The arid and semi-arid drylands of the world are increasingly recognized for their role in the terrestrial net carbon dioxide (CO ) uptake, which depends largely on plant litter decomposition and the subsequent release of CO back to the atmosphere. Observed decomposition rates in drylands are higher than predictions by biogeochemical models, which are traditionally based on microbial (biotic) degradation enabled by precipitation as the main mechanism of litter decomposition. Consequently, recent research in drylands has focused on abiotic mechanisms, mainly photochemical and thermal degradation, but they only partly explain litter decomposition under dry conditions, suggesting the operation of an additional mechanism. Here we show that in the absence of precipitation, absorption of dew and water vapor by litter in the field enables microbial degradation at night. By experimentally manipulating solar irradiance and nighttime air humidity, we estimated that most of the litter CO efflux and decay occurring in the dry season was due to nighttime microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the daytime. In a complementary study, at three sites across the Mediterranean Basin, litter CO efflux was largely explained by litter moisture driving microbial degradation and ultraviolet radiation driving photodegradation. We further observed mutual enhancement of microbial activity and photodegradation at a daily scale. Identifying the interplay of decay mechanisms enhances our understanding of carbon turnover in drylands, which should improve the predictions of the long-term trend of global carbon sequestration.

Litter production in many drought-affected ecosystems coincides with the beginning of an extended season of no or limited rainfall. Because of lack of moisture litter decomposition during such periods has been largely ignored so far, despite potential importance for the overall decay process in such ecosystems. To determine drivers and extent of litter decay in rainless periods, a litterbag study was conducted in Mediterranean shrublands, dwarf shrublands and grasslands. Heterogeneous local and common straw litter was left to decompose in open and shaded patches of various field sites in two study regions. Fresh local litter lost 4-18% of its initial mass over about 4 months without rainfall, which amounted to 15-50% of total annual decomposition. Lab incubations and changes in chemical composition suggested that litter was degraded by microbial activity, enabled by absorption of water vapor from the atmosphere. High mean relative humidity of 85% was measured during 8-9 h of most nights, but the possibility of fog deposition or dew formation at the soil surface was excluded. Over 95% of the variation in mass loss and changes in litter nitrogen were explained by characteristics of water-vapor uptake by litter. Photodegradation induced by the intense solar radiation was an additional mechanism of litter decomposition as indicated by lignin dynamics. Lignin loss from litter increased with exposure to ultraviolet radiation and with initial lignin concentration, together explaining 90%-97% of the variation in lignin mass change. Our results indicate that water vapor, solar radiation and litter quality controlled decomposition and changes in litter chemistry during rainless seasons. Many regions worldwide experience transient periods without rainfall, and more land area is expected to undergo reductions in rainfall as a consequence of climate change. Therefore, absorption of water vapor might play a role in decomposition and nutrient cycling in an increasing number of ecosystems.

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The hydrophobicity of Bordetella pertussis was assayed by measuring the ability of cells in suspension to adhere to a polystyrene surface. The quantity of adhered bacteria was measured by the binding of enzyme-conjugated anti B. pertussis antibodies. Hydrophobic adherence of non-pathogenic variant strains was about 20% of that exhibited by pathogenic strains. Hydrophobicity was a stable trait as it did not change with passaging or storage. Assays of a series of characterized stable variants suggested that the Filamentous Hemagglutinin (FHA) is the cell surface moiety responsible for hydrophobic adherence in B. pertussis.

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