Photochemical effects on microbial activity in natural waters: the interaction of reactive oxygen species and dissolved organic matter

Abstract

Bacterial utilization of dissolved organic matter (DOM) in surface waters is closely linked to photochemical transformations of DOM. Photochemically produced reactive oxygen species (ROS) play a central role in many photochemical reactions, but the role of ROS for the photochemical facilitation of bacterial utilization of DOM is previously not known. We exposed lake water with high DOM concentrations to simulated sunlight, with and without the addition of ROS scavengers, and quantified the effect on the production of CO2, the loss of DOM absorbance, and bacterial growth. The photodegradation of DOM through microbial-photochemical interactions was dependent on the action of ROS. The use of ROS scavengers in irradiations of the lake water revealed that photobleaching below 300 nm and the production of CO2 are highly dependent on the action of ROS. Photobleaching and CO2 production in irradiated waters decreased significantly with the addition of ROS scavengers, but post-irradiation bacterial growth in the samples containing an ROS scavenger increased significantly above those without. The decrease in ROS activity (CO2 production) likely caused an accumulation of bioavailable DOM and enhanced microbial processes. Rapid degradation of DOM through the action of ROS would be especially important in high DOM systems. The high photochemical ROS activity may counterbalance the positive effects on bacterial activity of DOM photolysis into bioavailable molecules.

Effects of sunlight and hydroxyl radical on dissolved organic matter: Bacterial growthefficiency and production of carboxylic acids and other substrates

“…Our results conflict with the recent results of Scully et al (2003a), who attempted to study the effect of photoproduced reactive oxygen species on DOM bioavailability. They found that the use of a ROS scavenger (furfuryl alcohol) during sunlight irradiation increased subsequent bacterial growth (measured as cell abundance only) in incubations, relative to samples irradiated without the scavenger.…”

Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

“…For example, Febria et al (2006) found that the production of hydrogen peroxide (H 2 O 2 ) in two delta lakes that were also sampled herein (CON and TK) outpaced the rate of removal during a period of uninterrupted sunshine in 2004, leading to an accumulation of H 2 O 2 within the water columns. Production of ROS during previous studies of DOM photodegradation has been observed to inhibit community-level rates of carbon production (Scully et al 2003;Anesio et al 2005;Glaeser et al 2014) and result in changes in bacterial community composition (Glaeser et al 2010(Glaeser et al , 2014. Further, in a prior study by Lund and Hongve (1994), declines in BA of up to 60% were observed after only an hour when UV-irradiated DOM was mixed with bacteria.…”

“…These by-products of the photodegradation process are short-lived, but nevertheless inhibit bacterial growth and production (Lund and Hongve 1994;Henle and Linn 1997;Anesio et al 2005). To some extent, therefore, irradiation of DOM by sunlight simultaneously stimulates and inhibits heterotrophic bacterial production (BP) (Scully et al 2003;Ruiz-Gonzalez et al 2013), leading to complex interactions that can result in enhanced, negative, mixed, or no effect on bacterial community metabolism (Lonborg et al 2016). In addition, the bacterial community composition may be altered either by exposure to ROS (Glaeser et al 2010;Glaeser et al 2014) or in response to the increased lability of the pool of DOM substrate (Judd et al 2007;Piccini et al 2009;Paul et al 2012;Ward et al 2017), giving rise to a species assemblage that is better suited to the ambient conditions.…”

UV–vis spectral modifications of water samples under irradiation: Lake vs. subterranean water

“…Indeed, CDOM protects the aquatic biota from exposure to UV radiation [2], which can be very significant during summertime in CDOM-poor environments such as mountain lakes located above the tree-line [3]. Another important issue is that radiation absorption by CDOM yields reactive species, such as • OH, 1 O 2 and the triplet states 3 CDOM*, which can be involved into transformation of dissolved compounds, including xenobiotics, as well as into the photoprocessing of CDOM itself [4][5][6][7].…”