Respiratory Kinetics of Marine Bacteria Exposed to Decreasing Oxygen Concentrations

Abstract

O xygen (O 2 ), the second most abundant gas in the atmosphere on Earth, plays a critical role in nature, including aquatic environments. As the most favorable electron acceptor, O 2 drives the degradation of organic matter and influences the cycling of other elements (e.g., nitrogen, sulfur, phosphorus) (1). For decades, aerobic respiration has been studied by different methods in various environments (2-4). Due to the limit of detection by traditional methods (Winkler methods and methods that use electrochemical sensors and optodes), the measurement of respiratory activity in low-O 2 environments, such as oceanic oxygenminimum zones (OMZs), is rare (5), and even in fully oxygenated waters, the direct measurement of oxygen dynamics is difficult and the more indirect method of measurement of formazan formation has therefore been applied (6). The lack of detailed data on aerobic respiration in low-O 2 environments restricts the understanding of carbon budgets (7) and the prediction of the development of O 2 within such environments (8). Recently, STOX oxygen sensors have been applied to quantify O 2 respiration rates in OMZs (9), and by this technique, it was possible to measure rates down to about 1 nmol Ϫ1 liter Ϫ1 h Ϫ1 , which is a level of resolution that is a factor of 10 higher than that obtained by traditional methods (10). Half-saturation constants (apparent K m values) for microbial communities were estimated from the data obtained with STOX oxygen sensors, but highly variable values ranging from 30 to 200 nmol liter Ϫ1 were obtained. Another study indicated high half-saturation O 2 concentrations of several micromolar, supposedly caused by diffusion limitation around and within aggregates (11).The final step of aerobic respiration is conducted by a terminal cytochrome oxidase, a membrane-associated protein which transfers electrons to O 2 (12). Two main families of terminal oxidases have been classified on the basis of structural and functional differences (13-16): the heme-copper oxidases (HCOs) and the cytochrome bd-type oxidases. Furthermore, three classes have been identified within HCOs according to the combination of different heme subunits (classes A, B, and C). The kinetic parameters maximum respiration rate (V max ) and K m can be used to describe respiratory activity as a function of the O 2 concentration, although it should be kept in mind that the Michaelis-Menten equation strictly applies only to the kinetics of single enzymes. According to the affinity of O 2 , these terminal oxidases can be classified as high-affinity terminal oxidases (with K m values of about 3 to 8 nmol liter Ϫ1 ) (17) and low-affinity terminal oxidases (with K m values of about 200 nmol liter Ϫ1 ) (18). Respiration rates in aquatic environments have been estimated by different methods, and the kinetics have been estimated using several models. Different equations have been proposed to estimate V max and K m values, such as fitting of the data directly to the Michaelis-Menten equation by computer-aided iterative regres...

Determination of Respiration Rates in Water with Sub-Micromolar Oxygen Concentrations

“…It is important to stress that these values are apparent K m -values for the whole community and not enzymatic K m -values. Aggregation of cells leads to higher apparent K m -values due to diffusion limitation (Kalvelage et al, 2015;Gong et al, 2016), and the higher than the enzymatic value obtained from Figure 5 may thus be caused by diffusion limitation. Although low affinity terminal oxidases are most widely distributed (Kalvelage et al, 2015), high affinity terminal oxidases may be important for the oxygen depletion below about 50 nmol O 2 L −1 , as they have K m -values of only 3-8 nmol L −1 (Morris and Schmidt, 2013).…”

“…All incubations were performed as described previously by Tiano et al (2014b) in modified 1-L Blue-Cap glass bottles allowing O 2 monitoring by both STOX sensors (Tiano et al, 2014b) and high-sensitivity optodes (Lehner et al, 2015;Gong et al, 2016). Immediately after reaching the laboratory, the O 2 present in the water was removed by bubbling with a N 2 : 0.05% CO 2 mixture for 20 min.…”

Cryptic oxygen cycling in anoxic marine zones

“…Tracking O 2 consumption during our experiments allowed for estimates of aerobic respiration rates. O 2 consumption curves were linear down to about 50 nmol·L −1 during dark incubations ( (19), and a linear O 2 decrease may thus be expected down to about 50 nmol O 2 ·L −1 . Therefore, O 2 consumption rates (referred to as respiration for simplicity) obtained at concentrations >50 nmol·L −1 represent potential respiration rates (R*) because they were measured above the threshold of O 2 limitation.…”

In Situ Hydrogen Dynamics in a Hot Spring Microbial Mat during a Diel Cycle

“…However, it cannot be ruled out that the H 2 production continued to be high later in the morning also, but that autotrophic or mixotrophic anoxygenic phototrophy by Chloro- flexus-like organisms (17,23) and oxic metabolism based on cryptic oxygenic photosynthesis consumed this H 2 . Prokaryotes having high-affinity terminal oxidases are not restricted in their O 2 consumption until they experience concentrations of a few nanomoles of O 2 per liter (41), and an environment with the simultaneous presence of H 2 and cryptic O 2 production by oxygenic photosynthesis would select for knallgas bacteria (i.e., bacteria oxidizing H 2 with O 2 ) having such high-affinity terminal oxidases. High levels of nitrogenase gene expression (16,29,30) and N 2 fixation activity (30) have been observed at sunset in the Octopus Spring and Mushroom Spring mats.…”