硅的生物地球化学过程及其在养殖碳汇形成过程中的作用和影响

The biogeochemical cycle of silicon and its role during the formation of an aquaculture carbon sink

中文关键词: 硅循环  硅藻  植硅体  生物硅  碳汇  贝类养殖

英文关键词: Silicon cycle  Diatom  Phytolith  Biogenic silica  Carbon sink  Shellfish culture

中文摘要:

      硅(Si)是地球上丰度仅次于氧的第二大元素，其生物地球化学过程与大气CO2变化、海洋生物泵以及海岸带富营养化等过程密切相关，因此，硅循环也成为全球环境变化研究关注的核心问题之一。本文在总结已有研究的基础上，论述了硅生物地球化学过程及其对碳循环的调节及影响，进一步聚焦近海规模化贝类养殖活动，分析了贝类养殖生态系统硅循环与碳循环的耦合作用及机理，展望了未来值得关注的关键科学问题。研究表明，地质尺度上，硅酸盐矿物风化是地表所有次生硅的来源，风化过程也是一个重要的碳汇过程。陆地生态系统中，植硅体较难降解，在其形成过程中会包裹有机碳而形成稳定的有机碳库，因而植硅体碳具有重大的碳汇潜力，很有可能成为全球碳汇的重要组成部分。海洋生态系统中，作为初级生产主要贡献者的硅藻(Bacillariophyta)吸收硅酸盐合成有机碳并将其打包在生源硅颗粒中向深层海洋输送并埋藏，埋藏量可占海洋碳埋藏总量的40%，生物硅泵驱动了生物碳泵；在近海贝类养殖区，通过硅藻作为滤食性贝类的饵料来源，硅酸盐成为渔业碳汇重要的物质支撑。因此，研究硅生物地球化学循环过程中，综合考虑各过程及其耦合作用是非常必要的，对深入了解其在贝类养殖碳汇中的作用及探究潜在养殖增汇途径具有重要意义。

英文摘要:

      In the context of global climate change, one central interest is an improved understanding of the global carbon cycle. A large number of studies have investigated carbon cycling and associated elements, mainly nitrogen and phosphorus. However, as an essential element for diatom growth, Si has been largely ignored. Si is the second most abundant element and is widely distributed on Earth. The chemical weathering of silicates on land and photosynthesis of diatoms in the ocean play an important role in atmospheric CO2 levels at various timescales. Diatoms are the primary producers in the ocean and account for as much as 40% of the annual ocean carbon fixation, which have an absolute requirement for Si to form siliceous cells. The main mechanism underlying ocean carbon sinks is a “biological pump.” The biological pump is driven by the biological Si pump to a large extent. Therefore, the biogeochemical process of Si has become one of the key research issues for global environmental change. Based on previous studies, the regulation and influence of the Si biogeochemical cycle on the carbon cycle are discussed in this review. The coupling effect and mechanism of the Si and carbon cycles in shellfish culture ecosystems were analyzed and the key research questions were explored. Chemical weathering of silicates and the cycling of their products form the basis of Si biogeochemistry. CO2 is consumed during weathering reactions. Therefore, silicate weathering on land represents an important sink for atmospheric CO2. Furthermore, at the geological timescale, primary silicate mineral weathering is the source of secondary silicate. Terrestrial plants absorb soluble silica through their root system during growth. Amorphous silica deposited in plant tissue after maturity is called phytolith. Phytoliths have excellent geochemical stability and occlude a certain amount of organic carbon during the formation process. The organic carbon occluded within phytolith is called phytolith-occluded carbon (PhytOC) and is buried in the soil. PhytOC is released into the soil with phytolith and may be preserved in soils for several thousands of years. As a consequence, PhytOC in terrestrial ecosystems could be significant potential carbon sinks globally due to the refractory phytolith. Primarily through river input, the dissolved silicate (DSi) is transported into the coastal ocean (approximately 84% of DSi input to the oceans). As the major primary producer, diatoms absorb DSi during growth and account for a large fraction of the total carbon fixation in the modern oceans. DSi is converted into biogenic silica via biological processes, is transported to the deep ocean, and is finally buried into sediments with organic carbon in the marine ecosystem. Thus, by controlling the contribution of diatoms to the total primary production, DSi can affect the carbon cycle in oceans. The carbon pump is driven by the Si pump. Mariculture has developed quickly in recent decades. Shellfish, which are dominated by filter-feeding species, are the main mariculture species. The filter-feeding shellfish consume particulate organic carbon as phytoplankton and use dissolved inorganic carbon to build their shell during growth. Filter-feeding shellfish are an import fishery carbon sink. As one of the important feed sources of filter-feeding shellfish, diatoms form fishery carbon sinks in coastal shellfish culture areas. Silicate is an essential salt for diatom growth. Consequently, the carbon sink of filter-feeding shellfish culture is connected with DSi through diatoms. Si could play an important role in driving the formation of carbon sinks in filter-feeding shellfish culture. Hence, it is necessary to consider all processes and coupling effects in the study of the Si biogeochemical cycle. It is important to understand its role in the carbon sinks of shellfish culture. Nowadays, in many systems, human perturbation has resulted in a decline in the ratio of Si:N to 1:1 or less, with severe impacts on the quality and structure of aquatic ecosystems. DSi limitation has been reported in many studies, in both coastal and marine waters. DSi limitation causes shifts from diatoms to non-siliceous algae and is supposedly related to the decreasing export of carbon. A shift from diatoms to other species would enhance the recycling of organic matter in the upper water column because diatoms are very effective in carbon sequestration. DSi limitation has also appeared in some aquaculture bays in China, such as Jiaozhou Bay and Laizhou Bay, in spring. Regarding future directions, it is suggested that more research be conducted on Si biogeochemistry in shellfish culture systems and coupling with the carbon cycle. The subsequent results could evaluate the role of Si in the carbon sink of filter-feeding shellfish culture. Future studies are expected to provide ideas for alleviating Si deficiency in the aquaculture bay and exploring the expansion path in shellfish farming.