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.