Methane production by three widespread marine phytoplankton species: release rates, precursor compounds, and potential relevance for the environment

Abstract

Abstract. Methane (CH4) production within the oceanic mixed layer is a widespread phenomenon, but the underlying mechanisms are still under debate. Marine algae might contribute to the observed CH4 oversaturation in oxic waters, but so far direct evidence for CH4 production by marine algae has only been provided for the coccolithophore Emiliania huxleyi. In the present study we investigated, next to E. huxleyi, other widespread haptophytes, i.e., Phaeocystis globosa and Chrysochromulina sp. We performed CH4 production and stable carbon isotope measurements and provide unambiguous evidence that all three investigated marine algae are involved in the production of CH4 under oxic conditions. Rates ranged from 1.9±0.6 to 3.1±0.4 µg of CH4 per gram of POC (particulate organic carbon) per day, with Chrysochromulina sp. and E. huxleyi showing the lowest and highest rates, respectively. Cellular CH4 production rates ranged from 16.8±6.5 (P. globosa) to 62.3±6.4 ag CH4 cell−1 d−1 (E. huxleyi; ag = 10−18 g). In cultures that were treated with 13C-labeled hydrogen carbonate, δ13CH4 values increased with incubation time, resulting from the conversion of 13C–hydrogen carbonate to 13CH4. The addition of 13C-labeled dimethyl sulfide, dimethyl sulfoxide, and methionine sulfoxide – known algal metabolites that are ubiquitous in marine surface layers – resulted in the occurrence of 13C-enriched CH4 in cultures of E. huxleyi, clearly indicating that methylated sulfur compounds are also precursors of CH4. By comparing the algal CH4 production rates from our laboratory experiments with results previously reported in two field studies of the Pacific Ocean and the Baltic Sea, we might conclude that algae-mediated CH4 release is contributing to CH4 oversaturation in oxic waters. Therefore, we propose that haptophyte mediated CH4 production could be a common and important process in marine surface waters.

A decade of methane measurements at the Boknis Eck Time-series Station in the Eckernförde Bay (Southwestern Baltic Sea)

“…The occurrences of the unusual CH 4 profiles were accompanied by the enhanced Chlorophyll a concentrations in the upper waters. CH 4 productions by widespread marine phytoplankton have been reported and might be potential sources of surface CH 4 supersaturations (Lenhart et al, 2016;Klintzsch et al, 2019). However, spring or autumn algae blooms at the BE time-series station were often observed without CH 4 accumulation and surface CH 4 contribution from phytoplankton remains to be proven.…”

Discovery and quantification of a widespread methane ebullition event in a coastal inlet (Baltic Sea) using a novel sonar strategy

“…520% 70 , also providing evidence that a substantial fraction of dissolved methane is liberated to the atmosphere 69,70 . Still, a contribution of pelagic methane sources like phytoplankton 71,72 or zooplankton 73,74 to the observed supersaturation within the oxic waters of the Baltic Sea is likely. Even if not studied in detail in the present field campaign, it might be speculated that the dissolved methane plume is transported out of the bay by currents and partially mixed into the surface waters and liberated to the atmosphere.…”

The Contribution of Methane Photoproduction to the Oceanic Methane Paradox

“…More recently, Zhang and Xie (2015) observed CH 4 production from photodegradation of chromophoric dissolved organic matter (CDOM) in a highly colored river water sample. Despite evidence that CH 4 can be produced under aerobic conditions by various biotic and abiotic processes, their quantitative relevance to the methane paradox remains weak (Bange & Uher, 2005; Zhang & Xie, 2015), difficult to evaluate (Althoff et al, 2014; Karl et al, 2008; Klintzsch et al, 2019), or has not been assessed on large scales (Repeta et al, 2016).…”