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Marine macroalgae are an overlooked sink of silicon in coastal systems
New Phytologist  (IF10.151),  Pub Date : 2021-12-01, DOI: 10.1111/nph.17889
Mollie R. Yacano, Sarah Q. Foster, Nicholas E. Ray, Autumn Oczkowski, John A. Raven, Robinson W. Fulweiler

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 DW), 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 per gram 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, RI, and Waquoit Bay, MA, 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.