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 cycling of silicon 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 superficial 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 years indicates no major changes of the sponge stocks, our results do not allow 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 silicon major users need to be integrated and their connections revealed, if we aim to reach a full understanding of the silicon cycling in coastal ecosystems.

Nudibranch predation boosts sponge silicon cycling

“…These species coexist at the maerl bed of Lomergat (48° 17.197’ N; 4° 21.279’ W), a shallow-water ecosystem (depth=3-11 m) located in the Bay of Brest (France). In this ecosystem, which occupies a small area of the Bay (1.44 km 2 of the total 133.13 km 2 ), assemblages of the maerl species Lithothamnion corallioides serve as the substrate for a highly diverse benthic community, including more than 20 sponge species 14 . Among them, H. perlevis is the dominant species in terms of both abundance (53%) and biomass (64%) 14 .…”

“…In this ecosystem, which occupies a small area of the Bay (1.44 km 2 of the total 133.13 km 2 ), assemblages of the maerl species Lithothamnion corallioides serve as the substrate for a highly diverse benthic community, including more than 20 sponge species 14 . Among them, H. perlevis is the dominant species in terms of both abundance (53%) and biomass (64%) 14 . 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 32 .…”

“…for reasons yet to be determined 12 . Their biological and skeletal characteristics make siliceous sponges major silicon sinks in marine ecosystems, accumulating vast stocks of biogenic silica within their tissues and in the sediments beneath them 12,13 , and recycling their silicon at significantly slower rates (200 to 1000 times slower) than that of other short-living, unicellular silicifiers such as diatoms 14 . Still, in areas where they form large aggregations (e.g., sponge grounds), sponge silicon cycling can enrich in dissolved silicon demersal layers on an oceanographic scale 15 .…”

“…These skeletons are more resistant to dissolution than those of other silicifiers, for reasons yet to be determined 12 . Their biological and skeletal characteristics make siliceous sponges major silicon sinks in marine ecosystems, accumulating vast stocks of biogenic silica within their tissues and in the sediments beneath them 12,13 , and recycling their silicon at significantly slower rates (200 to 1000 times slower) than that of other short-living, unicellular silicifiers such as diatoms 14 . Still, in areas where they form large aggregations (e.g., sponge grounds), sponge silicon cycling can enrich in dissolved silicon demersal layers on an oceanographic scale 15 .…”

“…Such consumption resulted in a mean normalized predation rate of 0.23 ± 0.11 mL sponge d -1 mL -1 nudibranch. Sponges incubated without predators (controls; n=3) showed no change in volume 14.…”