Marine macroalgae are a key primary producer in coastal ecosystems, but are often overlooked in blue carbon inventories. Large quantities of macroalgal detritus deposit on beaches, but the fate of wrack carbon (C) is little understood. If most of the wrack carbon is respired back to CO₂, there would be no net carbon sequestration. However, if most of the wrack carbon is converted to bicarbonate (alkalinity) or refractory DOC, wrack deposition would represent net carbon sequestration if at least part of the metabolic products (e.g., reduced Fe and S) are permanently removed (i.e., long-term burial) and the DOC is not remineralised. To investigate the release of macroalgal C via porewater and its potential to contribute to C sequestration (blue carbon), we monitored the degradation of Ecklonia radiata in flow-through mesocosms simulating tidal flushing on sandy beaches. Over 60 days, 81% of added E. radiata organic matter (OM) decomposed. Per 1 mol of detritus C, the degradation produced 0.48 ± 0.34 mol C of dissolved organic carbon (DOC) (59%) and 0.25 ± 0.07 mol C of dissolved inorganic carbon (DIC) (31%) in porewater, and a small amount of CO₂ (0.3 ± 0.0 mol C; ca. 3%) which was emitted to the atmosphere. A significant amount of carbonate alkalinity was found in porewater, equating to 33% (0.27 ± 0.05 mol C) of the total degraded C. The degradation occurred in two phases. In the first phase (days 0–3), 27% of the OM degraded, releasing highly reactive DOC. In the second phase (days 4–60), the labile DOC was converted to DIC. The mechanisms underlying E. radiata degradation were sulphate reduction and ammonification. It is likely that the carbonate alkalinity was primarily produced through sulphate reduction. The formation of carbonate alkalinity and semi-labile or refractory DOC from beach wrack has the potential to play an overlooked role in coastal carbon cycling and contribute to marine carbon sequestration.

Graphical abstract

中文翻译：

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海滩大型藻类残骸降解过程中溶解碳和碱度的产生：对蓝碳有影响的中宇宙实验

海洋大型藻类是沿海生态系统的主要初级生产者，但在蓝碳清单中经常被忽视。大量大型藻类碎屑沉积在海滩上，但对残骸碳 (C) 的命运知之甚少。如果大部分残骸碳被呼吸回 CO ₂，​​则不会有净碳封存。然而，如果大部分残骸碳转化为碳酸氢盐（碱度）或难降解的 DOC，如果至少部分代谢产物（例如，还原的 Fe 和 S）被永久去除（即长长期埋葬）并且 DOC 没有再矿化。为了研究通过孔隙水释放的大型藻类 C 及其对 C 封存（蓝碳）的贡献，我们监测了Ecklonia radiata在流过的中宇宙模拟沙滩上的潮汐冲刷。超过 60 天，81% 添加的E. radiata有机物 (OM) 分解。每 1 mol 碎屑 C，降解在孔隙水中产生 0.48 ± 0.34 mol C 的溶解有机碳 (DOC) (59%) 和 0.25 ± 0.07 mol C 的溶解无机碳 (DIC) (31%)，以及少量CO _2的(0.3 ± 0.0 mol C; ca. 3%) 排放到大气中。在孔隙水中发现了大量的碳酸盐碱度，相当于总降解 C 的 33% (0.27 ± 0.05 mol C)。降解发生在两个阶段。在第一阶段（第 0-3 天），27% 的 OM 降解，释放出高活性的 DOC。在第二阶段（第 4-60 天），不稳定的 DOC 转化为 DIC。E. radiata降解的机制是硫酸盐还原和氨化。碳酸盐碱度可能主要是通过硫酸盐还原产生的。来自海滩残骸的碳酸盐碱度和半不稳定或难熔 DOC 的形成有可能在沿海碳循环中发挥被忽视的作用，并有助于海洋碳封存。

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