Sodium sulphate

Toxicity to aquatic plants other than algae

Administrative data

Endpoint:

toxicity to aquatic plants other than algae

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Literature data

Data source

Reference

Reference Type:

publication

Title:

Sulphate toxicity to the aquatic moss, Fontinalis antipyretica

Author:

Davies Trevor D.

Year:

2007

Bibliographic source:

Chemosphere 66 (2007) 444–451

Materials and methods

Principles of method if other than guideline:

The aquatic moss, Fontinalis antipyretica was exposed to elevated sulphate concentrations for 21-days. Gametophores were sectioned to 2 cm lengths and exposed to sulphate concentrations up to 1500 mg/L, in waters of different water hardness.

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

Sodium sulphate

EC Number:

231-820-9

EC Name:

Sodium sulphate

Cas Number:

7757-82-6

Molecular formula:

H2O4S.2Na

IUPAC Name:

disodium sulfate

Details on test material:

- Name of test material: anhydrous sodium sulphate

Test solutions

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION
- Stock solution: a concentrated sulphate stock solution was made using Na2SO4 added to test waters in various amounts to achieve the desired sulphate concentrations.
- Confirm of concentration: concentrations were confirmed through ion chromatography for anions (ICA) by the Pacific Environmental Sciences Centre located in North Vancouver, BC.
- Storage: at 8°C in opaque 1L plastic bottles

Test organisms

Test organisms (species):

other: Fontinalis antipyretica

Details on test organisms:

- Class: moss
- Reproductive structure: sporophyte, lacks a distinct stalk and emerges among its leaves.
- Source: moss was collected near the University of British Columbia (UBC) campus at Vancouver, BC. Identification was confirmed by Wilf Schofield, Professor Emeritus of the Botany department at UBC.
- Natural environment: the moss was located in a shallow, slow-moving stream approximately 20–30 cm deep. The water exhibited a brownish colour and substantial suspended sediment, and a hardness of 19 mg/L as CaCO3.
- Collection: moss was collected from the field site in 20 L plastic buckets and kept submerged in ambient stream water during transport from the field site to the test lab. There was substantial overhanging canopy with little or no direct sunlight.

Study design

Test type:

semi-static

Water media type:

freshwater

Total exposure duration:

21 d

Test conditions

Hardness:

Test on sulphate toxicity: hardness 19 mg/L as CaCO3

Test temperature:

Controlled temperature of 15 ± 1°C

Nominal and measured concentrations:

- Sulphate concentrtions: 200, 400, 600, 800, 1000 and 1500 mg/L
- Exposure concentrations: were determined to be within 10% of nominal concentrations; therefore, nominal concentrations were used for statistical analysis. All exposure concentrations were formulated at the beginning of each test

Details on test conditions:

Two separate tests were conducted: the first to assess toxicity of sulphate (as Na2SO4); and the second to assess the effects of water hardness on sulphate toxicity.

1 - SULPHATE TOXICITY
TEST SYSTEM
- Plants: moss was collected the day prior to the start of the first test and stored overnight at 15 C in collection buckets.
- Test vessel: 250 ml Erlenmeyer flasks filled to 125 ml.
- Type: open; left uncovered for the duration of the test to ensure sufficient gas exchange.
- Samples: individual gametophores (referred to here as shoots) were taken from the moss clumps, and 2 cm apical segments were excised and temporarily stored in control water. After enough shoots were collected, they were randomly assigned to test concentrations.
- No. of replicates: four replicates of approximately 10 shoots were placed in each a flask at each test concentration and allowed to float freely.

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: stream water obtained from the F. antipyretica collection site as the test medium. Stream water was passed through a glass–fiber filter to remove organisms and suspended solids that could potentially affect moss growth and survival.
- Water change: water changes occurring every five days.

OTHER TEST CONDITIONS
- Humidity: humidity in the room was 100% and evaporation was not significant during the course of the experiment.
- Light source and intensity: light was provided by four full-spectrum halogen light bulbs supplying 1500–2000 Lux measured at the level of the exposure flasks.
- Photoperiod: lights were left on continuously.
- Other: flasks were gently gyrated on a shaker table to ensure sufficient nutrient and gas exchange for the duration of the experiments.

2 - EFFECTS OD WATER HARDNESS
TEST SYSTEM
- Plants: fresh moss samples could not be collected for this experiment due to extremely dry conditions at the moss collection site. Therefore, moss collected for the first test (sulphate toxicity) also was used for the second experiment.
- Test vessel: 250 ml Erlenmeyer flasks filled to 125 ml.
- Type: open; left uncovered for the duration of the test to ensure sufficient gas exchange.
- Samples: individual gametophores (referred to here as shoots) were taken from the moss clumps, and 2 cm apical segments were excised and temporarily stored in control water. After enough shoots were collected, they were randomly assigned to test concentrations.
- No. of replicates: four replicates of approximately 10 shoots were placed in each a flask at each test concentration and allowed to float freely.

