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

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.

Bacterial biosilicification: A new insight into the global silicon cycle

Biosilicification is the process by which organisms incorporate soluble, monomeric silicic acid, Si(OH)4, in the form of polymerized insoluble silica, SiO2. Biosilicifying eukaryotes, including diatoms, siliceous sponges, and higher plants, have been the targets of intense research to study the molecular mechanisms underlying biosilicification. By contrast, prokaryotic biosilicification has been less well studied, partly because the biosilicifying capability of well-known bacteria was not recognized until recently. This review summarizes recent findings on bacterial extracellular and intracellular biosilicification, the latter of which has been demonstrated only recently in bacteria. The topics discussed herein include bacterial (and archaeal) extracellular biosilicification in geothermal environments, encapsulation of Bacillus spores within a silica layer, and silicon accumulation in marine cyanobacteria. The possible contribution of bacterial biosilicification to the global silicon cycle is also discussed.

Observations Inform Improvements in Model Silicon Cycling in a Semi-enclosed Coastal Sea

<p>We have developed a coupled physical-biological model representing plankton and nutrient dynamics of the Strait of Georgia, a fjord-like semi-enclosed coastal sea on the west coast of Canada. The nutrient-phytoplankton-zooplankton-detritus (NPZD)-type biological model is based on nitrogen uptake and remineralization with a coupled silicon cycle and includes both diatom and non-siliceous phytoplankton functional groups. The Strait of Georgia exhibits an estuarine circulation driven by input from the Fraser River as well as many smaller rivers and streams. It has high levels of dissolved silica (can be >50 μM even at the surface). Silicon-replete conditions shape key characteristics of the local ecosystem, which include heavily silicified glass sponge reefs as well as frequent diatom and occasional silicoflagellate blooms. We therefore consider the ability of the model to match observed silicon levels an indicator of the fidelity of its representation of local biogeochemistry. Silicon in the model may be in the form of dissolved silica, living diatoms, or particulate biogenic silica, and model diatom growth may be limited by nitrogen, light, or dissolved silica availability. We will discuss the challenges involved in accurately representing important drivers of the regional silicon cycle. These include accurately capturing the division of primary productivity between diatoms and non-siliceous phytoplankton functional groups, as well as uncertainties in the magnitude of terrestrial inputs and sediment fluxes. We will show how evaluating the model functional groups by comparison with phytoplankton community composition determined by high performance liquid chromatography (HPLC) has informed our interpretation of model results and provided direction for efforts at improving model performance. We will discuss the impact of targeted adjustments to model parameters on the model silicon cycle in light of comparisons to observations.</p>

Contribution of terrestrially derived phytolith as a marine silicon sink

<p>Silicon is important as a nutrient for phytoplankton (diatom, radiolarian, silicoflagellates and sponges) and for the phytolith production by terrestrial vegetation. Silicon also contributes in removing carbon dioxide from the atmosphere through silicate weathering.  Hence it is important to understand the behavior of the silicon cycle throughout earth history. Silica is the second most abundant element in the earth's crust and the concentration of silicic acid in the marine environment has not changed since the past 10,000 years. Phytolith plays an important role in the silicon cycle. While the phytoplankton in marine environment bioengineers silica within the water column, phytolith transports terrestrial biogenic silica into the marine environment and act as a silicon sink. Though astonishingly, very few researches have been carried out in the field of marine phytolith sink and also on the phytoliths in the marine environment.</p><p>For this study, we have chosen the world highest terrestrial sediment receiving submarine fan, the Bengal fan. The core sample was extracted at a water depth of 3520m at 85.960985 N, 9.99351 E. 24 phytolith types were identified and all the morphotypes were counted dividing into three size classes. These size classes were specific to considering morphotypes. Most related simple geometries were used to calculate the volume of phytolith cells and these volume data were used in calculating the total volume of phytolith in one gram of sediment by combining with an absolute abundance of phytolith data for each size class, which were later used to calculate the total weight of phytolith in one gram of marine sediment. According to the results in deep oceanic sediment at the core, the location contains ⁓0.15mg/g phytolith during the low phytolith flux periods (ex. Late Holocene) and ⁓2.678mg/g of phytolith during the high phytolith flux periods such as 25ka to 30ka B.P. and around the beginning of deglaciation. After removing 10% from the total weight as phytolith occluded carbon (PhytOC), phytolith derived biogenic silica content in sediment varies from ⁓0.135mg/g - ⁓2.41mg/g. Thus, phytolith in marine sediment contributes as a permanent silicon and carbon sink. By considering average marine sediment density as 1.7g/cm<sup>3</sup>, in a 1cm thick, one square km sediment layer contains ⁓2 to 40 metric tons of biogenic silica derived from phytolith, during low and high phytolith flux periods. This study serves as the pioneer of this field of study and further it is important to investigate the release of biogenic silica in to marine environment by phytolith and PhytOC content in different morphotypes and in different geological regions, for better understanding the contribution of phytolith to the biogenic silicon cycle in the marine environment.</p><p>Keywords: Marine phytolith, Deep oceanic sediment, Silicon cycle, Phytolith Flux, Silicon sink.</p><p><strong>Acknowledgements</strong></p><p>This work was funded by the National Natural Science Foundation of China (NSFC 41876062) and Key Special Project for Introduced Talents Team of Southern Marine Science and EngineeringGuangdong Laboratory (Guangzhou) (GML2019ZD0206).</p><p> </p>

Plants sustain the terrestrial silicon cycle during ecosystem retrogression

The biogeochemical silicon cycle influences global primary productivity and carbon cycling, yet changes in silicon sources and cycling during long-term development of terrestrial ecosystems remain poorly understood. Here, we show that terrestrial silicon cycling shifts from pedological to biological control during long-term ecosystem development along 2-million-year soil chronosequences in Western Australia. Silicon availability is determined by pedogenic silicon in young soils and recycling of plant-derived silicon in old soils as pedogenic pools become depleted. Unlike concentrations of major nutrients, which decline markedly in strongly weathered soils, foliar silicon concentrations increase continuously as soils age. Our findings show that the retention of silicon by plants during ecosystem retrogression sustains its terrestrial cycling, suggesting important plant benefits associated with this element in nutrient-poor environments.

New Recognition for Major Players in the Ocean’s Silicon Cycle

Tiny, shelled protists known as Rhizaria may be responsible for up to one fifth of the total amount of silica produced by the world’s oceanic organisms.