Hydrogen sulfide in marine recirculating aquaculture systems and the effects of exposure on the metabolism, welfare, and production performance of Atlantic salmon (Salmo salar)

The development of environmentally sustainable aquaculture to meet the growing global demand for healthy protein sources is of increasing importance. One approach to support production without further impact on natural resources is by land-based recirculating aquaculture systems (RAS). RAS allows for a large production output with little water usage, and the closed system design decreases the risk of biological contamination of water bodies and impacts on wild fish populations. The reuse of water within a RAS relies on biofilters to clean the water from the metabolic waste products excreted by the cultured fish. However, some of the microbes that reside in the biofilters can generate negative impacts. The microbes, referred to as sulfate-reducing bacteria, can metabolize sulfate and this process produces hydrogen sulfide (H₂S) as a byproduct. H₂S is a gas known for its characteristic odor of rotten eggs, and its high toxic potential to aerobic organisms. This toxic effect has proven to be a challenge within the aquaculture industry, particularly for Atlantic salmon (Salmo salar) production facilities, as several mass mortality events in RAS have been ascribed to the presence of H2S. The main objectives of this Ph.D. project were to advance the knowledge of the metabolic and physiological effects of acute and continuous H₂S exposure in Atlantic salmon post-smolt. Furthermore, the project sought to identify efficient water treatment strategies for chemical H₂S removal in RAS.

Therefore, four studies were conducted, the first of which aimed to examine the kinetics of two common oxidants used in RAS facilities, oxygen (O₂) and hydrogen peroxide (H₂O₂), to evaluate their effectiveness in removing H₂S. Removal kinetics for H₂S in seawater and industrial RAS water were determined for both oxidants, and the influence of the stoichiometry of the added H₂O₂ to H₂S was evaluated, by testing three mole ratios of H₂O₂:H₂S (1:1, 2:1, 4:1). Oxygen was less efficient in removing H2S from RAS water compared to air-equilibrated seawater. The addition of H₂O₂ significantly increased the removal rate of H₂S in both seawater and RAS water, with higher H₂O₂:H₂S ratios leading to a faster removal. The organic matter in the water influenced the dose of H2O₂ required to keep up the H₂S removal rate, and the water quality should be considered when determining the necessary dosage. The study demonstrated that the addition of H₂O2 is an efficient water treatment technology for removing H₂S in RAS and that a half-life of less than 30 min can be achieved, by H₂O₂ dosages adjusted according to system water parameters and H₂S concentrations. Removal of H2S was not affected by different concentrations of nitrate (NO₃⁻), and NO₃⁻ was not found to contribute to the chemical oxidation of sulfide.

Study 2 examined the metabolic effects of acute H₂S exposure and critical concentrations in three size groups of Atlantic salmon. The toxic effects of H₂S are exerted in the mitochondria, where H₂S can inhibit cellular respiration. This response was assessed by means of intermittent flow- through respirometry using oxygen uptake (MO₂) as a proxy for the effect of H₂S on cellular respiration. MO₂ was measured in fish exposed to progressively increasing H₂S concentrations until the fish reduced MO₂ below the standard metabolic rate (SMR) or fish lost equilibrium (LOE), and this point was defined as the critical H₂S concentration (H₂S_crit). The results showed that the tolerance to H₂S in Atlantic salmon is lower than previously reported and that H₂S_crit was independent of size. Following H₂S exposure, the estimated excess oxygen consumption (EOC) greatly exceeded the accumulated oxygen deficit (DO₂) in all groups. Smaller fish exhibited a significantly larger EOC than the other larger groups, but the duration of the recovery phase post-exposure did not differ among the different sizes of fish.

To better understand the impact of acute H₂S exposure and the capacity to recover post-exposure, study 3 was conducted. Here, fish were exposed to critical H₂S concentrations, as determined in study 2, and the time course of the response to H₂S as a stressor was investigated. Fish were sampled before, during, and in the following 30 min, 1, 2, 6, and 24 h post exposure. The magnitude and severity of the stress response in Atlantic salmon to acute H₂S exposure was evaluated through the measurement of primary and secondary stress indicators. Exposure to H₂S caused a drop in blood pH and hemoglobin levels during exposure, and increased plasma cortisol, glucose, and lactate levels significantly. Lactate and glucose levels remained elevated for 2 and 6h post-expousre before returning to baseline lavels. The results of the study showed that during exposure to critical levels of H₂S, energy production in Atlantic salmon is supported by anaerobic pathways, as indicated by the elevated levels of lactate and glucose.

Though acute exposure to H₂S has received the most attention due to the lethal consequences of exposure to high H₂S concentrations, it has been shown that H₂S can also be present in RAS in low sublethal but constant concentrations. Study 4 tested if such conditions affect the digestibility and production performance of Atlantic salmon, and fish were exposed to one of two different sublethal H₂S concentrations, or H₂S-free water. The continuous exposure to sublethal H₂S concentrations over 10 days did not influence the growth rate, feed intake, or nutrient digestibility, and revealed that continuous low levels of H₂S may not jeopardize production.