4.3 Metabolic temperature response between heat-tolerant and heat-sensitive Brachionus species
Metabolism in ectotherms is inextricably linked to environmental temperature and its rate is accelerated by temperature increase. Genes belonging to core metabolic pathways had, in general, the same expression pattern between the heat-tolerant B. calyciflorus s.s. and the heat sensitive B. fernandoi, showing an increased expression with temperature induction. However, there were significant differences between the species in genes involved in oxidative phosphorylation, lipid metabolism, and carbohydrate metabolism.
Oxidative stress is related to the production of toxic compounds that are called reactive oxygen species (ROS) which cause cellular response by modifying proteins and nucleic acids and contributes to cellular damage (Halliwell & Whiteman, 2004). Exposure of ectothermic organisms to elevated temperatures accelerates mitochondrial respiration and potentially increases ROS formation (Heise, Puntarulo, Pörtner, & Abele, 2003; Keller, Sommer, Pörtner, & Abele, 2004). ROS formation blocks heat shock response and refolding activity under heat stress, thereby leading to increasing cellular stress and ultimately heat sensitivity (Adachi et al., 2009). Genes related to oxidative stress were significantly up-regulated over temperature increase in heat-sensitive B. fernandoi. In contrast, these genes were significantly down-regulated over temperature increase in the heat-tolerant B. calysiflorus s.s.. Here, oxidative stress response was induced at the lowest temperature tested here (20oC). Among the differentially expressed genes were NADH dehydrogenase and glutathione S-transferase (GST). For the B. calyciflorus s.s., a significant induction (7x fold change) of NADH dehydrogonase has been reported after sustained cold stress on 14 °C for 30 days (Paraskevopoulou, Dennis, Weithoff, Hartmann, & Tiedemann, 2019). Transcriptional regulation of GST genes also differed in two copepod species of the genus Tigriopus . For the species T. japonicas, GST genes were significantly down-regulated in response to temperature elevation up to 35 °C (Han, Jeong, Byeon, & Lee, 2018), In Pacific oyster, both of the above genes were down-regulated, with diversity in their expression according to the duration of heat exposure. This indicates that repression of oxidation stress likely acts as a protective mechanism of the cells and potentially enhances heat tolerance (Lim et al, 2016).
Lipid and carbohydrate metabolism are highly conserved processes that affect nearly all aspects of an organism’s biology. The consumed lipids and carbohydrates are broken down during digestion into fatty acids and simple sugars, providing the essentials to produce a wide range of metabolites that are required for development and survival. Genes related to lipid and carbohydrate metabolism were up-regulated from mild to high heat in heat-tolerant, B. calyciflorus s.s. and down-regulated in heat-sensitive B. fernandoi . Up-regulation with heat of genes related to carbohydrate metabolism was also found in a teleost fish, Gillichthys mirabilis (Buckley, Gracey, & Somero, 2007) and in the Pacific oyster, Crassostrea gigas , after 6h of acute heat exposure (Yang, Gao, Liu, Wang, & Zhou, 2017). This suggests a need for rapid production of ATP under increasing temperatures. Apparently, B. calyciflorus s.s. has adapted to maintain its metabolism under high heat, while the heat-sensitive species B. fernandoi apparently shuts down costly metabolic processes (indicated by down-regulation of the majority of metabolic related pathways) in order to allocate available resources to survival.