2025-W13 Physiological consequences of cyanobacterial lipid remodelling under changing ocean conditions

Overview

Due to anthropogenic activities the world’s oceans are changing dramatically. Increased atmospheric carbon dioxide (CO2) concentrations are partly absorbed in the oceans affecting several biogeochemical processes and contributing to ocean acidification (Caldeira and Wicket, 2003; Heinze et al., 2015). Additionally, human-induced climate change triggered changing nutrient distributions, a sea surface temperature (SST) increase and higher radiation in the photic zone constituting a challenge for marine life particularly phytoplankton. Yet effects of these multiple stressors on their physiology are limited. 

Cells possess lipid membranes as a barrier towards such ever changing environments. In phototrophs, they further act as a key component of the thylakoid membrane embedding the photosynthetic machinery and hence are critical for photosynthesis. However, lipid membranes undergo changes due to prevailing environmental conditions like the external nutrient concentration. For example, if phosphorus (P), an essential macronutrient, is limited, P-free lipids replace phospholipids to provide P for other cellular functions (Van Mooy et al., 2009). Oceanic regions, typically characterised by low nutrient concentrations with P in the low nanomolar range such as the large central gyre systems, are expanding (Polovina et al. 2008). Yet, these nutrient poor and challenging environments are dominated by the cyanobacterial genera Synechococcus and Prochlorococcus, one of the main global primary producers fixing 25% of atmospheric CO2. Due to global warming their contribution is even expected to increase by a further 15-30% (Visintini et al., 2021). 

Previous work in our group showed that lipid remodelling induced by P limitation affects carbon fixation in several Synechococcus spp. (Mausz et al. in prep., Figure 1). Nevertheless, how these environmentally important cyanobacteria will cope with changes in their lipid membrane while additionally facing multi-stressor conditions in a future ocean, remains yet to be resolved. 

This project has two major aims: First it wants to establish the ecophysiological consequences of P-limitation induced lipid remodelling in cyanobacteria in combination with future ocean conditions such as increased sea surface temperature, high irradiance and ocean acidification. Second, it wants to explore the function of major membrane lipids in cyanobacteria, which undergo lipid remodelling, sulfoquinovosyl diacylglycerol (SQDG) and phosphatidylglycerol (PG). 

Figure 1: Scheme of a cyanobacterial cell with its lipid membranes. (a) Overview of a cyanobacterial cell and (b) its lipid membranes with the different protein complexes embedded in the cytoplasmic membrane and thylakoid membrane. (c) Transmission electron image of Synechococcus sp. PCC 7002 showing membranes of thylakoid stacks. Scale bar equals 500 nm. Abbreviations: RubisCO, ribulose-1,5-bisphosphate carboxylase/oxygenase; PC, plastocyanin; PQ, plastoquinone; PSI and PSII, photosystem I and II.