Photochemical alteration of dissolved organic matter and the subsequent effects on bacterial carbon cycling and diversity

Abstract

The impact of solar radiation on dissolved organic matter (DOM) derived from 3 different sources (seawater, eelgrass leaves and river water) and the effect on the bacterial carbon cycling and diversity were investigated. Seawater with DOM from the sources was first either kept in the dark or exposed to sunlight (4 days), after which a bacterial inoculum was added and incubated for 4 additional days. Sunlight exposure reduced the coloured DOM and carbon signals, which was followed by a production of inorganic nutrients. Bacterial carbon cycling was higher in the dark compared with the light treatment in seawater and river samples, while higher levels were found in the sunlight-exposed eelgrass experiment. Sunlight pre-exposure stimulated the bacterial growth efficiency in the seawater experiments, while no impact was found in the other experiments. We suggest that these responses are connected to differences in substrate composition and the production of free radicals. The bacterial community that developed in the dark and sunlight pre-treated samples differed in the seawater and river experiments. Our findings suggest that impact of sunlight exposure on the bacterial carbon transfer and diversity depends on the DOM source and on the sunlight-induced production of inorganic nutrients.

Feasting on terrestrial organic matter: Dining in a dark lake changes microbial decomposition

“…The ability for bacteria in the dark lake sediments to break down increasing fractions of terrestrially derived organic matter was possible because they had the genes to produce hydrolyzing and oxidizing enzymes (Judd et al, 2007;Ward & Cory, 2016). Although this may have allowed the dark lake bacteria to access nutrients and bioavailable growth-promoting amino acids associated with t-OM, the metabolic cost of producing enzymes likely resulted in less growth (Logue et al, 2016;López-Urrutia & Morán, 2007;Roiha, Peura, Matheiu, & Rautio, 2016;Yamashita, Fichot, Shen, Jaffé, & Benner, 2015). Additionally, bacteria in the dark lake had a limited availability of LMW substrates from photosynthetically derived and photo-oxidized OM (Bertilsson & Tranvik, 1998;Karlsson et al, 2009).…”

Validation of the in vivo Iodo-Nitro-Tetrazolium (INT) Salt Reduction Method as a Proxy for Plankton Respiration

“…The "test" data set was then used to compare the predictive capacity of the (i) regression model derived in section "Maximum Incubation Time for the INT Reduction Rates"; (ii) the Martínez-García et al (2009) regression model derived from natural plankton communities (logCR O2 = 0.99log INT +1.23, R 2 = 0.80, n = 72, hereafter referred to as MG-natural); and (iii) the 12.8 conversion factor obtained in Martínez-García et al (2009) (hereafter referred to as MG-12.8) and used in previous studies (Martínez-García and Karl, 2015;Lønborg et al, 2016). The slope of the regression line between the estimated O 2 consumption rates with the former regression lines and the measured O 2 consumption rates were compared to the equation of a line with slope 1 using a Clarke test (Clarke, 1980).…”

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

“…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.…”