Sponge contribution to the silicon cycle of a diatom‐rich shallow bay

Abstract

In coastal systems, planktonic and benthic silicifiers compete for the pool of dissolved silicon, a nutrient required to make their skeletons. The contribution of planktonic diatoms to the silicon cycle in coastal systems is often well characterized, while that of benthic silicifiers such as sponges has rarely been quantified. Herein, silicon fluxes and stocks are quantified for the sponge fauna in the benthic communities of the Bay of Brest (France). A total of 45 siliceous sponge species living in the Bay account for a silicon standing stock of 1215 tons, while that of diatoms is only 27 tons. The silicon reservoir accumulated as sponge skeletons in the surface sediments of the Bay rises to 1775 tons, while that of diatom skeletons is only 248 tons. These comparatively large stocks of sponge silicon were estimated to cycle two orders of magnitude slower than the diatom stocks. Sponge silicon stocks need years to decades to be renewed, while diatom turnover lasts only days. Although the sponge monitoring over the last 6 yr indicates no major changes of the sponge stocks, our results do not allow us to conclude if the silicon sponge budget of the Bay is at steady state, and potential scenarios are discussed. The findings buttress the idea that sponges and diatoms play contrasting roles in the marine silicon cycle. The budgets of these two major silicon users need to be integrated and their connections revealed, if we aim to improve our understanding of the silicon cycling in coastal ecosystems.

Nudibranch predation boosts sponge silicon cycling

“…This habitat occupies a small area of the Bay (1.44 km 2 of the total 133.13 km 2 , i.e., 1.1%). In this ecosystem, assemblages of maerl (coralline red algae) serve as substrate for a highly diverse benthic community, including more than 20 species of sponges 13 . Among them, H. perlevis (Fig.…”

“…Among them, H. perlevis (Fig. 1 a) is the dominant species in terms of both abundance (53%) and biomass (64%) 13 . It has a moderate content of siliceous skeleton derived from the production of a single, mid-size (175–475 µm long) type of needle-like silica spicule, which is homogenously distributed throughout the sponge body 31 .…”

“…We scaled this measure to the area of study using the sponge biomass (mL m −2 ) measured in situ during this study. Silicon deposition in non-predated sponges was estimated from the average sponge silicon deposition measured in field (0.9 ± 0.2 g Si m −2 y −1 ) 13 and by assuming that the contribution of sponge silicon by H. perlevis should be proportional to its standing stock in the sponge community per m 2 , which is 65.5 ± 21.0%. Silicon standing stock of non-predated sponges during the year was estimated by adding silicon production to the silicon standing stock measured in situ in mid-June 2020 and subtracting the silicon deposition.…”

“…These skeletons are more resistant to dissolution than those of other silicifiers, for reasons yet to be determined 11 . Their biological and skeletal characteristics make siliceous sponges major silicon sinks in marine ecosystems, accumulating stocks of biogenic silica within their tissues and in the sediments beneath them 11 , 12 , and recycling their silicon at significantly slower rates (200–1000 times slower) than that of other short-lived, unicellular silicifiers such as diatoms 13 . Still, in areas where they form large aggregations (e.g., sponge grounds), sponge silicon cycling can enrich dissolved silicon demersal layers on an oceanographic scale 14 .…”

On the dissolution of sponge silica: Assessing variability and biogeochemical implications

“…More questionable proposals suggest some Precambrian fossil vestiges to be sponge silica skeletons (Muscente et al, 2015). The sponge skeletons accumulated in marine sediments make important local to regional silica reservoirs (Rützler and Macintyre, 1978;Bavestrello et al, 1996;Chu et al, 2011;Gutt et al, 2013;Murillo et al, 2016;Maldonado et al, 2021;Loṕez-Acosta et al, 2022). A first quantification of the deposition and burial of sponge skeletons, which occurs more markedly on continental margins and around seamounts, revealed that these processes lead to a substantial Si sink in the global ocean (Maldonado et al, 2019).…”