The influence of lipid content and taxonomic affiliation on methane and carbon dioxide production from phytoplankton biomass in lake sediment

Minna Hiltunen,Hannu Nykänen,Jari Syväranta

doi:10.1002/lno.11732

Minna Hiltunen, Hannu Nykänen + Show 1 more

Open Access

https://doi.org/10.1002/lno.11732

Copy DOI

Abstract

AbstractThe greenhouse gases methane (CH4) and carbon dioxide (CO2) are end products of microbial anaerobic degradation of organic matter (OM) in lake sediments. Although previous research has shown that phytoplankton lipid content influences sediment methanogenesis, current understanding on how OM quality affects methanogenesis is still limited. Such information is needed to more accurately assess how lake greenhouse gas emissions may change in response to anthropogenic activities. We cultured 11 phytoplankton species from five classes and studied how taxonomic identity, C : N ratio, lipid content, and fatty acid composition of phytoplankton biomass affects the CH4 and net CO2 production in anaerobic lake sediments with an incubation experiment that lasted > 100 d. The carbon‐normalized potential CH4 (0.09–0.23 μmol mg C−1 d−1) and net CO2 (0.09–0.28 μmol mg C−1 d−1) production rates were not related to phytoplankton taxonomic affiliation (e.g., class, species), C : N ratio, or fatty acid composition of algal biomass. Methane or net CO2 production potentials did not increase with higher lipid content (10–30%); however, total fatty acid content had a weak correlation with CH4 production potential. In contrast to previous research, our results suggest that lipid content is of minor importance in determining methanogenesis rates from the biomass of multispecies phytoplankton communities settling on sediments. The decrease in CO2 concentration and the correlation between stable carbon isotope signatures of CH4 and molar ratio of CH4 and CO2 at the end of the experiment may indicate that importance of hydrogenotrophic methanogenesis, which uses CO2 when other substrates become limiting, increased during the long incubation.

Highlights

Minor part of the variation was related to species and the interaction of the factors species and N treatment (F6,45 = 12.66, p < 0.001, R2 = 0.02), but as expected, class identity was the major determinant of phytoplankton fatty acid composition
The results presented here offer very little support for this hypothesis as only total fatty acid content was weakly related to CH4 production while the lipid content (10–30%) of algae was not, meaning that the biomass from highly productive eutrophic lakes cannot be considered a lower quality substrate for CH4 production
There were no clear differences in potential gas production among phytoplankton classes, suggesting that fatty acid composition or cell-wall structure did not influence methanogenesis

Summary

Introduction

The main production pathways for CH4 during anoxic degradation of OM in freshwater sediments are acetoclastic methanogenesis (acetate as precursor) and hydrogenotrophic methanogenesis (H2 and CO2 as precursors) (Whiticar et al 1986; Conrad 2020). The relative importance of these pathways is determined by temperature, microbial community composition, and the characteristics of the substrate, which influences the production rates of acetate, H2, and CO2 during the initial hydrolysis and fermentation step of complex organic compounds (Conrad 2020). The estimation of pathways is rather difficult due to variability between microbial taxa and environmental conditions in isotopic fractionation of carbon during the OM degradation processes (Conrad 2005; Goevert and Conrad 2009; Heuer et al 2010)

Results

Discussion

Conclusion