The influence of elevated SiO2 (aq) on intracellular silica uptake and microbial metabolism

Seasonal dynamics and nutrient controls of biogenic silica in Baltic Sea surface microplankton and picoplankton communities

In recent years, new contributors to the marine silica cycle have emerged, including pico-sized phytoplankton (<2-3 µm in size) such as Synechococcus and picoeukaryotes. Their contribution and relevance to silica cycling are still under investigation. Field studies reporting the biogenic silica (bSi) standing stock in the pico-sized fraction are limited to silica-poor oligotrophic environments, and the mechanism of bSi accumulation in picoplankton remains unknown. We investigated the variability of bSi standing stocks in two size fractions (picoplankton, 0.22-3 µm and microplankton, >3 µm) in the dissolved silica-replete Baltic Sea via biweekly time series samplings spanning 2 years. Time series data showed that the large changes in bSi standing stock in the Baltic Proper were primarily related to microplankton biomass and community composition. Meanwhile, picoplankton were, at times, surprisingly high contributors to total bSi year-round (up to 21.6%). Simultaneously, we performed microcosm incubation experiments with natural phytoplankton communities in each season to examine how nutrient additions affected bSi concentrations. In these experiments, increases in microplankton bSi were directly correlated to increases in diatom biomass, highlighting their influential role in the Baltic Sea silica cycle. Meanwhile, phosphorus additions triggered an increase in picoplankton bSi accumulation in all experiments. This uncovers a potential control of bSi accumulation in picoplankton, which can help identify the cellular mechanisms behind this process and uncover their role in silica cycling. The results link phytoplankton community composition and silica cycling, which is important for understanding the consequences of organism shifts due to climate change.IMPORTANCEThe marine carbon and silica cycles are tightly intertwined and largely controlled by diatoms. Nevertheless, recent studies, mostly in oligotrophic waters, have proposed new contributors to the marine silica cycle: picoplankton. Here, we report the first study of seasonal dynamics of biogenic silica (bSi) standing stock in microplankton and picoplankton in the silica-replete Baltic Sea. Microplankton bSi dynamics were correlated with changes in composition and biomass. Picoplankton were consistent contributors to bSi, and for the first time in diverse natural communities, we found a direct correlation between phosphorus and bSi accumulation. The results are important for understanding how climate change-predicted phytoplankton composition shifts will affect carbon and silica cycling and provide a direction for future research on nutrient controls of silica accumulation in picoplankton.

Effects of ambient silicic acid concentration on the physiology of marine cyanobacterial Synechococcus

The marine cyanobacterium Synechococcus can accumulate significant amounts of silicon (Si) within its cells, potentially contributing to the vertical transport of biogenic Si in the ocean. This has prompted interest in understanding the uptake, subcellular distribution, and chemical forms of Si in Synechococcus. The surprisingly high intracellular Si concentration could have important impacts on Synechococcus physiology, but these effects have been relatively unexplored. In this study, we investigated the effects of external Si concentrations, [Si(OH)4], on the cell physiology of two strains of Synechococcus (CC9311 and WH7803). Increased ambient [Si(OH)4] significantly enhanced total cellular and water-soluble Si content in Synechococcus, but had little effect on their growth. However, the cell size, volume and photosynthetic performance were significantly reduced in strain WH7803, indicating some harmful effects in the cell. Increases in ambient [Si(OH)4] also altered the Derjaguin-Muller-Toporov (DMT) modulus, adhesion, surface potential and elemental composition of the Synechococcus cells. Notably, significant changes in area-specific adsorbed metals and volume-specific metal content were observed at higher [Si(OH)4] levels, suggesting Si availability in seawater could markedly impact the interactions between Synechococcus cell and environment. Overall, this study provides new insights into the physiological adaptations of marine cyanobacterium Synechococcus to varying environmental silicic acid concentrations.

The evaluation of biogenic silica in brackish and freshwater strains reveals links between phylogeny and silica accumulation in picocyanobacteria

Through biosilicification, organisms incorporate dissolved silica (dSi) and deposit it as biogenic silica (bSi), driving the silicon (Si) cycle in aquatic systems. While Si accumulation in marine picocyanobacteria has been recently observed, its mechanisms and ecological implications remain unclear. This study investigates biosilicification in marine and brackish picocyanobacteria of the Synechococcus clade and two model freshwater coccoid cyanobacteria. Brackish strains showed significantly higher Si quotas when supplemented with external dSi (100 µM) compared to controls (up to 60.0 ± 7.3 amol Si.cell-1 versus 9.2 to 16.3 ± 2.9 amol Si.cell-1). Conversely, freshwater strains displayed no significant differences in Si quotas between dSi-enriched treatments and controls, emphasizing that not all phytoplanktons without an obligate Si requirement accumulate this element. The Si-accumulating marine and brackish picocyanobacteria clustered within the Synechococcus clade, whereas their freshwater counterparts formed a distinct sister group, suggesting a link between phylogeny and silicification. Rapid culture growth caused increased pH and led to dSi precipitation, influencing apparent dSi uptake; this was mitigated by pH control through bubbling. This phenomenon has significant implications for natural systems affected by phytoplankton blooms. In such environments, pH-induced silicon precipitation may reduce dSi availability impacting Si-dependent populations like diatoms. Our findings suggest brackish picocyanobacteria could significantly influence the Si cycle through at least two mechanisms: cellular Si accumulation and biologically induced changes in dSi concentrations.IMPORTANCEThis work provides the first evidence of biogenic silica accumulation in brackish picocyanobacteria and uncovers a link between phylogeny and biosilicification patterns. Our findings demonstrate that picocyanobacterial growth induces pH-dependent silica precipitation, which could lead to overestimations of cellular Si quotas by up to 85%. This process may drive substantial silica precipitation in highly productive freshwater and coastal marine systems, with potential effects on silica cycling and the population dynamics of Si-dependent phytoplankton. The extent of biosilicification in modern picocyanobacteria offers insights into the rock record, shedding light on the evolutionary and ecological dynamics that influence sedimentary processes and the preservation of biosilicification signatures in geological formations. Overall, this research adds to the significant impact that microorganisms lacking an obligate silica requirement may have on silica dynamics.

