Simultaneous exposure to nitrate and low pH reduces the blood oxygen-carrying capacity and functional performance of a freshwater fish

Abstract

Human activities present aquatic species with numerous of environmental challenges, including excessive nutrient pollution (nitrate) and altered pH regimes (freshwater acidification). In isolation, elevated nitrate and acidic pH can lower the blood oxygen-carrying capacity of aquatic species and cause corresponding declines in key functional performance traits such as growth and locomotor capacity. These factors may pose considerable physiological challenges to organisms but little is known about their combined effects. To characterise the energetic and physiological consequences of simultaneous exposure to nitrate and low pH, we exposed spangled perch (Leiopotherapon unicolor) to a combination of nitrate (0, 50 or 100 mg L−1) and pH (pH 7.0 or 4.0) treatments in a factorial experimental design. Blood oxygen-carrying capacity (haemoglobin concentration, methaemoglobin concentrations and oxygen equilibrium curves), aerobic scope and functional performance traits (growth, swimming performance and post-exercise recovery) were assessed after 28 days of exposure. The oxygen-carrying capacity of fish exposed to elevated nitrate (50 and 100 mg L−1) was compromised due to reductions in haematocrit, functional haemoglobin levels and a 3-fold increase in methaemoglobin concentrations. Oxygen uptake was also impeded due to a right shift in oxygen–haemoglobin binding curves of fish exposed to nitrate and pH 4.0 simultaneously. A reduced blood oxygen-carrying capacity translated to a lowered aerobic scope, and the functional performance of fish (growth and swimming performance and increased post-exercise recovery times) was compromised by the combined effects of nitrate and low pH. These results highlight the impacts on aquatic organisms living in environments threatened by excessive nitrate and acidic pH conditions.

Exposure to Nitrate Increases Susceptibility to Hypoxia in Fish

“…DOI: https://doi.org/10.1086/713252 kPa) and their blood-carrying capacity (lower hemoglobin concentration, HB]; and hematocrit, HCT) was marginally impaired. Exposure to other nitrogenous waste products (nitrite, ammonia) have also been shown to disrupt regular cardiorespiratory function (causing hyperventilation, tachycardia; Aggergaard and Jensen 2001; Williams et al 1997), while nitrate exposure has been shown to lower the blood-carrying capacity of various fishes via the formation of methemoglobin, and reductions in functional [HB] and HCT (Aboagye and Allen 2014;Gomez Isaza et al 2020b;Monsees et al 2017;Yang et al 2019). Indeed, in our previous study, we found that silver perch exposed to identical nitrate concentrations (50 and 100 mg NO 3 − L -1 ) experienced significant methemoglobin formation (20 -34% methemoglobin in the blood) and indicates that silver perch are susceptible long-term nitrate exposures (Gomez Isaza et al 2021).…”

“…High levels of nitrate also reduce concentrations of functional hemoglobin, lower hematocrit, and alter hemoglobinoxygen binding affinities (i.e. reduce affinity for oxygen at the gills; Gomez Isaza et al 2021; Monsees et al 2017;Yang et al 2019), effectively lowering blood oxygen carrying capacity (Gomez Isaza et al 2020b;Monsees et al 2017), raising energetic costs (Gomez Isaza et al 2018, 2020c and leading to respiratory collapse and disrupted physiological function (Jensen 2003;Jensen et al 1987). As such, organisms exposed to elevated nitrate concentrations and hypoxia are confronted with both environmental hypoxia and hypoxemia and may be unable to adequately respond to both stressors (Rodgers et al 2020).…”

Toxicidade de compostos nitrogenados em peixes influenciada por parâmetros físico-químicos da água: uma revisão narrativa

“…O baixo pH é um fator ambiental capaz de aumentar a absorção de alguns poluentes causando o acúmulo desses compostos no fígado e nas brânquias (Çoǧun & Kargin, 2004). Para avaliar como o pH influencia na toxicidade do nitrato, indivíduos de Leiopotherapon unicolor foram expostos simultaneamente ao nitrato e diferentes níveis de pH (pH 4,0 ou 7,0) por 28 dias de exposição (Gomez Isaza et al, 2020). Os resultados mostraram que, a exposição ao nitrato levou ao comprometimento da capacidade de transporte de oxigênio, independentes da concentração.…”

Use of complex physiological traits as ecotoxicological biomarkers in tropical freshwater fishes

“…Nitrate is the most abundant nutrient contaminant globally; although less toxic than nitrite it nonetheless shows persistently high concentrations in freshwaters due to agricultural runoff from nitrogen‐based fertilizers, fossil fuel combustion, and wastewater treatment plants. Nitrate also causes the formation of methemoglobin, compromising blood oxygen‐carrying capacity (Gomez Isaza et al, 2020b). Gomez Isaza et al (2020a) investigated whether nitrate exposure would increase susceptibility to elevated temperatures, measured as CT max , and limit aerobic performance at warmer temperatures, measured as AS by swimming respirometry.…”

“…This absence of effects of nitrate in fish acclimated to 32°C was unexpected but paralleled the findings for CT max and aerobic performance that are reported above. In a study on the subtropical highly eurythermal spangled perch Leiopotherapon unicolor , Gomez Isaza et al (2020b) investigated effects on U crit of combinations of sublethal nitrate and low pH (acidification). The U crit was impaired by both nitrate and acid pH, with most severe impairment being in fish at the highest nitrate concentration and low pH, although the combined effect was additive rather than synergistic.…”