Joint research project

Methan paradox in the Black Sea: role of picocyanobacteria in the aerobic production of methane

Project leaders
Cristiana Callieri, Snejana Moncheva
Agreement
BULGARIA - BAS - Bulgarian Academy of Sciences
Call
CNR/BAS triennio 2019-2021 2019-2021
Department
Earth system science and environmental technologies
Thematic area
Earth system science and environmental technologies
Status of the project
New

Research proposal

Methane (CH4) is a potent greenhouse gas, responsible for up to 20% of global warming, having 28 times the warming potential of CO2 (IPCC 2007). Oxic lake-surface has been found to be oversaturated with CH4, contributing to almost 90% of CH4 emission to the atmosphere (Donis et al. 2017). The appearance of CH4 within oxic surface-water has been termed the "oceanic methane paradox" and has been observed in the oceans and lakes where microbes can metabolize methylated compounds and release CH4 even aerobically (Karl et al. 2008).The maximal reported CH4 concentrations in the oxic layer are much higher in freshwater than in seawater, but considering the larger cover area of oceans on Earth and the thickness of the mixing layer the contribution of marine and lake methane to the atmosphere is roughly equal (Tang and Grossart, 2016).
In many oceans the oversaturated methane concentrations have been quantified, but to date no report has been published on a meromictic sea like the Black Sea. The Black Sea is the largest surface water reservoir of methane (6000 Gmol), which is contained in the anoxic zone of the main basin. The estimated CH4 flux to the atmosphere from the basin is about 4.1 Gmol yr-1 (Reeburgh et al 1991). The characteristic of a meromictic sea is the presence of a permanent thermocline (or chemocline) that constitutes a density barrier for diffusion from the meso and bathypelagic waters to the oxic waters. Therefore, the methane produced in the anoxic layer from Archaea and Bacteria is oxidated in deep waters and does not reach easily the upper layer, like in the majority of the oceans. The study of the oxic methane production in a meromictic sea is therefore a unique occasion to have low interference with the methane coming from below.
Different explanation for the presence of methane in the oxic waters have been proposed. For example: 1) methanogenesis can occur in anaerobic microsites, where Archaea attached to autotrophs can use the methyl coenzyme M reductase A (mcrA) gene to produce methane; 2) organosulphur compounds (methionine, dimethyl sulphoxide and DMSP) can be chemically converted to methane by microalgae; 3) the coccolithophorid Emiliania huxleyi produces CH4 during all growth phases as part of its normal metabolism; 4)picocyanobacteria contain a phosphonatase pathway that exclusively hydrolyzes 2-aminoethyl phosphonate (2-AEP;Ilikchyan et al. 2009) producing CH4.
This last process is particularly interesting and evidences have been found of the presence of picocyanobacteria of the genus Synechococcus in the correspondence of methane increase (Weller et al. 2013). Detection of phnD expression indicate a capacity of picoplankton to utilize phosphonates, a refractory form of phosphorus that can represent 25% of the high-molecular-weight dissolved organic phosphorus pool in marine systems (Ilikchyan et al. 2009), also derived from the herbicide glyphosate.
In particular the research here proposed has the aim to study the relation between cyanobacteria (Synechococcus or other) and the presence of methane in the Black Sea, the largest meromictic sea in the world. Synechococcus is the main genus composing the picocyanobacterial community of aquatic environments. It is an autotrophic microorganism with cell size < 2µm, considered as the main primary producers in the oceans and in many large lakes (Scanlan 2012, Callieri et al. 2012). Their size and adaptation ability are the reasons of their success on our planet, where they evolved colonizing aquatic environments. Picocyanobacteria play a key role in the organic carbon cycle, supplying energy and matter to higher level of the trophic chain and recently have been proposed as the possible actors in the methanogenesis in surface lakes.
The presence in the euphotic layers of the Black Sea (up to 50m) of Synechococcus has been observed in different studies (Uysal 2006, Callieri et al. submitted). Nevertheless data on their relation with the methane was never studied in the Black Sea. Therefore, we want: 1) to quantify the presence of methane and CO2 within the euphotic layers of Black Sea and measure the delta 13C values of these two dissolved gases 2) to study how these geochemical parameters can be related to the Synechococcus activity.
The CNR-ISE has the expertise on the ecology and genomics of picocyanobacteria (Synechococcus) in lakes and in the Black Sea. The CNR-IGG has the competence on dissolved gas measurement in different aquatic environments. The Bulgarian Academy of Sciences (BAS) of Varna is studying the biogeochemistry of the west basin of Black Sea using profilers for the measurements of main chemical, physical and biological parameters from the surface to 1000m since many years. CNR-ISE has been the Italian project leader in the bilateral CNR BAS 2016-2018 focussed on the presence of picocyanobacteria in the mesopelagic Black Sea. The cooperation with the BAS Institute of Varna, with their infrastructures, like the vessel for sampling is a rare opportunity to the Italian researchers to study the Black Sea and its role in methane production. The Black Sea is a unique environment, often overlooked and not sufficiently studied in the cruise of the Mediterranean Sea. The paradox of methane should be investigated also in this environment, focussing of the possible actors of methane production and outgassing in the atmosphere, particularly urgent in an era of climate change.
Ref
IPCC (2007) Climate Change 2007.Cambridge University Press / Donis et al. 2017. Nature communications / Karl et al 2008. Nat Geosci / Tang et al. 2016 Environ Sci Tech Letters/ Weller et al. 2013/ Ilikchyan et al. 2009. Environ Microbiol / Scanlan D. 2012. In: Ecology of Cyanobacteria: Their Diversity in Time and Space. Springer Publishers: 503-533 / Callieri C. et al. 2012. In: Ecology of Cyanobacteria: Their Diversity in Time and Space. Springer Publishers: 229-269 / Uysal 2006. Deep Sea Res./ Reeburgh et al 1991 Deep-Sea Res

Research goals

The aims of this project is to study, for the first time in the oxic superficial waters of Black Sea, the methane production and to identify the main biological actors of this process not yet clear both in marine and freshwaters. The knowledge of the methanogenesis pathways in the oxic superficial water of a meromictic sea could change the current view of methane global dynamics in the oceans.
Among the different hypotheses of methane production in oxic waters described above, we will try to verify the possible role of picocyanobacteria of Synechococcus genus in methane production with the decomposition of methylphosphonate, through the activation of the phnD gene. The clarification of the role of picocyanobacteria could give an impulse in the interpretation of the climate models for future predictions, given the ubiquity of such picocyanobacteria in surface waters.
One of the characteristic of the bilateral projects is the absence of the research funds itself and therefore it is important to have the possibility to joint to already planned sampling campaign otherwise not affordable. In our case we will have the chance to participate to the Bulgarian cruise with the research vessel "Akademik" and sampling for the chemical and isotopic analysis of methane and CO2 as well as for the cyanobacteria counting and genetic analyses. This in an opportunity that Italian research had not to lose.

Last update: 29/03/2024