Understanding the role of biofilms in forming clay coatings on sand grains.

Sandstone deposits held together with clay matrixes have been reported from almost all geological time periods. This is somewhat surprising, as the physical processes which form sand deposits are usually very good at separating particles by grain sizes, hence we have sandy beaches and sand dune fields in deserts, rather than sand-and-clay deposits in these places. A number of possible explanations for the formation of clay matrixes in sandstone deposits have been put forward, including subsequent infilling, simultaneous deposition of clay particles, and bioturbation, but none of these has been generally accepted.

In a paper published in the journal Geology on 17 August 2017, Luke Woodridge, Richard Worden, Josh Griffiths, and Anu Thompson of the School of Environmental Sciences at the University of Liverpool, and Peter Chung of the School of Geographical and Earth Sciences at the University of Glasgow, describe a possible method by which clay particles may become adhered to sand grains by the formation of biofilms in intertidal environments, thereby giving a method for the formation of sandstones held together by clay matrixes.

A variety of organisms form biofilms in intertidal sandy sediments, including Diatoms (single-celled Algae with silica shells), Euglenids (single-celled flagelate Algae), Crysophyceans (Golden Algae), Dinoflagellates (shelled flagellate Algae), and a variety of Bacteria. Woodridge et al. concentrated on the role of motile epipelic Diatoms, the dominant group of biofilm-forming organisms in sediments in northeastern Europe.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells.

Motile epipelic Diatoms excrete strands of extracellular polymeric substances, which they used to attach themselves to grains of sand. This both serves to anchor them within the sediments, and facilitates their movement within the sediment, enabling them to move up and down in response to environmental stimuli, such as daylight. These strands eventually become detached from the Diatoms, but remain attached to the sand grains, forming a web of mucus strands (biofilm) that bind the sand together. Clay particles are known to adhere to biofilms, particularly in the presence of divalent cations such as Magnesium (Mg²⁺) and Calcium (Ca²⁺), both of which are present in seawater, thereby providing a potential method for the formation of clay coatings on sand grains.

Woodridge et al. collected sand samples from the Ravenglass Estuary in Cumbria, northwest England, which is considered to be an analogue for the environments in which about 54% of the sandstones with clay matrixes found in the rock record formed. These grains were found to be partially covered by a network of fibrous filaments, to which were adhered a variety of silt and clay particles, as well as organic material, including Diatoms.

Backscattered electron and environmental scanning electron microscope (SEM) images of clay-coated grains. Arrows indicate clay coatings. Dashed lines outline the extent of the biofilm coats on the grain surface. (A) SEM image of loose sediment. (B) Thin section of clay-coated sand grains from an intertidal estuarine setting. (C) Environmental SEM image of hydrated sediment. Triangle in top right points to a diatom with excreted extracellular polymeric substance grain attachment outlined. Woodridge et al. (2017).

Woodridge et al. found that the grains were covered by the biofilm to a variable extent, with grains having as little as 0.5% to as much as 87% of their surface covered. They then mapped the distribution of particles with different coverages within the estuary, which showed that coverage was most extensive in the upper and middle estuary, on tidal bars and tidal flats that correspond to tidally exposed or shallow waters. They also mapped the distribution of Chlorophyll-a concentration in sediments (a proxy for the presence of Diatoms) across the estuary, finding a strong correlation with the distribution of biofilms, strongly supporting the idea that the Diatoms are responsible for the biofilms covering the sand grains.

Maps of clay-coated sand grain distribution and biomarker proxy for tidal flat biofilm abundance on sand grains. (A) Clay-coat coverage of sand grains established from quantitative petrography. Rivers Irt, Mite, and Esk are indicated. (B) Chlorophyll-a concentrations. Darker shades in (A) and (B) represent greater extent of clay-coat coverage and greater abundance of sediment biofilm (chl-a—chlorophyll-a). Woodridge et al. (2017).

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Microplastics in deep-sea marine sediments.

Plastics are considered to be one of the major environmental challenges of our time. They are highly durable synthetic polymers, around 30% of which are produced for short-life purposes, such as disposable packaging, and are discarded within a year of being manufactured. Despite the large number of plastic items being manufactured and then thrown away, and visible evidence of plastic debris in ecosystems from pole to pole, environmental scientists for a long time struggled to find evidence of plastic accumulation (rather than presence) in natural ecosystems, until they began to examine microplastic particles (tiny plastic fragments, generally formed from the break-down of larger items) in sediments and ocean waters, where a steady build-up of plastics over time has been confirmed. However even these studies have failed to account for the amount of plastic thought likely to be present in the environment, leading scientists to suspect that a large amount of plastic is present but unaccounted for somewhere in the natural environment.

