Plants in aquatic ecosystems: current trends and future directions

Aquatic plants fulfil a wide range of ecological roles, and make a substantial contribution to the structure, function and service provision of aquatic ecosystems. Given their well-documented importance in aquatic ecosystems, research into aquatic plants continues to blossom. The 14th International Symposium on Aquatic Plants, held in Edinburgh in September 2015, brought together 120 delegates from 28 countries and six continents. This special issue of Hydrobiologia includes a select number of papers on aspects of aquatic plants, covering a wide range of species, systems and issues. In this paper we present an overview of current trends and future directions in aquatic plant research in the early 21st century. Our understanding of aquatic plant biology, the range of scientific issues being addressed and the range of techniques available to researchers have all arguably never been greater; however, substantial challenges exist to the conservation and management of both aquatic plants and the ecosystems in which they are found. The range of countries and continents represented by conference delegates and authors of papers in the special issue illustrate the global relevance of aquatic plant research in the early 21st century but also the many challenges that this burgeoning scientific discipline must address

Osmoregulatory Physiology of Pupfish (Cyprinodon spp.) in Freshwater

Active uptake of Na+ at the gill by fish is fundamental to their osmoregulation and thus survival in freshwater environments. Studies to date have demonstrated several different mechanisms by which fish can accomplish this important physiological function. This dissertation provides a comparative characterization of Na+ uptake in the coastal pupfish, Cyprinodon variegatus variegatus (Cvv), the closely related Lake Eustis pupfish, C. v. hubbsi (Cvh), and the endangered desert pupfish, C. macularius (Cm). When acclimated to slightly saline freshwater (7 mM Na+) all three fish use a low affinity Na+/H+ exchanger (NHE) for apical Na+ uptake. Cvv also uses a Na+:K+:2Cl- co-transporter under these conditions, the first time this has been functionally demonstrated in a freshwater fish. When acclimated to 2 mM Na+, all three fish strictly rely on a low affinity NHE for Na+ uptake. Only Cvh, which naturally occurs in eight dilute freshwater lakes in central Florida, was able to acclimate to 0.1 mM Na+ and under these conditions switches to a high affinity NHE for Na+ uptake. In other fish studied to date that use a high affinity NHE in dilute freshwater, the NHE operates in a metabolon with an Rh glycoprotein (ammonia transporter) that provides H+ for the operation of NHE in this thermodynamically constrained environment. This dissertation confirmed that Cvh does not use an NHE-Rh metabolon, but instead acquires H+ from carbonic anhydrase mediated hydration of CO2 to allow for NHE operation in dilute freshwater. This is the first time this has been demonstrated in a fish exposed to low Na+ water. Finally, because Cvh only occurs in eight lakes in central Florida, all of which are suffering from habitat loss and water quality degradation through urban development, this dissertation evaluated whether Cvh should be designated a separate species from Cvv, allowing for greater environmental protection. While there is no clear consensus on the definition of a species, within the regulatory framework of the Endangered Species Act, this dissertation demonstrated heritable physiological differences between Cvv and Cvh, partial pre- and post-zygotic isolation between the two subspecies, and likely monophyletic origin for Cvh, all of which support the designation of Cvh as an evolutionarily significant unit, and likely a separate species

Characterization of Na+ uptake in the endangered desert pupfish, Cyprinodon macularius (Baird and Girard)

This study investigated the mechanism of Na+ uptake in freshwater by the endangered pupfish, Cyprinodon macularius. Cyprinodon macularius exhibits a low-affinity uptake system and appears to be relatively inflexibile with respect to mechanisms of Na+ uptake compared with most freshwater species. This study provided an initial characterization of Na+ uptake in saline freshwater by the endangered pupfish, Cyprinodon macularius. This species occurs only in several saline water systems in the southwestern USA and northern Mexico, where salinity is largely controlled by water-management practices. Consequently, understanding the osmoregulatory capacity of this species is important for their conservation. The lower acclimation limit of C. macularius in freshwater was found to be 2 mM Na+. Fish acclimated to 2 or 7 mM Na+ displayed similar Na+ uptake kinetics, with K m values of 4321 and 3672 M and V max values of 4771 and 3602 nmol g−1 h−1, respectively. A series of experiments using pharmacological inhibitors indicated that Na+ uptake in C. macularius was not sensitive to bumetanide, metolazone, or phenamil. These results indicate the Na+-K+-2Cl− cotransporter, Na+-Cl− cotransporter, and the Na+ channel-H+-ATPase system are likely not to be involved in Na+ uptake at the apical membrane of fish gill ionocytes in fish acclimated to 2 or 7 mM Na+. However, Na+ uptake was sensitive to 1 × 10−3 M amiloride (not 1 × 10−4 or 1 × 10−5 M), 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), and ethoxzolamide. These data suggest that C. macularius relies on a low-affinity Na+-H+ exchanger for apical Na+ uptake and that H+ ions generated via carbonic anhydrase-mediated CO2 hydration are important for the function of this protein

