Response of three krill species to hypoxia and warming: an experimental approach to oxygen minimum zones expansion in coastal ecosystems

Abstract

To understand the adaptation of euphausiid (krill) species to oxygen minimum zones (OMZs), respiratory response and stress experiments combining hypoxia‐reoxygenation exposure with warming were conducted. Experimental krill species were obtained from the Antarctic (South Georgia area), the Humboldt Current System (HCS, Chilean coast) and the Northern California Current System (NCCS, Oregon). Euphausia mucronata from the HCS showed oxyconforming pO2‐dependent respiration below 80% air saturation (18 kPa). Normoxic subsurface oxygenation in winter posed a ‘high oxygen stress’ for this species. The NCCS krill, Euphausia pacifica, and the Antarctic krill, Euphausia superba, maintained respiration rates constant down to low critical pO2 values of 6 kPa (30% air saturation) and 11 kPa (55% air saturation), respectively. Antarctic krill had low antioxidant enzyme activities, but high concentrations of the molecular antioxidant glutathione (GSH) and was not lethally affected by 6 h exposure to moderate hypoxia. The temperate krill species (E. pacifica) had higher superoxide dismutase (SOD) values in winter than in summer, which relates to a higher winter metabolic rate. In all species, antioxidant enzyme activities remained constant during hypoxic exposure at the typical temperature for their habitat. Warming by 7 °C above habitat temperature in summer increased SOD activities and GSH levels in E. mucronata (HCS), but no oxidative damage occurred. In winter, when the NCCS is well mixed and the OMZ is deeper, +4 °C of warming combined with hypoxia represents a lethal condition for E. pacifica. In summer, when the OMZ expands upwards (100 m subsurface), antioxidant defences counteracted hypoxia and reoxygenation effects in E. pacifica, but only at mildly elevated temperature (+2 °C). In this season, experimental warming by +4 °C reduced antioxidant activities and the combination of warming with hypoxia again caused mortality of exposed specimens. We conclude that a climate change scenario combining warming and hypoxia represents a serious threat to E. pacifica and, as a consequence, NCCS food webs.

Glacial melting: an overlooked threat to Antarctic krill

“…Surface water warming could be an additional source of stress for krill. However, early experiments with temperatures varying from −1 to 10 °C have shown that krill are able to tolerate some exposure to high temperatures by lowering their metabolic rate 37 . The mortality observed in this study should therefore not be related to these stressors but to the amount and the size of glacially derived lithogenic particles.…”

Coping with Salt Water Habitats: Metabolic and Oxidative Responses to Salt Intake in the Rufous-Collared Sparrow

“…Oxidative metabolism has been associated with ROS production, which leads to cumulative damage of molecules, such as DNA, proteins, and lipids that can result in increased disease risk and death (Dowling and Simmons, 2009 ; Monaghan et al, 2009 ; Costantini, 2010 ; van de Crommenacker et al, 2010 ; Selman et al, 2012 ). While it has been shown that increased osmoregulatory function is positively associated with stress in invertebrates and fish (Martínez-Álvarez et al, 2002 ; Tremblay and Abele, 2016 ; Velez et al, 2016 ; Rivera-Ingraham and Lignot, 2017 ), we do not currently understand how salt intake and changes in osmotic conditions may increase the ROS production and cause oxidative damage in birds. In addition to changes in metabolic rates and tissue biochemical activities, a common response to high osmotic loads is the increase in urine concentration mediated by a complex interplay of hormones (Braun and Dantzler, 1984 ; McCormick and Bradshaw, 2006 ).…”

Approaches for testing hypotheses for the hypometric scaling of aerobic metabolic rate in animals

“…For most animals, MR/g will increase with availability of O 2 up to some limit, above which saturation occurs, and MR/g is limited by another process (Fig. 3) (71,138,143,146,197). If the MR/g of larger animals is directly limited by O 2 supply, we would predict that larger animals would require a higher environmental PO 2 to attain maximal MR/g for that physiological state (Fig.…”