Dietary carbohydrates induce a higher SDA than lipids in rainbow trout (Oncorhynchus mykiss) based on environmental temperature

As global temperatures increase so do the needs to investigate how the energy metabolism of fish responds to a broad range of thermal condition. Limited resources make it additionally important to use them sustainably in the feeds for aquaculture. Here we investigated the use of three different carbohydrate to lipid ratios (1:1; 1: 0.6; 1.4: 1 as non-protein energy substrates (NPES) in diets for rainbow trout (Oncorhynchus mykiss) under five different thermal regimes (12; 14; 16; 18; 20 °C) in a bioenergetic approach using a group respirometer. The results showed that the diet with carbohydrate as the main NPES resulted in a quadratic relationship of the specific dynamic action (SDA) values to temperature while diets with lipid as main NPES or a balanced ratio did not show such a response. SDA values in the diet with carbohydrate as the main NPES were significantly higher at temperatures around the optimum (15-17 °C) than the diets with lipid as the main NPES or with a balanced carbohydrate to lipid ratio. The retained energy (RE) was highly dependent on the standard metabolic rate (SMR) and SDA values did not carry over onto them. The protein utilization for energy combustion was significantly lower at 12 °C in the diet with carbohydrate as the main NPES than in the diet with lipid as the main NPES thus indicating that carbohydrates hold a relevant nutritional value especially at lower temperatures.

The thermal dependence of the protein-sparing effect in rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

We performed an instantaneous bioenergetic study with rainbow trout (Oncorhynchus mykiss) of 206.3 g ± 2.9 g in a group respirometer of nine 250 L tanks at five different water temperatures (12; 14; 16; 18; 20°C) in order to determine the optimal thermal condition for a maximal visualization of the protein-sparing effect. Twelve fish per tank were tested at a stocking density of 9.94 kg m^-3 ± 0.14 kg m^-3 and fed three low-protein/high-energy diets with constant crude protein content of approximately 35% and three different energy contents (17.35; 18.76; 20.50 MJ kg^-1 ) once daily at a ration of 1.3% body weight (n = 3). Energy levels were increased by adding gelatinized wheat starch as a carbohydrate source and fish oil, canola oil and palmitin as lipid sources. Three different dietary digestible protein/digestible energy ratios (DP/DE; 20.38; 19.08; 18.09 mg kJ^-1 ) were achieved by replacing bentonite as a non-nutritive filler with carbohydrates and lipids. Oxygen consumption and ammonia excretion were assessed to obtain the potentially retainable energy (RE) and ammonia quotient (AQ) as benchmarks for potential growth and protein-sparing effect. The results showed the lowest relative metabolic combustion of protein at 16.9°C ± 0.1°C. We determined this temperature to set the optimal thermal condition for the induction of a maximum protein-sparing effect in juvenile rainbow trout. Increasing the DP/DE ratio significantly altered the magnitude of the relative metabolic protein use but had no effect on its interactions with temperature. We were able to reduce average metabolic fuel use of protein across diets from 16.2% ± 2.3% at 12°C to 8.0% ± 1.2% at 16°C. This study found no relevant significant differences of RE with the environmental temperature.

SDA coefficient is temperature dependent in rainbow trout (Oncorhynchus mykiss, Walbaum 1792) in a practical approach using group respirometry

Rising global temperatures have raised the need for detailed knowledge of the effects of rising temperatures on the physiology of animals used in aquaculture. Here we used a multifactorial bioenergetic approach using groups of rainbow trout (Oncorhynchus mykiss) with an average single fish weight of 183.75 g ± 0.65 g to investigate the interactions of feeding and temperature with key metabolic variables. We used a recirculating aquaculture respirometry system (RARS) to test three ration sizes (0.65; 0.975; 1.3% of live body weight (BW)) over a range of three consecutive temperatures (14; 17; 20 °C). The fish were fed once per day for 6 days at each temperature and subsequently starved for 5 days to return to standard metabolic rate (SMR). This study aimed to answer the highly discussed topic of the temperature dependency of key metabolic specific dynamic action (SDA)-variables SDA_coef and SDA_dur. We were able to provide evidence, that in rainbow trout the SDA_coef is highly dependent on the environmental temperature in the first ever approach to assess these variables in a group respirometer with this species. We compared the results of this study with a sophisticated bioenergetic model by Elliot and Hurley (2002) and thereby provide evidence for the practicability of group respirometry as a method to assess bioenergetic data under culture like conditions.

The C. elegans GATA transcription factor elt-2 mediates distinct transcriptional responses and opposite infection outcomes towards different Bacillus thuringiensis strains

The nematode Caenorhabditis elegans has been extensively used as a model for the study of innate immune responses against bacterial pathogens. While it is well established that the worm mounts distinct transcriptional responses to different bacterial species, it is still unclear in how far it can fine-tune its response to different strains of a single pathogen species, especially if the strains vary in virulence and infection dynamics. To rectify this knowledge gap, we systematically analyzed the C. elegans response to two strains of Bacillus thuringiensis (Bt), MYBt18247 (Bt247) and MYBt18679 (Bt679), which produce different pore forming toxins (PFTs) and vary in infection dynamics. We combined host transcriptomics with cytopathological characterizations and identified both a common and also a differentiated response to the two strains, the latter comprising almost 10% of the infection responsive genes. Functional genetic analyses revealed that the AP-1 component gene jun-1 mediates the common response to both Bt strains. In contrast, the strain-specific response is mediated by the C. elegans GATA transcription factor ELT-2, a homolog of Drosophila SERPENT and vertebrate GATA4-6, and a known master regulator of intestinal responses in the nematode. elt-2 RNAi knockdown decreased resistance to Bt679, but remarkably, increased survival on Bt247. The elt-2 silencing-mediated increase in survival was characterized by reduced intestinal tissue damage despite a high pathogen burden and might thus involve increased tolerance. Additional functional genetic analyses confirmed the involvement of distinct signaling pathways in the C. elegans defense response: the p38-MAPK pathway acts either directly with or in parallel to elt-2 in mediating resistance to Bt679 infection but is not required for protection against Bt247. Our results further suggest that the elt-2 silencing-mediated increase in survival on Bt247 is multifactorial, influenced by the nuclear hormone receptors NHR-99 and NHR-193, and may further involve lipid metabolism and detoxification. Our study highlights that the nematode C. elegans with its comparatively simple immune defense system is capable of generating a differentiated response to distinct strains of the same pathogen species. Importantly, our study provides a molecular insight into the diversity of biological processes that are influenced by a single master regulator and jointly determine host survival after pathogen infection.