In a paper published in the journal Royal Society Open Science on 17 December 2014, Lucy Woodall of the Department of Life Sciences at The Natural History Museum, Anna Sanchez-Vidal and Miquel Canals of the Departament d’ Estratigrafia, Paleontologia i Geociències Marines at the Universitat de Barcelona, Gordon Paterson, also of the Department of Life Sciences at The Natural History Museum, Rachel Coppock and Victoria Sleight of the Marine Biology and Ecology Research Centre at Plymouth University, Antonio Calafat, also of the Departament d’ Estratigrafia, Paleontologia i Geociències Marines at the Universitat de Barcelona, Alex Rogers of the Department of Zoology at the University of Oxford, Bhavani Narayanaswamy of the Scottish Association for Marine Science and Richard Thompson, again of the Marine Biology and Ecology Research Centre at Plymouth University, discus the presence of microplastic particles in deep-marine sediment samples collected from a series of sites in the North Atlantic, Mediterranean and southern Indian Ocean.

The samples examined were taken from the upper portions of cores gathered for other studies by the Universitat de Barcelona and the Natural History Museum over a twelve year period. Because of this the samples were gathered following different procedures, limiting the amount of comparison that can be made between the samples. Nevertheless it was possible to establish the presence of plastics in areas not previously sampled, and compare the proportions of different plastics within individual samples.

Locations of sampling sites of bottom sediment and deep-water coral where content of microplasticswas investigated. Sample depth ranged down to 3500 m, for details see table 1. Sediment was collected by the University of Barcelona (circles) and the Natural History Museum (filled squares), and deep-water corals were collected by the Natural History Museum (open squares). Bathymetry corresponds to ETOPO1Global Relief Model. Woodall et al. (2014).

The areas sampled included open slopes in the subpolar North Atlantic, the northeast Atlantic and the Mediterranean, canyons in the northeast Atlantic and Mediterranean, basins in the Mediterranean and Corals from seamounts in the southwest Indian Ocean.

All of the samples were found to contain microplastics in the form of fibres 2-3 mm in length and ~0.1 mm in width. The most abundant fibre was rayon, which is not strictly speaking a plastic (it is made from dissolved and resolidified cellulose from wood-pulp, rather than hydrocarbons) and which comprised 56.9% of all the fibres found in the study; this is comparable to results for rayon in previous studies of synthetic fibres ingested by Fish (where 57.8% of all fibres were rayon) and in ice cores (where 54% of all fibres were rayon). Of actual plastics sampled 53.4% were polyester, 34.1% were 'other plastics (including polyamides and acetate) and 12.4% were acrylic.

Plastics were found at comparable levels to those seen in intertidal and shallow-marine sediments, and at a rate roughly a thousand times higher than found in surface waters. Given the vast areas covered by the deep ocean floor, this is likely to account for a substantial proportion of the 'missing' plastic predicted to be present in the natural environment.

All of the plastics found were heavier than water. At first sight this is what would be expected as such plastics should sink whereas plastics lighter than water should not, however for microplastics the situation is more complex, as such plastics will tend to be held at the surface by surface-tension, only sinking after becoming colonized by marine organisms, adhered to phytoplankton and the aggregated with organic debris and small particles in the form of marine snow.

The impact of microplastics on deep-sea ecosystems is unclear. In surface and shallow-marine organisms such plastics have been shown to have adverse effects both due to their physical and toxicological properties, and this is likely to be the case also with deep-marine organisms, but this cannot be asserted confidently without further study.

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Counting floating plastics in the world’s oceans.                                                   Floating plastic is considered to be a major pollutant in the world’s oceans. It enters the oceans in large quantities from shipping, coastal communities...

 

 

Marine litter on the European seafloor.   Manmade rubbish (litter) is known to be extremely harmful to aquatic lifeforms, both as a direct physical hazard (such as nets which continue to trap and kill Fish long after they have become detached from fishing vessels or plastic items which resemble food and...

 

Plastic contamination in Lake Garda, Italy. Plastic contaminants are known to present a threat in many ecosystems, with particular concern being raised about the oceans, where large accumulations of plastic are known to be found on ocean gyres (large rotating currents) and where damage to wildlife from plastic ingestion is well documented. The effect of plastic contamination on freshwater ecosystems is less well documented, though studies of...

 

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