Controls on authigenic mineralization in experimental Ediacara-style preservation

The earliest evidence of complex macroscopic life on Earth is preserved in Ediacaran-aged siliciclastic deposits as three-dimensional casts and molds, known as Ediacara-style preservation. The mechanisms that led to this extraordinary preservation of soft-bodied organisms in fine- to medium-grained sandstones have been extensively debated. Ediacara-style fossilization is recorded in a variety of sedimentary facies characterized by clean quartzose sandstones (as in the eponymous Ediacara Member) as well as less compositionally mature, clay-rich sandstones and heterolithic siliciclastic deposits. To investigate this preservational process, we conducted experiments using different mineral substrates (quartzose sand, kaolinite, and iron oxides), a variety of soft-bodied organisms (microalgae, cyanobacteria, marine invertebrates), and a range of estimates for Ediacaran seawater dissolved silica (DSi) levels (0.5-2.0 mM). These experiments collectively yielded extensive amorphous silica and authigenic clay coatings on the surfaces of organisms and in intergranular pore spaces surrounding organic substrates. This was accompanied by a progressive drawdown of the DSi concentration of the experimental solutions. These results provide evidence that soft tissues can be rapidly preserved by silicate minerals precipitated under variable substrate compositions and a wide range of predicted scenarios for Ediacaran seawater DSi concentrations. These observations suggest plausible mechanisms explaining how interactions between sediments, organic substrates, and seawater DSi played a significant role in the fossilization of the first complex ecosystems on Earth.

Picocyanobacteria-A non-negligible group for the export of biomineral silica to ocean depth

Diatoms have long been thought to dominate the marine silicon (Si) cycle, as well as play an important role in the ocean's carbon (C) export, due to density-driven particle sedimentation. Research in the past decade has shed new light on the potential importance of picocyanobacteria to C export, although the sinking mechanism is still unclear. Interestingly, the recent discovery of Si accumulation by picocyanobacteria of the genus Synechococcus has strong implications for the marine Si cycle, which may also have profound influence on the oceanic C export. Understanding the mechanisms of Synechococcus Si accumulation and its ecological effects are therefore critical for addressing wider issues such as Si and C exports by small cells via biological pump. Here, we show that recent advances in process studies indicate that the presence of Si within picocyanobacteria may be a common and universal feature. Subsequently, we generalize four biochemical forms of Si potentially present in picocyanobacterial cells, which are all different from diatomaceous opal-A, and hypothesize that these various structures of Si phases may be several stage products of Si precipitation. At the same time, several aspects of Si dynamics in Synechococcus are also discussed emphatically. In addition, we provide a first estimate of picocyanobacteria Si stock and production for the global ocean, accounting for 12% of the global Si inventory and 45% of the global annual Si production in the surface ocean, respectively. The implication is that picocyanobacteria may exert a significant influence on the marine Si cycle, which is likely to alter our understanding of the long-term control of the oceanic Si cycling by diatoms. Finally, we summarize three possible mechanisms and pathways through which picocyanobacteria-derived Si can be transported to the deep ocean. Altogether, marine picocyanobacteria, despite very small in cell size, are a non-negligible group for the export of biomineral Si to deeper waters and ocean sediments.

Biogenic silica accumulation in picoeukaryotes: Novel players in the marine silica cycle

It is well known that the biological control of oceanic silica cycling is dominated by diatoms, with sponges and radiolarians playing additional roles. Recent studies have revealed that some smaller marine organisms (e.g. the picocyanobacterium Synechococcus) also take up silicic acid (dissolved silica, dSi) and accumulate silica, despite not exhibiting silicon dependent cellular structures. Here, we show biogenic silica (bSi) accumulation in five strains of picoeukaryotes (<2-3 μm), including three novel isolates from the Baltic Sea, and two marine species (Ostreococcus tauri and Micromonas commoda), in cultures grown with added dSi (100 μM). Average bSi accumulation in these novel biosilicifiers was between 30 and 92 amol Si cell^-1 . Growth rate and cell size of the picoeukaryotes were not affected by dSi addition. Still, the purpose of bSi accumulation in these smaller eukaryotic organisms lacking silicon dependent structures remains unclear. In line with the increasing recognition of picoeukaryotes in biogeochemical cycling, our findings suggest that they can also play a significant role in silica cycling.