Physiological protective action of dissolved organic carbon on ion regulation and nitrogenous waste excretion of zebrafish (Danio rerio) exposed to low pH in ion-poor water

Abstract

Dissolved organic carbon (DOC) represents a heterogeneous group of naturally-occurring molecules in aquatic environments, and recent studies have evidenced that optically dark DOCs can exert some positive effects on ionoregulatory homeostasis of aquatic organisms in acidic waters. We investigated the effects of Luther Marsh DOC, a dark allochthonous DOC, on ion regulation and N-waste excretion of zebrafish acutely exposed to either neutral or low pH in ion-poor water. In the first experiment, simultaneous exposure to pH 4.0 and DOC greatly attenuated the stimulation of Na diffusive losses (J ), and prevented the blockade of Na uptake (J ) seen in zebrafish exposed to pH 4.0 alone, resulting in much smaller disturbances in Na net losses (J ). DOC also attenuated the stimulation of net Cl losses (J ) and ammonia excretion (J ) during acidic challenge. In the second experiment, zebrafish acclimated to DOC displayed similar regulation of J and J , and, therefore, reduced J at pH 4.0, effects which persisted even when DOC was no longer present. Protective effects of prior acclimation to DOC on J and J at pH 4.0 also occurred, but were less marked than those on Na balance. Urea fluxes were unaffected by the experimental treatments. Overall, these effects were clearly beneficial to the ionoregulatory homeostasis of zebrafish at low pH, and were quite similar to those seen in a recent parallel study using darker DOC from the upper Rio Negro. This suggests that dark allochthonous DOCs share some chemical properties that render fish tolerant to ionoregulatory disturbances during acidic challenge.

Does dissolved organic carbon from Amazon black water (Brazil) help a native species, the tambaqui Colossoma macropomum to maintain ionic homeostasis in acidic water?

“…Interestingly, in that study, increases in ammonia net fluxes were facilitated in reference water by the addition of Ca 2+ , demonstrating that native DOC molecules may interact with gill membranes in the same manner as Ca 2+ ions with respect to ammonia excretion mechanisms (Wood et al ., ). In D. rerio , a stimulation in J Amm.net was seen in fish exposed to pH 4.0 in IPW in the presence of two different sources of DOC, which was thought to be essential for animals to keep Na + uptake coupled to ammonia excretion under acidic conditions (Duarte et al ., , ). This coupling was thought to be associated with an upregulation of the Rh‐protein–Na + –NH 4 + exchange metabolon in the gills (Weihrauch et al ., ; Wright & Wood, ).…”

“…ions with respect to ammonia excretion mechanisms (Wood et al, 2003). In D. rerio, a stimulation in J Amm.net was seen in fish exposed to pH 4.0 in IPW in the presence of two different sources of DOC, which was thought to be essential for animals to keep Na + uptake coupled to ammonia excretion under acidic conditions (Duarte et al, 2016(Duarte et al, , 2018. This coupling was thought to be associated with an upregulation of the Rh-protein-Na + -NH 4 + exchange metabolon in the gills (Weihrauch et al, 2009;Wright & Wood, 2009).…”

The physiology of fish in acidic waters rich in dissolved organic carbon, with specific reference to the Amazon basin: Ionoregulation, acid–base regulation, ammonia excretion, and metal toxicity

“…With respect to paracellular junctions, the presence of DOC (6–15.5 mg/L) has been shown to reduce the efflux rates of Na + and Cl − across the gills in a variety of species, both at circumneutral and acidic pHs, but often with more pronounced effects in the latter (Al‐Reasi et al, 2016; Duarte et al, 2016, 2018; Gonzalez et al, 2002, 2006; Wood et al, 2002, 2003). In freshwater stingrays, the protective effects of the natural DOC of Rio Negro blackwater at pH 4.0 were mimicked by a 10‐fold increase in environmental Ca 2+ concentration (in the absence of DOC), suggesting that DOC had the same “tightening” actions on the paracellular junctions as those of Ca 2+ discussed earlier (Wood et al, 2003).…”

“…The dissociation of protons from allochthonous DOC is the major cause of acidity in peat bogs and many other natural blackwaters. However, there is now extensive evidence in freshwater fish that natural DOCs may actually mitigate the ionoregulatory disturbances of the ambient low pH which they help to create (Duarte et al, 2016, 2018; Gonzalez et al, 2002; Wood et al, 2003, 1998). It has even been suggested that DOCs may affect ventilation (Holland et al, 2014; Perry et al, 2021) and respiratory gas exchange (O 2 , CO 2 , and ammonia) at the gills (Matsuo et al, 2004), and at least for ammonia, there are data that support this (Duarte et al, 2016, 2018; Wood et al, 2003).…”

“…However, there is now extensive evidence in freshwater fish that natural DOCs may actually mitigate the ionoregulatory disturbances of the ambient low pH which they help to create (Duarte et al, 2016, 2018; Gonzalez et al, 2002; Wood et al, 2003, 1998). It has even been suggested that DOCs may affect ventilation (Holland et al, 2014; Perry et al, 2021) and respiratory gas exchange (O 2 , CO 2 , and ammonia) at the gills (Matsuo et al, 2004), and at least for ammonia, there are data that support this (Duarte et al, 2016, 2018; Wood et al, 2003). Here, our major focus is on ionoregulatory physiology and associated functions (acid–base balance, ammonia excretion), where there remain significant gaps in our understanding of the effect(s) of DOC on ion transport in freshwater fish.…”