Introduction

Across the marine landscape, from estuaries to the open ocean, biota take up silicon (Si) as monosilicic acid and deposit it into their tissues as biogenic silica (BSi). Along the coast, vegetated ecosystems, such as salt marshes and mangroves, sequester a significant amount of Si in their tissues and likely help regulate the availability of Si in surrounding waters (Carey & Fulweiler, 2014; Elizondo et al., 2021). Si is also accumulated by sponges, euglyphid amoebae, radiolarians, silicoflagellates, and choanoflagellates, as well as a few coccolithophores, Prasinophyceae, and picocyanobacteria (Raven & Giordano, 2009; Gadd & Raven, 2010; Baines et al., 2012). The dominant driver of coastal (and open ocean) Si cycling, however, is generally thought to be diatoms. These siliceous phytoplankton require Si on a 1 : 1 molar ratio with nitrogen (N). Diatoms are responsible for 40–50% of global marine primary production (Field et al., 1998; Rousseaux & Gregg, 2013) and form the base of the marine food web in many parts of the ocean, especially coastal temperate regions (Irigoien et al., 2002).

Macroalgae are also important primary producers, particularly in shallow coastal marine ecosystems, with global net primary production of 80–210 Tmol C yr⁻¹ (Raven, 2018). Macroalgae act as a food source for grazers (Horne et al., 1994) and play a large role in altering the cycling of nutrients, such as N and phosphorus (P) (Hersh, 1995). Many estuaries have experienced a shift towards macroalgae as the dominant group of primary producers over the past several decades (Valiela et al., 1992; Hauxwell et al., 2001; Potter et al., 2021). This is due to the ability of macroalgae to thrive in nutrient-rich systems, displacing other primary producers by way of rapid uptake of N and P and shading of photosynthetic organisms below (e.g. the seagrass Zostera; Valiela et al., 1992, 1997; Peckol et al., 1994).

The role of macroalgae on Si availability, however, is largely unconstrained, with only a few published studies reporting BSi concentrations. Work on freshwater macrophytes found BSi concentrations ranged from 0.2% to 2.8% (by dry weight), and the percentage of BSi was positively correlated to water flow (Schoelynck et al., 2010, 2012). Research from four decades ago on freshwater macroalgae demonstrated more rapid growth in Cladophora glomerata when Si was added to the growth medium (Moore & Traquair, 1976). The stipe of Ecklonia cava was reported to contain BSi concentrations of 13.35 μg g⁻¹ dry mass (0.0013% BSi), and the tissue of Delisea fimbriaia contained 1530 μg g⁻¹ dry mass (0.15% BSi) (Fu et al., 2000). More recently, BSi concentrations of the sporophytes of kelp, Saccharian japonica, were found to vary by location on the blade (Mizuta & Yasui, 2012) and to increase when S. japonica experienced various stresses (Mizuta et al., 2021). Similarly, BSi concentrations of Pyropia yezoensis increased when exposed to increased temperature and reached a concentration of 30% BSi (Le et al., 2019).

We hypothesized that marine macroalgae may contain significant amounts of Si in their biomass and thus could impact the Si cycle of coastal ecosystems. To determine the extent to which marine macroalgae are a reservoir of Si, we quantified BSi concentrations from 12 macroalgae genera from two temperate estuaries (Narragansett Bay, Rhode Island, and Waquoit Bay, Massachusetts, USA), and from a subset of samples we measured macroalgae percentage carbon (C). Finally, from one of the estuaries, we used macroalgae δ¹³C values, which can be used as a proxy for identifying CO₂ or bicarbonate source in photosynthesis, to infer the presence/absence of C concentration mechanisms (Raven et al., 2002) and as an indicator of productivity (Oczkowski et al., 2010). We then examined the relationship between BSi concentrations and macroalgae δ¹³C to better understand mechanisms driving BSi uptake.