High net calcium uptake explains the hypersensitivity of the freshwater pulmonate snail, Lymnaea stagnalis, to chronic lead exposure

Previous studies have shown that freshwater pulmonate snails of the genus Lymnaea are exceedingly sensitive to chronic Pb exposure. An EC20 of <4 μg l −1 Pb for juvenile snail growth has recently been determined for Lymnaea stagnalis, which is at or below the current USEPA water quality criterion for Pb. We characterized ionoregulation and acid–base balance in Pb-exposed L. stagnalis (young adults ∼1 g) to investigate the mechanisms underlying this hypersensitivity. After 21-day exposure to 18.9 μg l −1 Pb, Ca 2+ influx was significantly inhibited (39%) and corresponding net Ca 2+ flux was significantly reduced from 224 to −23 nmol g −1 h −1. An 85% increase in Cl − influx was also observed, while Na + ion transport appeared unaffected. Finally, a marked alkalosis of extracellular fluid was observed with pH increasing from 8.35 in the control to 8.65 in the 18.9 μg l −1 Pb-exposed group. Results based on direct measurement of Ca 2+ influx in 1 g snails gave an influx nearly an order of magnitude higher (750 nmol g −1 h −1) than in comparably sized fish in similar water chemistry. Under control conditions, specific growth rate in newly hatched snails was estimated at 16.7% per day over the first 38-day post-hatch and whole body Ca 2+ concentrations were relatively constant at ∼1100 nmol g −1 over this period. Based on these data, it is estimated that newly hatched snails have net Ca 2+ uptake rates on the order of 7600 nmol g −1 h −1. A model was developed integrating these data and measured inhibition of Ca 2+ influx rates of 13.4% and 38.7% in snails exposed to 2.7 and 18.9 μg l −1 Pb, respectively. The model estimates 45% and 83% reductions in newly hatched snail growth after 30-day exposure in these two Pb-exposed groups. These results compare well with previous direct measurements of 47% and 90% reductions in growth at similar Pb concentrations, indicating the high net Ca 2+ uptake is the controlling factor in observed Pb hypersensitivity

Comparative characterization of Na+ transport in Cyprinodon variegatus variegatus and Cyprinodon variegatus hubbsi: a model species complex for studying teleost invasion of freshwater

The euryhaline fish Cyprinodon variegatus variegatus is capable of tolerating ambient salinities ranging from 0.3 to 160 PSU, but is incapable of long-term survival in freshwater (<2 mmol l(-1) Na(+)). A population isolated in several freshwater (0.4-1 mmol l(-1) Na(+)) lakes in central Florida is now designated as a subspecies (Cyprinodon variegatus hubbsi). We conducted a comparative study of Na(+) transport kinetics in these two populations when acclimated to different ambient Na(+) concentrations. Results reveal that the two subspecies have qualitatively similar low affinity Na(+) uptake kinetics (K(m)=7000-38,000 μmol l(-1)) when acclimated to 2 or 7 mmol l(-1) Na(+), but C. v. hubbsi switches to a high affinity system (K(m)=100-140 μmol l(-1)) in low-Na(+) freshwater (≤1 mmol l(-1) Na(+)). Inhibitor experiments indicate that Na(+) uptake in both subspecies is EIPA-sensitive, but sensitivity decreases with increasing external Na(+). EIPA induced a 95% inhibition of Na(+) influx in C. v. hubbsi acclimated to 0.1 mmol l(-1) Na(+), suggesting that this subspecies is utilizing a Na(+)/H(+) exchanger to take up Na(+) in low-Na(+) environments despite theoretical thermodynamic constraints. Na(+) uptake in C. v. hubbsi acclimated to 0.1 mmol l(-1) Na(+) is phenamil-sensitive but not bafilomycin-sensitive, leading to uncertainty about whether this subspecies also utilizes Na(+) channels for Na(+) uptake. Experiments with both subspecies acclimated to 7 mmol l(-1) Na(+) also indicate that a Cl(-)-dependent Na(+) uptake pathway is present. This pathway is not metolazone-sensitive (NCC inhibitor) in either species but is bumetanide-sensitive in C. v. variegatus but not C. v. hubbsi. This suggests that an apical NKCC is increasingly involved with Na(+) uptake for this subspecies as external Na(+) increases. Finally, characterization of mitochondria-rich cell (MRC) size and density in fish acclimated to different ambient Na(+) concentrations revealed significant increases in the number and size of emergent MRCs with decreasing ambient Na(+). A linear relationship between the fractional area of emergent MRCs and Na(+) uptake rate was observed for both subspecies. However, C. v. variegatus have lower Na(+) uptake rates at a given MRC fractional area compared with C. v. hubbsi, indicating that the enhanced Na(+) uptake by C. v. hubbsi at low ambient Na(+) concentrations is not strictly a result of increased MRC fractional area, and other variables, such as differential expression of proteins involved in Na(+) uptake, must provide C. v. hubbsi with the ability to osmoregulate in dilute freshwater

