicted functional genes based on 16S rRNA gene data showed that methanol oxidation, methylotrophy, and methanotrophy were significant in the bottom seawater of the cultivation area. The carbon (C), nitrogen (N), and sulfur (S) cycle functional genes in the sediment environment of the kelp cultivation area were more active than those in other areas, especially in which sulfate reduction and denitrification were the two main processes. Furthermore, a DOM priming effect was identified in the cultivated sediment environment, where kelp-released labile dissolved organic matter (LDOM) stimulates rapid degradation of the original RDOM, potentially enhancing C sequestration.

Diversity, composition and ecological networks of bacterial communities in response to a full cultivation cycle of the seaweed, Gracilariopsis lemaneiformis

Cultivation of the seaweed, Gracilariopsis lemaneiformis, supports environmental bioremediation and the aquaculture economy in coastal ecosystems, and microorganisms play important roles during the cultivation process. In this study, we aimed to understand the response of bacterial communities through a full cultivation cycle of G. lemaneiformis. We analyzed the bacterial communities using 16S rRNA gene amplicon sequencing and defined the environmental factors of 144 water samples from the Nan'ao Island, South China Sea. Community diversity, keystone species and ecological networks of bacterial communities shifted markedly in the cultivation zone largely due to changes in the environmental factors, seaweed biomass and cultivation stages. The bacterial communities at the seaweed zone have lower species richness, more seaweed-associated taxa and simpler but more stable co-occurrence networks compared to the control zone. Persistent microbial groups such as Aquimarina, Formosa, Glaciecola and Marinobacter exhibited a strong association with seaweed during the growth and maturity stages. We describe a conceptual model to summarize the changes in the bacterial community composition, its diversity and the ecological networks in seaweed cultivation zone. Overall, this study provides new perspectives on the dynamic interaction of seaweed cultivation, bacterial communities and environment factors and their potential ecosystem services as observed in the example of the G. lemaneiformis cultivation ecosystem.

Particulate Organic Carbon Released during Macroalgal Growth Has Significant Carbon Sequestration Potential in the Ocean

Substantial amounts of particulate organic carbon (POC) are released during macroalgal growth; however, the fate of these POCs and their carbon sequestration effects remain unclear. Here, field investigations found that Ulva prolifera caused a significant increase of POC in seawater below the surface during a macroalgal bloom. However, laboratory simulations revealed that 77.6% of these POC was easily degraded by microorganisms in a short period of time, concurrently resulting in the production of dissolved organic carbon (DOC) from POC transformation. Over a period of 3 months, the bioavailable components of macroalgae-released POC and POC-transformed DOC were degraded, leaving 39.6% of the antibiodegradable substances composed of biorecalcitrant POC and biorecalcitrant DOC. However, although the biorecalcitrant POC was rich in humic-like components resisting biodegradation, the biorecalcitrant POC exhibited greater sensitivity to photodegradation than biorecalcitrant DOC. The photodegradation removal rate of biorecalcitrant POC (14.1%) was more than 10 times that of biorecalcitrant DOC (1.2%). Ultimately, a substantial portion (36.3%) of the POC released by growing macroalgae could potentially perform long-term carbon sequestration after conversion to recalcitrant POC and recalcitrant DOC, and these inert carbons derived from macroalgal POC have been previously ignored and should also be included in macroalgal carbon sequestration accounting.

Dissolved organic nitrogen cycling revealed at the molecular level in the Bohai and Yellow Sea

Marginal seas play a crucial role in the cycling of dissolved organic nitrogen (DON) between the terrestrial and marine environments. However, very few studies have considered the molecular transformation of DON in marginal seas, leaving the DON molecular modifications in its cycling largely unknown. Therefore, this study examined DON cycling in the Bohai Sea and Yellow Sea, two semi-closed marginal seas in northern China, using stable isotopes (δ15N and δ13C), optical characteristics, and molecular compositions. Compared to the Yellow Sea, the Bohai Sea had a weaker exchange with the open ocean, resulting in higher concentrations, lower δ15N, and more recalcitrant properties in DON. The DON cycling showed significant differences inside and outside the Yellow Sea Cold Water (YSCW). Degradation was the major sink of DON in the YSCW, during which more highly unsaturated compounds and carboxyl-rich alicyclic molecules were produced. Nitrogen atoms were found to be removed from the molecules with more N atoms to those with fewer ones during the DON degradation. This study discovered the molecular modifications in DON cycling and highlighted the intrinsic mechanisms in the cycling of DON in marginal seas.