Tuesday 27 January 2015

The fate of soil microbes during the End Permian Extinction.

The End Permian Extinction is the most severe extinction event recorded in the fossil record, with the loss of around 96% of all known species, and many dominant Palaeozoic groups of organisms. The event led to the effective resetting of the Earth’s biosphere, enabling the emergence of a radically different Mesozoic Biota. The event is thought to have been caused by massive volcanism in the Siberian Traps, which not only produced huge emissions of lava and volcanic gasses at the surface, but ignited vast areas of buried Palaeozoic coals and hydrocarbons, leading to massive emissions of acidic and halogenic gas, which in turn resulted in bouts of severe acid rain and the breakdown of the ozone layer, allowing harmful ultraviolet light to reach the Earth’s surface.

A widespread increase in the mutation rate seen in Lycopsid microspores at the end of the Permian has long been seen as evidence of the mutagenic influence of ultraviolet radiation on End Permian ecosystems, but widespread soil acidification is harder to detect. Such an event would be expected to wash acid-soluble metallic plant nutrients such as aluminium, calcium and magnesium out of soils, as well as cause a rapid increase in erosion (and marine sedimentation) rates, due to a loss of soil cohesion; however while both of these have been recorded, they could both also be caused by a variety of other phenomena, making them inconclusive evidence.

In a paper published in the journal Geology on 7 January 2015, Mark Sephton and Dan Jiao of the Department of Earth Science and Engineering at Imperial College London, Michael Engel of the School of Geology and Geophysics at The University of Oklahoma, Cindy Looy of the Department of Integrative Biology and Museum of Paleontology at the University of California–Berkeley and Henk Visscher of the Laboratory of Palaeobotany and Palynology at the Department of EarthSciences at Utrecht University, describe the results of an investigation into the breakdown of lignin at the Vigo Meano Section in northern Italy.

The Vigo Meano Section is thought to provide the most detailed record of the molecular composition of solvent-extractable organic matter from across the Permian/Triassic boundary. The section comprises organic-rich marls (calcium-rich clays, likely to have been formed in an inshore marine environment with high ground runoff) that are not thought to have been influenced by any subsequent metamorphic heating. This section has been used in several prior studies of geochemistry across the Permian/Triassic boundary, and has extremely well constrained dates.

Lignin (the major component of plant fibres) is largely broken down by enzymes excreted into the environment by Fungi and Bacteria. This breakdown process results in lignin breaking down to vanillin (4-hydroxy-3-methoxybenzaldehyde), which then breaks down to vanillic acid (4-hydroxy-3-methoxybenzoic acid), which is further broken down into protocatechuic acid (3,4-dihydroxybenzoic acid), which can be broken down further into a variety of products by a range of soil microbes. Because vanillin is widely used as flavouring in the food industry (vanilla), this breakdown process has been extensively studied in investigations into food spoiling and is very well understood.

Soil bacteria are known to be very sensitive to fluctuations in pH (in chemistry the pH is a reflection of the acidity or alkalinity of a substance, with neutral substances such as pure water having a pH of 7 and more acid substances having lower pHs), with even small changes in acidity leading to major differences in species composition, and much poorer and less diverse bacterial flora found in acid soils. Fungi are far more tolerant, with many common soil species able to survive large fluctuations in soil pH. However the enzymes used to break down vanillin are at their most effective when the soil pH is about 8, and cease activity at about pH 4, with the effect that few soil fungi can survive long at pHs below about 4.5.

Sephtonet al. reasoned that since this is the case, it should be possible to detect any sudden and dramatic increase in soil acidity acid during the end-Permian biotic crisis due to an increase in vanillin and vanillic acid in sediments at this time. Moreover since both compounds are readily biodegradable even under anaerobic conditions, they should not persist for long in the marine environment, making for a close relationship between spikes in soil acidity and sediment composition.

Results mass spectrography analysis for vanillin and vanillic acid across the Permian/Triassic boundary at Vigo Meano show a number of peaks in presence of the two chemicals, including a sustained peak across the major extinction episodes and Permian/Triassic boundary, which suggests the occurrence of pulses of soil acidification so severe that an almost complete cessation of biodegradation must have occurred not just within the soil, but during the transportation and sedimentation process, strongly supporting the idea that the soils were exposed to bouts of rainfall with pHs as low as 4 and possibly lower than 2 (strong enough to cause acid burns to exposed skin) during the end Permian biotic crisis.

Ratios of vanillic acid to vanillin [acid to aldehyde ratio, (Ad/Al)v] in latest Permian and earliest Triassic organic matter assemblagesfrom the VigoMeano section (southern Alps, Italy), providing proxy evidence for pulses of soil acidification (pH < 4 ).VG, Val Gardena Formation; PTB, approximate positionof the Permian-Triassic boundary; extinctions, interval of principal marine extinction and floral turnover in southern Alps; d13C, position ofend-Permian negative carbon-isotope shift in southern Alps. Letters at bottom of stratigraphic column, from left to right, correspond to clay, silt, fine sand, medium sand, and coarse sand, respectively. Sephton et al. (2015).

See also…

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