Silicate deposition during decomposition of cyanobacteria may promote export of picophytoplankton to the deep ocean

Abstract

Marine particles transport organic matter through the water column to the sediment where the organic matter can be buried. This pathway is one of the few natural removal mechanisms of CO 2 from the atmosphere over geological time. Picophytoplankton, major primary producers in the ocean, have until recently been thought unimportant regarding particle transport. Here we provide evidence that silicate is deposited on extracellular polymeric substance (EPS) associated with decomposing picophytoplankton. We also find that Si is enriched in a previously unexplored group of marine particles (called micro-blebs) from the deep-water column. The surprising similarity in morphology and composition between EPS-Si and micro-blebs suggests that EPS-Si may be a precursor of micro-blebs observed in the deep ocean. This previously unexplored source of silicon may be important to silicon cycling and may further enhance export of picophytoplankton to the deep ocean.

Picoplankton contribution to biogenic silica stocks and production rates in the Sargasso Sea

“…Thus, unlike diatoms, detrital picoplankton may continue to accumulate bSi via abiotic processes, potentially mimicking bSi production. However, this abiotic enrichment of bSi relative to C occurred on the order of 1–4 weeks [ Tang et al ., ] and would be slower than the quantified V b <3μm (Figure H). Additionally, if Synechococcus produces a solid form of amorphous silica, then presumably its specific surface area would be high, favoring rapid dissolution post mortem.…”

“…These factors may include the following: Other subsurface picoplankton could accumulate Si, e.g., Prochlorococcus and picoeukaryotes. At depth, the proportion of siliceous detritus in the <3 μm fraction was lower than in the surface and this disparity was greater than the differences in Synechococcus abundance with depth. Note: siliceous detritus biases V b to be lower than that for an assemblage where most of the bSi is associated with living cells. The process of Si accumulation in Synechococcus is not like diatoms and may occur when cells are living or dead, i.e., detrital Synechococcus may be a sink for Si [ Tang et al ., ]. …”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…Indeed, the water column inventory of Si in Synechococcus can exceed that of diatoms in some cases, although in today's DSi depleted oceans it is believed that most of the nanoparticles produced by Synechococcus are recycled within surface waters (Baines et al, 2012). However, BSi-like material deposited on fast settling particles containing extracellular polymeric substances associated with decomposing picophytoplankton (Figure 3) has been shown to be a relevant source of sinking particulate amorphous Si and could account for as much as 43% of the C export production at the Bermuda Time Series station (Tang et al, 2014).…”

“…Synchrotron XRF microscopy of natural and cultured individual cells of the modern marine photosynthetic picocyanobacteria Synechococcus, suggests that they accumulate Si at Si:P levels approaching that of diatoms (Baines et al, 2012). Further, decomposition of Synechococcus has been shown to produce extracellular polymeric substances (EPS) that themselves drive the production of minuscule "microblebs" of silica that, by adding dense ballast to aggregates, may enhance the export of picoplankton-derived material from the euphotic zone (Tang et al, 2014). These findings were initially controversial, especially as they have potentially significant importance to both carbon and Si cycling.…”

“…The implication is that FIGURE 3 | Scanning transmission electron microscope images (A) and EDS spectra of decomposing Synechococcus cells (B) collected during dark incubation of cells after 15 days showing the association of silica with extracellular polymeric substance during the degradation of cyanobacterial cells (From Tang et al, 2014). Reprinted by permission from Macmillan Publishers Ltd: Tang et al (2014). Copyright © 2014. this removed the selective pressure for possessing a system for Si homeostasis and detoxification, causing the widespread, independent losses of Si transporters (SITs, SIT-Ls, and Lsi2-like transporters) across multiple different eukaryotic lineages (Marron et al, 2016).…”