Near-future ocean acidification does not alter the lipid content and fatty acid composition of adult Antarctic krill

Jessica A Ericson,So Kawaguchi,Stephen Nicol,Nicole Hellessey,Nils Hoem,Peter D Nichols,Patti Virtue

doi:10.1038/s41598-019-48665-5

Abstract

Euphausia superba (Antarctic krill) is a keystone species in the Southern Ocean, but little is known about how it will respond to climate change. Ocean acidification, caused by sequestration of carbon dioxide into ocean surface waters (pCO2), alters the lipid biochemistry of some organisms. This can have cascading effects up the food chain. In a year-long laboratory experiment adult krill were exposed to ambient seawater pCO2 levels (400 μatm), elevated pCO2 levels mimicking near-future ocean acidification (1000, 1500 and 2000 μatm) and an extreme pCO2 level (4000 μatm). Total lipid mass (mg g−1 DM) of krill was unaffected by near-future pCO2. Fatty acid composition (%) and fatty acid ratios associated with immune responses and cell membrane fluidity were also unaffected by near-future pCO2, apart from an increase in 18:3n-3/18:2n-6 ratios in krill in 1500 μatm pCO2 in winter and spring. Extreme pCO2 had no effect on krill lipid biochemistry during summer. During winter and spring, krill in extreme pCO2 had elevated levels of 18:2n-6 (up to 1.2% increase), 20:4n-6 (up to 0.8% increase), lower 18:3n-3/18:2n-6 and 20:5n-3/20:4n-6 ratios, and showed evidence of increased membrane fluidity (up to three-fold increase in phospholipid/sterol ratios). These results indicate that the lipid biochemistry of adult krill is robust to near-future ocean acidification.

Highlights

Euphausia superba (Antarctic krill, hereafter ‘krill’) is a highly abundant keystone species in the Southern Ocean food web[1]
We investigated whether lipid indicators of (a) homeoviscous adaptation (PUFA/SFA ratios, PL/ST ratios, and mean carbon chain length (MCL)) and (b) immune responses (n-3/n-6; 22:6n-3/20:4n-6 ratios and 18:3n-3/18:2n-6 ratios) in krill were altered by seawater pCO2
During weeks 26–43 (Fig. 1A), there was a fourfold increase in average total lipid in krill to 273.8 ± 75.4 mg/g dry mass (DM), and the effect of pCO2 on total lipid differed between weeks (Two Way ANOVA; pCO2*week, p = 0.052)

Summary

Introduction

Euphausia superba (Antarctic krill, hereafter ‘krill’) is a highly abundant keystone species in the Southern Ocean food web[1]. Organisms can alter their cell membrane fatty acid structure in response to environmental stressors[18]; a process termed ‘homeoviscous adaptation’. The effects of temperature on homeoviscous adaptation are well known[18], but recent studies find that the mechanisms of homeoviscous adaptation can be applied to other stressors such as pH Bacteria alter their cell membrane fatty acid saturation and chain length as the pH level (acidity) of their environment changes, which may alter the permeability of the cell membrane and control proton influx[19,20,21]. The effects of decreasing seawater pH (via ocean acidification), and temperature on homeoviscous adaptation have recently been studied in marine sponges, by measuring ratios of polyunsaturated/saturated fatty acids (PUFA/SFA), ratios of PL/ST, and mean carbon chain length (MCL) in these organisms[11]. Sterols can be incorporated into cell membranes and packed between PUFA to increase membrane thickness[18]

Methods

Results

Discussion

Conclusion