Evaluation of community respiratory mechanisms with oxygen isotopes: A case study in Lake Kinneret

Abstract

Gross and net O 2 production between May 1996 and February 1999 was determined in bottle incubation experiments with H 2 18 O spike and from the change in O 2 concentration. Carbon fixation rates were obtained from 14 C incubations. In general, production rates determined using the H 2 18 O-spike were about twice the primary production determined by the 14 C method, where the latter was close to net oxygen evolution. These relationships are similar to results for the open ocean. During the spring bloom, when the dinoflagellate Peridinium was abundant, the ratio of gross O 2 production to carbon fixation was about 7.5, and net O 2 production was greater than carbon fixation. The difference between O 2 gross production and carbon fixation results, at least in part, from uptake by Mehler reaction and from recycling of the 14 C tracer by dark respiration and the alternative oxidase (AOX). We used the difference in isotopic discrimination against 18 O, occurring during O 2 consumption by various biological pathways, to place constraints on the relative engagement of these pathways. We estimated the overall discrimination against 18 O in the lake from O 2 isotopic mass balance as 20.5-29‰. The only mechanism that can explain the strong overall fractionation in the lake is AOX, which strongly discriminates against 18 O (ϳ31‰). Our results show, for the first time, that uptake by AOX is widespread and quantitatively important to oxygen consumption in aquatic systems.Oxygen exchange by photosynthesis and respiration is the largest biogeochemical cycle in aquatic systems. In order to understand this cycle, it is necessary to know the gross rates of the major processes involved in oxygen production and uptake. Production of O 2 is known to occur in one process in photosystem II, but O 2 consumption in aquatic organisms is possible by several reactions. These include ordinary respiration through the cytochrome oxidase pathway, respiration by the alternative oxidase pathway, Mehler reaction, and photorespiration. The first two processes take place in light as well as in dark conditions, whereas the latter two occur only under illumination. Although the presence of the above mechanisms has been established in different studies, their quantitative importance in the overall O 2 uptake in aquatic systems is not well known, and it is necessary to assess their role in natural environments. In this respect, ordinary O 2 incubation methods, which are very useful for the assessment of photosynthetic production from light-and dark-incubation experiments (e.g., Williams and Purdie 1991), do not provide the necessary information. The main drawback of these methods is their inability to measure the rate of O 2 AcknowledgmentsWe thank Y. Geiffman and the Mekorot Watershed Unit for provision of wind data and extend special thanks to the Kinneret Limnological Laboratory skipper M. Hatab. We are grateful

Aerobic respiration and hypoxia in the Lower St. Lawrence Estuary: Stable isotope ratios of dissolved oxygen constrain oxygen sink partitioning

“…DO was determined by Winkler titration (Grasshoff et al 1983), with a relative standard deviation of better than 1%. For O 2 isotopic analysis, approximately 150 mL of water was collected in evacuated 300-mL bottles containing 100 mL of saturated HgCl 2 solution to stop biological activity (Emerson et al 1995;Luz et al 2002). The dissolved gases were allowed to equilibrate with the headspace over several days at room temperature.…”

“…Hence, respiration leads to a detectable increase in the 18 O : 16 O ratio of O 2 in oxygen-deficient marine and freshwater environments (Bender 1990;Kiddon et al 1993;Parker et al 2005). If the isotope fractionation during respiration (e O ) is known, the spatial distribution of the O 2 18 O : 16 O, or its temporal variation, can yield quantitative information on community respiration rates (Luz et al 2002;Tobias et al 2007). Previous work (Guy et al 1993;Kiddon et al 1993;Bender et al 1994) indicates that the O-isotope effect for dark respiration by bacteria, through the enzyme cytochrome oxidase, is relatively robust, with estimates clustering around 18%.…”

Technical note: Consistent calculation of aquatic gross production from oxygen triple isotope measurements

“…A similar mass balance equation was used by Hendricks et al (2004), but the latter study did not consider the influence of isotopic fractionation during oxygen invasion from and evasion to the atmosphere. Following Luz et al (2002), I include these fractionations here explicitly. I also show how the model could be extended to include bubble injection, which was mentioned but apparently disregarded in the gross production calculations of Stanley et al (2010).…”

Effects of biological and physical factors on seasonal oxygen dynamics in a stratified, eutrophic coastal ecosystem

“…For example, spatial patterns are not affected by e, but lower e values give higher P : R and productivity estimates when P : R is larger than 1. Even though our three independent estimates for e were very close to 222%, we explored the effects of alternative e values of 218% and 225%, covering the range commonly reported in previous studies (Kroopnick 1975;Luz et al 2002;Hendricks et al 2004). For the northern Gulf of Mexico shelf adjacent to the Mississippi and Atchafalaya Rivers, e values of 225% did not seem reasonable for either seasonal transects or summer cruises, because P : R values for most stations could not be calculated using e of 225%.…”

“…Decreasing d 18 O values in response to photosynthesis are due to the addition of isotopically depleted (light) oxygen, derived from ambient water, to the existing dissolved oxygen (DO) pool (Guy et al 1989(Guy et al , 1993, whereby the d 18 O of source water in coastal marine systems ranges from 0% to about 23%. Respiration preferentially removes light oxygen with a large fractionation factor (e of 215% to 225% ;Kroopnick 1975;Luz et al 2002;Hendricks et al 2004), which results in increased d 18 O values for the residual DO pool. Oxygen that is respired within bottom sediments (hereafter referred to as benthic respiration) has a small fractionation effect, ranging from 0% to 23% (Brandes and Devol 1997).…”