中文翻译：

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海洋大型藻类是沿海系统中被忽视的硅槽

介绍

在整个海洋景观中，从河口到开阔的海洋，生物群以单硅酸的形式吸收硅 (Si)，并将其作为生物二氧化硅 (BSi) 沉积到组织中。沿海地区的植被生态系统（例如盐沼和红树林）在其组织中隔离了大量的硅，并可能有助于调节周围水域中硅的可用性（Carey & Fulweiler，2014 年；Elizondo 等人，2021年）。Si 也被海绵、euglyphid 变形虫、放射虫、硅鞭毛虫和领鞭毛虫以及一些颗石藻、Prasinophyceae 和 picocyanobacteria 积累 (Raven & Giordano, 2009; Gadd & Raven , 2010 ; Baines et al ., 2012 )). 然而，沿海（和开阔海域）Si 循环的主要驱动因素通常被认为是硅藻。这些硅质浮游植物需要 Si 与氮 (N) 的摩尔比为 1:1。硅藻占全球海洋初级生产的 40-50%（Field等人，1998 年；Rousseaux 和 Gregg，2013 年），并在海洋的许多地方，尤其是沿海温带地区构成海洋食物网的基础（Irigoien等人）等人，2002 年）。

大型藻类也是重要的初级生产者，特别是在浅海沿海海洋生态系统中，全球净初级生产量为 80-210 Tmol C yr ⁻¹（Raven，2018 年）。大型藻类是食草动物的食物来源 (Horne et al ., 1994 )，并且在改变 N 和磷 (P) 等养分循环方面发挥重要作用 (Hersh, 1995 )。在过去的几十年里，许多河口都经历了向大型藻类转变，成为初级生产者的主要群体（Valiela等人，1992 年；Hauxwell等人，2001 年；Potter等人，2021 年）). 这是由于大型藻类能够在营养丰富的系统中茁壮成长，通过快速吸收 N 和 P 以及遮蔽下面的光合生物（例如海草 Zostera；Valiela 等人，1992年， 1997年； Peckol等人，1994 年）。

然而，大型藻类对 Si 可用性的作用在很大程度上不受限制，只有少数已发表的研究报告了 BSi 浓度。对淡水大型植物的研究发现 BSi 浓度范围为 0.2% 至 2.8%（按干重计），并且 BSi 的百分比与水流量呈正相关（Schoelynck 等人，2010年，2012年）。四十年前对淡水大型藻类的研究表明，当将 Si 添加到生长培养基中时，Cladophora glomerata的生长速度更快 (Moore & Traquair, 1976 )。据报道， Ecklonia cava的柄含有 13.35 μg g ⁻¹干重 (0.0013% BSi) 的 BSi 浓度，以及Delisea fimbriaia含有 1530 μg g ⁻¹干物质 (0.15% BSi)（Fu等人，2000）。最近，发现海带Saccharian japonica的孢子体 BSi 浓度随叶片上的位置而变化（Mizuta 和 Yasui， 2012 年），并且当S. japonica经历各种压力时会增加（Mizuta等人，2021 年）。类似地，当暴露于升高的温度并达到 30% BSi 的浓度时，Pypopia yezoensis的 BSi 浓度会增加（Le等人，2019 年）。

我们假设海洋大型藻类的生物量中可能含有大量的 Si，因此可能会影响沿海生态系统的 Si 循环。为了确定海洋大型藻类在多大程度上是 Si 的储存库，我们量化了来自两个温带河口（美国马萨诸塞州纳拉甘西特湾和 Waquoit 湾）的 12 个大型藻类的 BSi 浓度，以及我们测量的样本子集大型藻类百分比碳 (C)。最后，我们使用其中一个河口的大型藻类 δ ¹³ C 值来推断是否存在 C 浓度机制（Raven等人，2002 年），该值可用作识别光合作用中 CO _{2或碳酸氢盐来源的替代指标}) 并作为生产力的指标（Oczkowski等人，2010 年）。^{然后我们检查了 BSi 浓度与大型藻类 δ 13} C之间的关系，以更好地理解驱动 BSi 摄取的机制。