Metabolomic analysis of heat-hardening in adult green-lipped mussel (Perna canaliculus): A key role for succinic acid and the GABAergic synapse pathway.

We evaluated the thermotolerance (LT50) of adult green-lipped mussels (Perna canaliculus) following an acute thermal challenge in the summer of 2012 and the winter of 2013. Mussels were grouped into two treatments, naïve (N, no prior heat treatment) and heat-hardened (HH = 1 h at 29 °C, 12 h recovery at ambient) before being immersed for 3 h in water of varying temperature, i.e. Ambient (Control), 25, 29, 31, 33, and 35 °C with subsequent mortality monitored for 30 days. As expected, naïve mussels were less thermotolerant than heat-hardened i.e. Summer LT50, N = 31.9, HH = 33.5 °C; Winter LT50, N = 31.4, HH = 33.8 °C. Moreover, at 33 °C no heat-hardened mussels died compared to 100% mortality in naïve specimens. At 35 °C all mussels died regardless of treatment. For the 'Summer' mussels, metabolite abundances in gill tissues of both naïve and heat-hardened mussels were quantified. For mussels at 33 °C, succinic acid was significantly higher in naïve mussels than heat-hardened mussels, indicating perturbations to mitochondrial pathways in these thermally stressed mussels. Additionally, analysis of biochemical pathway activity suggested a loss of neural control i.e. significantly reduced GABAergic synapse activity, in naïve vs. heat-hardened mussels at 33 °C. Taken together these findings suggest that heat-hardening improves mussel survival at higher temperatures by delaying the onset of cellular anaerobic metabolism, and by maintaining inhibition of neural pathways. Such results offer new perspectives on the complex suite of sub-cellular stress responses operating within thermally stressed organisms.

Effect of tidal regime on the thermal tolerance of the marine gastropod Lunella smaragda (Gmelin 1791).

The tidal cycle around New Zealand results in spring low tides consistently occurring during the hottest part of the day (mid-afternoon) in north-eastern New Zealand, and during the cooler dawn/dusk periods in the north-west of the country. We hypothesised that due to mid-afternoon spring low tides, intertidal populations residing at north-eastern sites would show greater thermotolerance than their north-west conspecifics. To test this we used the marine gastropod, Lunella smaragda, which were collected from sites on both the East and West coasts of the Auckland region and exposed to an acute heat shock. Thermotolerance was measured as survivorship (LT50), drop down time (time to heat coma) and thermal stability of the anaerobic energy producing enzyme Tauropine dehydrogenase. Furthermore, temperature loggers were deployed at each site so as to record and compare thermal regimes among sites. A strong temperature spike associated with spring low tide was found at all sites, and maximal temperatures of all East coast sites were higher than West coast sites (in some case by up to 10°C). In terms of thermotolerance, mortality of L. smaragda occurred at 42°C leading to 100% mortality at 45°C. However, comparison of LT50 showed snails were equally thermotolerant regardless of site of collection. Similar results were found in TDH thermal stability with animals from all sites showing an approximately 80% decrease in enzyme activity after 10min exposure to 42°C. Whilst drop down times were different among sites these were correlated with animal size as opposed to site of collection. Thus, East coast populations of L. smaragda appear no more thermotolerant than their West coast counterparts. Such a result is concerning as maximal temperatures at East coast sites already exceed the LT50 values of L. smaragda recorded in the lab suggesting these populations have less of a thermal safety margin.

Potential impact of metabolic acidosis on the fixed-acid Bohr effect in snapper (Pagrus auratus) following angling stress.

Oxygen equilibrium data were obtained for intact erythrocytes from Pagrus auratus and the pH-dependence of the fixed-acid Bohr and Root effects described. Indicators of aerobic-anaerobic poise were then measured following a period of strenuous exercise induced by angling capture. Haematological indices and plasma metabolites were evaluated with respect to their potential impact on the blood oxygen transport mechanism. An increase in post-capture haemoglobin content, blood lactate, and glucose were observed. The fall in caudal venous pH from 7.53 to 7.41, reflecting the acid-base status of blood draining the working tail musculature, remained within the operational zone of the maximal fixed-acid Bohr effect (Phi(7.4-7.8)=-0.95), but above the critical pH at which the Root effect suppresses oxygen transport capacity. These stress-induced changes reflect a strategy of optimal O(2) unloading to tissues with the Root effect unlikely to be expressed in the swimming vasculature.