Thiocyanate, calcium and sulfate as causes of toxicity to Ceriodaphnia dubia in a hard rock mining effluent

A series of Toxicity Identification Evaluations (TIEs) to identify the cause(s) of observed toxicity to Ceriodaphnia dubia have been conducted on a hard rock mining effluent. Characteristic of hard rock mining discharges, the effluent has elevated (∼3000 mg l −1) total dissolved solids (TDS) composed primarily of Ca 2+ and SO 4 2−. The effluent typically exhibits 6–12 toxic units (TUs) when tested with C. dubia. Phase I and II toxicity identification evaluations (TIEs) indicated Ca 2+ and SO 4 2− contributed only ∼4 TUs of toxicity, but this was likely an underestimate due to problems with simulating the supersaturated CaSO 4 concentrations in the effluent. Treatment of the effluent with BaCO 3 to precipitate Ca 2+ and SO 4 2− revealed that these ions contribute ∼6 TUs of the observed toxicity, but the remaining source(s) of toxicity (up to 6 TUs) remained unidentified. Subsequent investigations identified thiocyanate (SCN −) in the effluent at 100–150 μM. Toxicity tests reveal that C. dubia are sensitive to SCN − with an estimated IC25 of 8.3 μΜ for reproduction in moderately hard water suggesting between 12 and 18 TUs of toxicity in the effluent. Additional experiments demonstrated that SCN − toxicity is reduced in the high TDS matrix of the mining effluent. Testing of a mock effluent simulating the major ion and SCN − concentrations resulted in 10.4 TUs, suggesting that Ca 2+, SO 4 2− and SCN − are the three toxicants present in this effluent. This research suggests SCN − may be a more common cause of toxicity in mining effluents than is generally recognized

Effects of chronic waterborne nickel exposure on growth, ion homeostasis, acid-base balance, and nickel uptake in the freshwater pulmonate snail, Lymnaea stagnalis

•Lymnaea stagnalis is extremely sensitive to waterborne nickel exposure.•Growth in Lymnaea is the most sensitive endpoint during chronic nickel exposure.•Nickel disrupts ion homeostasis and acid-base balance in Lymnaea.•Lymnaea accumulates nickel in a dose-dependent manner during chronic exposure.•No direct interactions between nickel and calcium occur in Lymnaea. The freshwater pulmonate snail, Lymnaea stagnalis, is the most sensitive aquatic organism tested to date for Ni. We undertook a series of experiments to investigate the underlying mechanism(s) for this observed hypersensitivity. Consistent with previous experiments, juvenile snail growth in a 21-day exposure was reduced by 48% relative to the control when exposed to 1.3μgl−1 Ni (EC20 less than the lowest concentration tested). Ca2+ homeostasis was significantly disrupted by Ni exposure as demonstrated by reductions in net Ca2+ uptake, and reductions in Ca2+ concentrations in the hemolymph and soft tissues. We also observed reduced soft tissue [Mg2+]. Snails underwent a significant alkalosis with hemolymph pH increasing from 8.1 to 8.3 and hemolymph TCO2 increasing from 19 to 22mM in control versus Ni-exposed snails, respectively. Unlike in previous studies with Co and Pb, snail feeding rates were found to be unaffected by Ni at the end of the exposure. Snails accumulated Ni in the soft tissue in a concentration-dependent manner, and Ni uptake experiments with 63Ni revealed a biphasic uptake profile – a saturable high affinity component at low exposure concentrations (36–189nM) and a linear component at the high exposure concentrations (189–1897nM). The high affinity transport system had an apparent Km of 89nM Ni2+ and Vmax of 2.4nmolg−1h−1. This equates to a logK of 7.1, significantly higher than logK's (2.6–5.2) for any other aquatic organisms evaluated to date, which will have implications for Biotic Ligand Model development. Finally, pharmacological inhibitors that block Ca2+ uptake pathways in snails did not inhibit Ni uptake, suggesting that the uptake of Ni does not occur via Ca2+ uptake pathways. As with Cu and Pb, the exact mechanism for the significant disruption in Ca2+ homeostasis and reduction in juvenile snail growth remains unknown