Abstract
Abstract. Lipids, in their function as trophic markers in food webs and organic matter source indicators in the water column and sediments, provide a tool for reconstructing the complexity of global change effects on aquatic ecosystems. It remains unclear how ongoing changes in multiple environmental drivers affect the production of key lipid biomarkers in marine phytoplankton. Here, we tested the responses of sterols, alkenones and fatty acids (FAs) in the diatom Phaeodactylum tricornutum, the cryptophyte Rhodomonas sp. and the haptophyte Emiliania huxleyi under a full-factorial combination of three temperatures (12, 18 and 24 ∘C), three N : P supply ratios (molar ratios 10 : 1, 24 : 1 and 63 : 1) and two pCO2 levels (560 and 2400 µatm) in semicontinuous culturing experiments. Overall, N and P deficiency had a stronger effect on per-cell contents of sterols, alkenones and FAs than warming and enhanced pCO2. Specifically, P deficiency caused an overall increase in biomarker production in most cases, while N deficiency, warming and high pCO2 caused nonsystematic changes. Under future ocean scenarios, we predict an overall decrease in carbon-normalized contents of sterols and polyunsaturated fatty acids (PUFAs) in E. huxleyi and P. tricornutum and a decrease in sterols but an increase in PUFAs in Rhodomonas sp. Variable contents of lipid biomarkers indicate a diverse carbon allocation between marine phytoplankton species in response to changing environments. Thus, it is necessary to consider the changes in key lipids and their consequences for food-web dynamics and biogeochemical cycles, when predicting the influence of global change on marine ecosystems.
Highlights
Ocean phytoplankton has profoundly responded to and driven natural climatic variability throughout Earth’s history (Riding, 1992; Falkowski and Oliver, 2007; Falkowski, 2015)
The alkenones in E. huxleyi consisted of four pairs of homologues, i.e., C37 methyl ketones (C37 Me including C37 : 4 Me, C37 : 3 Me and C37 : 2 Me), C38 methyl ketones (C38 Me) (C38 : 3 Me and C38 : 2 Me), C38 ethyl (C38 : 3 Et and C38 : 2 Et) and C39 Et ketones (C39 Et including C39 : 3 Et and C39 : 2 Et)
Higher per-cell contents of brassicasterol / epi-brassicasterol were observed at higher temperatures and higher N : P supply ratios in P. tricornutum (Fig. 1a; Table S3), under the lowest and highest N : P supply ratios in Rhodomonas sp. (Fig. 1c) and under higher N : P supply ratios in E. huxleyi (Fig. 1e)
Summary
Ocean phytoplankton has profoundly responded to and driven natural climatic variability throughout Earth’s history (Riding, 1992; Falkowski and Oliver, 2007; Falkowski, 2015). The ocean-related global change fundamentally affects marine ecosystems (HoeghGuldberg and Bruno, 2010). These include especially global phytoplankton biomass decreases (Boyce et al, 2010; Moore et al, 2018; Lotze et al, 2019) and plankton community changes (Richardson and Schoeman, 2004; Jonkers et al, 2019), which alters food-web dynamics (Kortsch et al, 2015; du Pontavice et al, 2020) and biogeochemical cycles (Hofmann and Schellnhuber, 2009; Gruber, 2011; Doney et al, 2012). Some of the phytoplankton-produced biomolecules (biomarkers), functioning as indicators of nutritional food quality (Müller-Navarra, 2008) and tracers of organic matter sources (Volkman et al, 1998), have provided crucial insight into the trajectory of ecological responses to changing environment along food webs in the present-day ocean (Ruess and Müller-Navarra, 2019), as well as over geological time (Brocks et al, 2017)
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