Methanotrophy within the water column of a large meromictic tropical lake (Lake Kivu, East Africa)

Abstract

Abstract. The permanently stratified Lake Kivu is one of the largest freshwater reservoirs of dissolved methane (CH 4 ) on Earth. Yet CH 4 emissions from its surface to the atmosphere have been estimated to be 2 orders of magnitude lower than the CH 4 upward flux to the mixed layer, suggesting that microbial CH 4 oxidation is an important process within the water column. A combination of natural abundance stable carbon isotope analysis (δ 13 C) of several carbon pools and 13 CH 4 -labelling experiments was carried out during the rainy and dry season to quantify (i) the contribution of CH 4 -derived carbon to the biomass, (ii) methanotrophic bacterial production (MBP), and (iii) methanotrophic bacterial growth efficiency (MBGE), defined as the ratio between MBP and gross CH 4 oxidation. We also investigated the distribution and the δ 13 C of specific phospholipid fatty acids (PLFAs), used as biomarkers for aerobic methanotrophs. Maximal MBP rates were measured in the oxycline, suggesting that CH 4 oxidation was mainly driven by oxic processes. Moreover, our data revealed that methanotrophic organisms in the water column oxidized most of the upward flux of CH 4 , and that a significant amount of CH 4 -derived carbon was incorporated into the microbial biomass in the oxycline. The MBGE was variable (2-50 %) and negatively related to CH 4 : O 2 molar ratios. Thus, a comparatively smaller fraction of CH 4 -derived carbon was incorporated into the cellular biomass in deeper waters, at the bottom of the oxycline where oxygen was scarce. The aerobic methanotrophic community was clearly dominated by type I methanotrophs and no evidence was found for an active involvement of type II methanotrophs in CH 4 oxidation in Lake Kivu, based on fatty acids analyses. Vertically integrated over the water column, the MBP was equivalent to 16-60 % of the average phytoplankton particulate primary production. This relatively high magnitude of MBP, and the substantial contribution of CH 4 -derived carbon to the overall biomass in the oxycline, suggest that methanotrophic bacteria could potentially sustain a significant fraction of the pelagic food web in the deep, meromictic Lake Kivu.

Nitrous oxide and methane seasonal variability in the epilimnion of a large tropical meromictic lake (Lake Kivu, East-Africa)

“…The present paper also complements the work of Morana et al (2015b) based on the same two-year sampling at Ishungu, which mainly focused on the biogeochemistry of organic matter.…”

Biogeochemistry of a large and deep tropical lake (Lake Kivu, East Africa: insights from a stable isotope study covering an annual cycle

“…Such large isotopic depletion of the POC pool in the water column has been reported by Blees et al (2014), who measured δ 13 C-POC as low as −49 ‰ in Lake Lugano, and it was related to high methanotrophic activity. In Lake Kivu, CH 4 concentrations were found to decrease sharply with decreasing depth at the oxic-anoxic transition (Borges et al, 2011), and the dissolved CH 4 that reached the oxycline via turbulent diffusivity and vertical advection (Schmid et al, 2005) is known to be isotopically light, with a δ 13 C signature of approximately −60 ‰ (Pasche et al, 2011;Morana et al, 2015). Therefore, the vertical patterns in CH 4 concentrations and δ 13 C-POC values observed during this study suggest that a substantial part of CH 4 was consumed and incorporated into the microbial biomass in the oxycline.…”

“…Therefore, the vertical patterns in CH 4 concentrations and δ 13 C-POC values observed during this study suggest that a substantial part of CH 4 was consumed and incorporated into the microbial biomass in the oxycline. Indeed, experiments carried out in Lake Kivu in February 2012 and September 2012 showed that microbial CH 4 oxidation was significant in the oxycline, and phospholipid fatty acid analysis revealed high abundance of methanotrophic bacteria of type I at the same depths (Morana et al, 2015). With es-timates of the isotope fractionation factor during microbial CH 4 oxidation (1.016, Morana et al, 2015), and of the δ 13 C-CH 4 at each sampling point, it is possible to estimate the theoretical δ 13 C signature of methanotrophic organisms at each depth.…”

“…Indeed, experiments carried out in Lake Kivu in February 2012 and September 2012 showed that microbial CH 4 oxidation was significant in the oxycline, and phospholipid fatty acid analysis revealed high abundance of methanotrophic bacteria of type I at the same depths (Morana et al, 2015). With es-timates of the isotope fractionation factor during microbial CH 4 oxidation (1.016, Morana et al, 2015), and of the δ 13 C-CH 4 at each sampling point, it is possible to estimate the theoretical δ 13 C signature of methanotrophic organisms at each depth. Note that the δ 13 C-CH 4 was not directly measured during this study but a very strong linear correlation between the log-transformed CH 4 concentrations and δ 13 C-CH 4 was found along vertical profiles performed in February and September 2012 in Lake Kivu (δ 13 C-CH 4 = −7.911 log(CH 4 ) − 13.027; r 2 = 0.87, n = 34; Morana et al, 2015).…”

“…With es-timates of the isotope fractionation factor during microbial CH 4 oxidation (1.016, Morana et al, 2015), and of the δ 13 C-CH 4 at each sampling point, it is possible to estimate the theoretical δ 13 C signature of methanotrophic organisms at each depth. Note that the δ 13 C-CH 4 was not directly measured during this study but a very strong linear correlation between the log-transformed CH 4 concentrations and δ 13 C-CH 4 was found along vertical profiles performed in February and September 2012 in Lake Kivu (δ 13 C-CH 4 = −7.911 log(CH 4 ) − 13.027; r 2 = 0.87, n = 34; Morana et al, 2015). Hence the δ 13 C-CH 4 at each sampling point between January 2012 and May 2013 can be approximated from the measured CH 4 concentrations using this empirical relationship.…”