Salinity stress from the perspective of the energy-redox axis: Lessons from a marine intertidal flatworm

Abstract

In the context of global change, there is an urgent need for researchers in conservation physiology to understand the physiological mechanisms leading to the acquisition of stress acclimation phenotypes. Intertidal organisms continuously cope with drastic changes in their environmental conditions, making them outstanding models for the study of physiological acclimation. As the implementation of such processes usually comes at a high bioenergetic cost, a mitochondrial/oxidative stress approach emerges as the most relevant approach when seeking to analyze whole-animal responses. Here we use the intertidal flatworm Macrostomum lignano to analyze the bioenergetics of salinity acclimation and its consequences in terms of reactive oxygen/nitrogen species formation and physiological response to counteract redox imbalance. Measures of water fluxes and body volume suggest that M. lignano is a hyper-/iso-regulator. Higher salinities were revealed to be the most energetically expensive conditions, with an increase in mitochondrial density accompanied by increased respiration rates. Such modifications came at the price of enhanced superoxide anion production, likely associated with a high caspase 3 upregulation. These animals nevertheless managed to live at high levels of environmental salinity through the upregulation of several mitochondrial antioxidant enzymes such as superoxide dismutase. Contrarily, animals at low salinities decreased their respiration rates, reduced their activity and increased nitric oxide formation, suggesting a certain degree of metabolic arrest. A contradictory increase in dichlorofluorescein fluorescence and an upregulation of gluthathione-S-transferase pi 1 (GSTP1) expression were observed in these individuals. If animals at low salinity are indeed facing metabolic depression, the return to seawater may result in an oxidative burst. We hypothesize that this increase in GSTP1 could be a “preparation for oxidative stress”, i.e. a mechanism to counteract the production of free radicals upon returning to seawater. The results of the present study shed new light on how tolerant organisms carry out subcellular adaptations to withstand environmental change.

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

“…As recently shown in Rivera-Ingraham et al (2016b), flatworms under hyposaline conditions not only decrease their activity and respiration rates, but also significantly increase very specific antioxidant defenses, namely the level of glutathione S-transferase (GST). Could this be interpreted as a POS mechanism?…”

“…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). However, this is far from a general trend; for example, the marine intertidal flatworm Macrostomum lignano is able to regulate its body volume at extremely low salinities while appearing to enter a state of metabolic arrest (Rivera-Ingraham et al, 2016b). So how can organisms control water and ion fluxes with limited energetic resources?…”

“…Live-imaging techniques and in vivo analysis of free radical production can be used with M. lignano, as this organism is small and transparent. We recently revealed that when exposed to hypersalinity, these flatworms increase respiration rates, which is accompanied by a dramatic increase in superoxide anion (O 2 · − ) production while H 2 O 2 and other ROS/RNS (specifically DCFH-oxidizing species; see Glossary) decreased (Rivera-Ingraham et al, 2016b). This hypersaline exposure is accompanied by upregulation of the gene expression of various antioxidants, although this does not always enable organisms to avoid increased apoptosis following exposure to high environmental salinities, a failure that is most probably due to dysfunctioning, worn-out cells.…”

“…Rivera-Ingraham et al, 2016b), and these pro-oxidant conditions, if not properly controlled, may also interfere with correct acclimation. The mechanisms used by renal cells to counteract hypersalinity are well studied; in this system, the synthesis or accumulation of FAA and other osmolytes is essential.…”

A novel flatworm-specific gene family implicated in reproduction inMacrostomum lignano

“…One of such organisms is the free-living flatworm Macrostomum lignano, which has been developed into an experimental platform to study various biological phenomena (Arbore et al, 2015;Rivera-Ingraham et al, 2016;Vellnow et al, 2017;Wasik et al, 2015). Several experimental protocols have been established for this animal, including transgenesis methods (Wudarski et al, 2017).…”