Loredana Saccone

Anthropogenic perturbations of the silicon cycle at the global scale: Key role of the land‐ocean transition

Silicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO(2)). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO(2). The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO(2) in the coastal zone are to be expected over the course of this century.

Constraining the role of early land plants in Palaeozoic weathering and global cooling.

How the colonization of terrestrial environments by early land plants over 400 Ma influenced rock weathering, the biogeochemical cycling of carbon and phosphorus, and climate in the Palaeozoic is uncertain. Here we show experimentally that mineral weathering by liverworts—an extant lineage of early land plants—partnering arbuscular mycorrhizal (AM) fungi, like those in 410 Ma-old early land plant fossils, amplified calcium weathering from basalt grains threefold to sevenfold, relative to plant-free controls. Phosphate weathering by mycorrhizal liverworts was amplified 9–13-fold over plant-free controls, compared with fivefold to sevenfold amplification by liverworts lacking fungal symbionts. Etching and trenching of phyllosilicate minerals increased with AM fungal network size and atmospheric CO2 concentration. Integration of grain-scale weathering rates over the depths of liverwort rhizoids and mycelia (0.1 m), or tree roots and mycelia (0.75 m), indicate early land plants with shallow anchorage systems were probably at least 10-fold less effective at enhancing the total weathering flux than later-evolving trees. This work challenges the suggestion that early land plants significantly enhanced total weathering and land-to-ocean fluxes of calcium and phosphorus, which have been proposed as a trigger for transient dramatic atmospheric CO2 sequestration and glaciations in the Ordovician.

Nanoscale Observations of Extracellular Polymeric Substances Deposition on Phyllosilicates by an Ectomycorrhizal Fungus

Microorganisms colonizing surfaces can exude a wide range of substances, generally called Extracellular Polymeric Substances (EPS). While EPS has often been visualized as thick mature strata embedding microbes, the initial phases of EPS production, its structure at the micro- and nanoscale and the microbial wall areas involved in its exudation are less known. In this work we use Atomic Force Microscopy to image EPS produced by the fungus Paxillus involutus on phyllosilicate surfaces. Hyphal tips initially deposit EPS which assumes the shape of a “halo” surrounding hyphae. The fusion of adjacent EPS halos is likely responsible for the creation of EPS monolayers covering mineral surfaces. It is also proposed that a specific region of hyphae initiates the formation of mineral channels produced by fungi. The results presented here permit for the first time to propose a model for the initial stages of EPS accumulation in fungi and filamentous microorganisms in general.