Atmospheric chemistry of carboxylic acids: microbial implication versus photochemistry

Abstract

The objective of this work was to compare experimentally the contribution of photochemistry vs. microbial activity to the degradation of carboxylic acids present in cloud water. For this, we selected 17 strains representative of the microflora existing in real clouds and worked on two distinct artificial cloud media that reproduce marine and continental cloud chemical composition. Photodegradation experiments with hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) as a source of hydroxyl radicals were performed under the same microcosm conditions using two irradiation systems. Biodegradation and photodegradation rates of acetate, formate, oxalate and succinate were measured on both media at 5 °C and 17 °C and were shown to be on the same order of magnitude (around 10<sup>−10</sup>–10<sup>−11</sup> M s<sup>−1</sup>). The chemical composition (marine or continental origin) had little influence on photodegradation and biodegradation rates while the temperature shift from 17 °C to 5 °C decreased biodegradation rates of a factor 2 to 5. <br><br> In order to test other photochemical scenarios, theoretical photodegradation rates were calculated considering hydroxyl (OH) radical concentration values in cloud water estimated by cloud chemistry modelling studies and available reaction rate constants of carboxylic compounds with both hydroxyl and nitrate radicals. Considering high OH concentration ([OH] = 1 × 10<sup>−12</sup> M) led to no significant contribution of microbial activity in the destruction of carboxylic acids. On the contrary, for lower OH concentration (at noon, [OH] = 1 × 10<sup>−14</sup> M), microorganisms could efficiently compete with photochemistry and in similar contributions than the ones estimated by our experimental approach. <br><br> Combining these two approaches (experimental and theoretical), our results led to the following conclusions: oxalate was only photodegraded; the photodegradation of formate was usually more efficient than its biodegradation; the biodegradation of acetate and succinate seemed to exceed their photodegradation

Potential impact of microbial activity on the oxidant capacity and organic carbon budget in clouds

“…This result has major consequences when considering the impact of microbial activity on carbon budgets; it suggests that microorganisms could play a role not only in cloud chemistry at night, as previously indicated (27)(28)(29), but also during the daytime, when…”

“…In the recent past, researchers investigated the microbial activity in cloud water containing organic compounds, such as carboxylic acids, formaldehyde, and methanol (25,26). Inferred estimates indicated that the activity of microorganisms was likely to affect the chemistry of these compounds in warm clouds and could even drive their reactivity during the night (27)(28)(29).…”

Survival of microbial isolates from clouds toward simulated atmospheric stress factors

“…Cells were suspended at a concentration of~10 6 cells mL À1 based on OD 600nm in 5 mL of artificial continental cloud water (see Vaïtilingom et al, 2011, Table 1, for chemical composition) supplemented with 0, 0.1, 0.25, 0.5 or 1 mM hydrogen peroxide (final concentration). Cultivable cell concentrations were determined at the beginning of the experiment and after a 90 min-exposure at 20 C in the dark.…”

H<sub>2</sub>O<sub>2</sub> modulates the energetic metabolism of the cloud microbiome

“…Particularly through measurements of adenosine triphosphate (ATP) and anabolic precursors or nutrient incorporation rates, it has been shown that cloud microorganisms are metabolically active and play an important role in cloud chemical reactivity (Sattler et al, 2001;Amato et al, 2007a;Hill et al, 2007;Vaïtilingom et al, 2012Vaïtilingom et al, , 2013. Several studies performed on simplified or real microcosms have demonstrated that cloud microorganisms are able to degrade carbon compounds (Ariya et al, 2002;Amato et al, 2005Amato et al, , 2007cHusarova et al, 2011;Vaïtilingom et al, 2010Vaïtilingom et al, , 2011Vaïtilingom et al, , 2013Matulovà et al, 2014); recent studies have also shown that this could be the case in the air (Krumins et al, 2014).…”

“…All incubation flasks contained 100 mL of artificial cloud solution under agitation (130 rpm); its composition was first described in Vaïtilingom et al (2011). This solution was mimicking cloud chemical composition from cloud samples classified as "marine" following the work from Deguillaume et al (2014) at the PUY station.…”