Methane formation from algae in oxic seawater
Final Report Abstract
Methane (CH4) production within the oceanic mixed layer is a widespread phenomenon, but the underlying mechanisms are still under debate. To explain the source of CH4 in surface waters, it has been suggested that methanogenesis takes place in anoxic microenvironments of organic aggregates. Other sources such as in-situ formation of CH4 by algae have also been suggested, however, a direct evidence of algae-derived CH4 formation from lab experiments with (axenic) algae cultures was missing thus far. The overall aim of this research project was the verification (proof of principle) and quantification of CH4 production by several species of marine algae such as haptophytes (e.g. Chrysochromulina spp., Phaeocystis globosa and Emiliania huxleyi). Generally, we closely followed the project according to the proposed research plan and most aspects of the work packages were addressed successfully. Most of our results were published in peer-reviewed journals or are currently in preparation. The following are the most notable achievements from the project: We provided the first isotope evidence that the marine algae E. huxleyi produce CH4 with bicarbonate and the sulfur-bound methyl group of methionine as carbon precursors. Since our results unambiguously showed that the common coccolithophore E. huxleyi is able to produce CH4 per se under oxic conditions, we suggested that algae living in marine environments might contribute to the regional and temporal oversaturation of surface waters. Next to E. huxleyi, other widespread haptophytes, i.e. P. globosa and Chrysochromulina sp. could be identified as CH4 producers and stable carbon isotope measurements provided unambiguous evidence that all three investigated marine algae produce CH4 per se under oxic conditions. The addition of 13C labelled dimethyl sulfide, dimethyl sulfoxide and methionine sulfoxide – known algal metabolites that are ubiquitous in marine surface layers - enabled us to monitor the occurrence of 13C-enriched CH4 in cultures of E. huxleyi clearly indicating that methylated sulphur compounds are also precursors of CH4. Environmental factors such as temperature and light intensity were shown to control CH4 emission rates of three widespread marine algal species (E. huxleyi, P. globosa and Chrysochromulina sp.). Based on the results, we concluded that extensive blooms of E. huxleyi could act as a main regional source of CH4 in surface water, since blooming of E. huxleyi is related to the seasonal increase in both light and temperature which also stimulate CH4 production. We further highlighted that under typical global change scenarios E. huxleyi will increase its CH4 production in the 21th century. The stable carbon isotope values of CH4 emitted by the three marine algae species E. huxleyi, Chrysochromulina sp. and P. globosa. were determined. The δ13CH4 values of algal derived CH4 were significantly enriched in 13C compared to atmospheric values and to other biotic sources (methanogenic archaea) as described in the literature. Thus, δ13CH4 values emitted from marine algae might provide new information for a better understanding of sinks and sources at the ecosystem level or even on a global scale. Finally, our project opened up new research avenues that have implications for our understanding of CH4 formation under aerobic conditions in several environments. We demonstrated that Cyanobacteria living in marine, freshwater and terrestrial environments produce CH4 at substantial rates under light, dark, oxic and anoxic conditions, linking CH4 production with light driven primary productivity in a globally relevant and ancient group of photoautotrophs. Finally, we showed that in a freshwater environment (Lake Stechlin) aquatic surface water CH4 accumulation originates from a highly dynamic interplay between (oxic) CH4 production and surface emission. Laboratory incubations of eight different limnic phytoplankton strains and application of stable isotope techniques provided the first unambiguous evidence that major phytoplankton classes in Lake Stechlin per se produce CH4 under oxic conditions.
Publications
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(2016). Evidence for methane production by marine algae (Emiliana huxleyi). Biogeosciences, 13, 3163-3174
K. Lenhart, T. Klintzsch, G. Langer, G. Nehrke, M. Bunge, S. Schnell, F. Keppler
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(2018). A fast and sensitive method for the ontinuous in-situ determination of dissolved methane and its δ13C-isotope ratio in surface waters. Limnology and Oceanography: Methods, 16, 273–285
J.F. Hartmann, T. Gentz, A. Schiller, M. Greule, H.-P. Grossart, D. Ionescu, F. Keppler, K. Martinez-Cruz, A. Sepulveda-Jauregui, M. Isenbeck-Schröter
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(2019). Methane production by three widespread marine phytoplankton species: release rates, precursor compounds, and relevance for the environment. Biogeosciences, 16, 4129-4144
T. Klintzsch, G. Langer, G. Nehrke, A. Wieland, K. Lenhart, F. Keppler
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(2020). Aquatic and terrestrial Cyanobacteria produce methane. Science Advances, 6, eaax5343
M. Bižić, T. Klintzsch, D. Ionescu, M. Hindiyeh, M. Günthel, A. Muro-Pastor, W. Eckert, T. Urich, F. Keppler, H.-P. Grossart
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(2020). Effects of temperature and light on methane production from widespread marine algae. Journal of Geophysical Research: Biogeosciences, 125, e2020JG005793
T. Klintzsch, G. Langer, A. Wieland, H. Geisinger, K. Lenhart, G. Nehrke, F. Keppler
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(2020). High Spatio-Temporal Dynamics of Production and Emission of Methane in Oxic Surface Water. Environmental Science & Technology, 54, 1451-1463
J.F. Hartmann, M. Günthel, T. Klintzsch, G. Kirillin, H.-P. Grossart, F. Keppler, M. Isenbeck-Schröter
