Fig. 3. Turbidity over time across three CO2 concentrations and temperatures (mean ±SEM). Plotted values represent non-transformed turbidity measurements in NTU (Nephelometric Turbidity Units). Slope estimates were calculated from non-transformed data, while statistical comparisons were performed on square root–transformed values using Tukey-adjusted post hoc tests. Letters that differ indicate statistical differences (p <0.05) across CO2 levels within each temperature and across temperatures within each CO2 level.
Abstract
Nephrocalcinosis (“kidney stones”) is a persistent health issue in intensive aquaculture systems, affecting fish welfare and productivity. However, its causes and risk factors remain poorly understood. Dissolved CO2 concentrations are typically elevated (hypercapnia) in intensive aquaculture systems, causing elevated blood PCO2 levels. Fish compensate for this hypercapnia-induced “respiratory acidosis” by elevating plasma bicarbonate (HCO3−) to restore pH. However, if returned to normocapnia (normal CO2), fish experience an alkalosis which we propose may induce nephrocalcinosis. We used an in vitro model to test whether a rapid rise in pH in the pre-urine, caused by an abrupt PCO2 drop when fish are transferred from hypercapnic to normocapnic levels, could be the trigger for kidney stone formation. Synthetic fish pre-urine was sequentially subjected to: (1) elevated PCO2 (1–3 %) to mimic hypercapnia-induced respiratory acidosis; (2) HCO3− addition to simulate metabolic compensation; and (3) a rapid reduction in PCO2 mimicking events that occur routinely during fish culture and handling (e.g., transfer, sorting/grading, fasting, vaccination). These exposures were conducted at 6, 12, and 18 °C. Precipitation in the pre-urine, measured by turbidity, was rapid and sustained following the drop in PCO2 (step 3 above). The magnitude of precipitation was strongly associated with pH rise (ΔpH), particularly at higher temperatures. These findings demonstrate that abrupt decreases in PCO2 can promote mineral precipitation under physiologically relevant ionic conditions, and that elevated temperature exacerbates this effect. The results support the hypothesis that nephrocalcinosis may result, in part, from acid-base disturbances commonly encountered under intensive aquaculture conditions.