Osmoregulation and salinity-induced oxidative stress: is oxidative adaptation determined by gill function?

Abstract

Osmoregulating decapods such as the Mediterranean green crab Carcinus aestuarii possess two groups of spatially segregated gills: anterior gills serve mainly respiratory purposes, while posterior gills contain osmoregulatory structures. The co-existence of similar tissues serving different functions allows the study of differential adaptation, in terms of free radical metabolism, upon salinity change. Crabs were immersed for 2 weeks in seawater (SW, 37 ppt), diluted SW (dSW, 10 ppt) and concentrated SW (cSW, 45 ppt). Exposure to dSW was the most challenging condition, elevating respiration rates of whole animals and free radical formation in hemolymph (assessed fluorometrically using C-H 2 DFFDA). Further analyses considered anterior and posterior gills separately, and the results showed that posterior gills are the main tissues fueling osmoregulatory-related processes because their respiration rates in dSW were 3.2-fold higher than those of anterior gills, and this was accompanied by an increase in mitochondrial density (citrate synthase activity) and increased levels of reactive oxygen species (ROS) formation (1.4-fold greater, measured through electron paramagnetic resonance). Paradoxically, these posterior gills showed undisturbed caspase 3/ 7 activity, used here as a marker for apoptosis. This may only be due to the high antioxidant protection that posterior gills benefit from [superoxide dismutase (SOD) in posterior gills was over 6 times higher than in anterior gills]. In conclusion, osmoregulating posterior gills are better adapted to dSW exposure than respiratory anterior gills because they are capable of controlling the deleterious effects of the ROS production resulting from this salinity-induced stress.

Osmoregulation, bioenergetics and oxidative stress in coastal marine invertebrates: raising the questions for future research

“…However, some of the FAA synthesized during exposure to hypersaline environments can also play a direct or indirect role in redox balance (see Yancey, 2005 and references therein). In C. aestuarii, increased respiration rate at low salinity is accompanied by increased ROS formation in hemolymph and tissues (Rivera-Ingraham et al, 2016a). But, as highlighted by Salin et al (2015b), respiration and ROS production are not necessarily linked.…”

“…Lynch et al, 1973;Rivera-Ingraham et al, 2016a;Young, 1979), isopods (e.g. Charmantier and Charmantier-Daures, 1994;Łapucki and Normant, 2008), amphipods (Morritt and Spicer, 1995) and cladocerans (Aladin, 1991).…”

“…The physiological mechanisms of hyper-regulation come at an energetic cost, which usually translates into higher respiration rates (Dehnel, 1960;Rivera-Ingraham et al, 2016a;Sabourin and Stickle, 1980) required to fuel the catabolism of osmotically active amino acids (Gilles, 1973) and the active uptake of salts (Willmer, 2001). As shown in the hyper-regulating isopod Idotea chelipes, metabolic rates increase linearly with increasing difference between the osmolality of the medium and the hemolymph (Łapucki and Normant, 2008).…”

“…2 could be related to differential oxidative adaptation between osmoregulatory and nonosmoregulatory organs. In Rivera-Ingraham et al (2016a), we used an estuarine hyper-/iso-osmoregulating decapod crab as a study model to demonstrate that non-osmoregulating (anterior) and osmoregulating (posterior) gill tissues facing hypo-osmotic shock can clearly differ in their metabolic and oxidative response. Posterior gills with osmoregulatory function respond by generating new mitochondria, thus producing the energy needed to fuel osmoregulatory structures as well as the antioxidant defenses (namely superoxide dismutase) that are required to counteract stress-induced ROS production.…”

“…Havird et al, 2013;Towle et al, 2011). This can be accompanied by mitochondrial biogenesis to meet the energy requirements to fuel osmoregulation (Rivera-Ingraham et al, 2016a). While small amounts of ROS are required to activate osmoregulatory pathways (Wagner et al, 2013), larger and uncontrolled quantities lead to the degeneration or inactivation of the membrane-bound iontransporting pumps such as NKA (Kim and Akera, 1987;Kukreja et al, 1990), thus reducing osmoregulatory capacities.…”