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: undiluted well-water and well-water diluted 3:1 with de-ionized water (105 mg/L and 26 mg/L hardness) to simulate medium and soft-water hardness respectively. Well water was not filtered and no additives such as plant nutrients were supplemented.
- Water change: water changes occurring every five days.

OTHER TEST CONDITIONS
- Humidity: humidity in the room was 100% and evaporation was not significant during the course of the experiment.
- Light source and intensity: light was provided by four full-spectrum halogen light bulbs supplying 1500–2000 Lux measured at the level of the exposure flasks.
- Photoperiod: lights were left on continuously.
- Other: flasks were gently gyrated on a shaker table to ensure sufficient nutrient and gas exchange for the duration of the experiments.

Results and discussion

Effect concentrations

Remarks on result:

not measured/tested

Details on results:

SHOOT LENGTH
All moss shoots displayed a reduction in final shoot length with increasing sulphate concentration.
Shoots in ambient stream control water had the greatest increase in length during the exposure period and showed a relatively linear reduction in mean final length with increasing sulphate concentration. Shoots in soft and medium-hardness waters grew less than those in ambient stream water up to 600 mg/L sulphate. Increasing sulphate concentrations had the greatest impact on shoot length in ambient stream water; soft-water treatments showed a lesser response; and the medium-hardness treatment did not show a significant reduction in shoot length until the highest sulphate exposure of 1500 mg/L sulphate.
 
DRY WEIGHT
Moss shoots in the ambient stream treatment displayed a relatively linear reduction in final dry weight in response to increasing sulphate concentrations, with the first significant difference detected at 600 mg/L sulphate. The soft-water treatment did not show a significant reduction in dry weight relative to the control until 1500 mg/L sulphate. Shoots in the medium-hardness treatment had the highest dry weight of all groups and showed a significant reduction in growth relative to the control group at 400 mg/L sulphate. However, dry weights did not decline significantly up to 1500 mg/L sulphate relative to the 600 mg/L sulphate (p < 0.05) exposure group. Mean dry weight was highest in the medium-hardness treatment, with a final mean weight of 2.1 mg, relative to the ambient-water and soft-water treatments of 1.3 and 1.6 mg respectively.
 
CHLOROPHYLL LEVELS
Shoots in all hardness treatments showed a reduction in chlorophyll a and b content (on a dry weight basis) with increasing sulphate exposure. Shoots grown in stream water had significantly higher chlorophyll a and b concentrations per gram dry weight (chl a and b/g DW) in the control groups in comparison to both well-water control groups. Shoots in the stream-water control had a mean chlorophyll content of 6.9 mg chl a and b/g DW and showed a continual decline with increasing sulphate exposure; a significant decline was first detected at 400 mg/L sulphate. Above 800 mg/L sulphate, chlorophyll levels were below 3 mg chl a and b/g DW and shoots began to show obvious signs of stress by beginning to turn brown.
Both soft- and medium-water treatments showed a comparable, yet limited, response to increasing sulphate exposure, up to the highest exposure sulphate treatment of 1500 mg/L. Although a statistically significant reduction in chlorophyll levels was observed at 600 mg/L sulphate relative to the control treatment in the soft well-water treatments (p < 0.05), no significant difference was observed between the 200 mg/L sulphate treatment and all other treatments until 1500 mg/L sulphate (p < 0.05), in which shoots appeared dead and brown. In the medium-hardness group, a significant reduction in chlorophyll levels was observed at 1000 mg/L sulphate, but not at 1500 mg/L sulphate.

Any other information on results incl. tables

DISCUSSION

Reduction in shoot length and chlorophyll content provides evidence that there is an inverse relationship between sensitivity of F. antipyretica to elevated sulphate concentrations and the hardness of the exposure water. The effect of increasing sulphate exposure on dry weight is difficult tointerpret because a consistent trend is lacking. Dry weight measurements from the medium-hardness water treatment suggest that these shoots exhibited the most growth; however, this differs from the total length results where the medium-hardness treatment exhibited the least overall growth up to 600 mg/L sulphate exposure. This may possibly be explained due to the higher ion content of the medium-hardness water, which had a hardness of 105 mg/L as CaCO3 in comparison to the soft water of 26 mg/L as CaCO3.

Differences in weights in controls were relatively large (1.83 versus 2.59 mg in soft and medium-hardness water respectively), the difference (<0.8 mg) may be a result of greater ion uptake by the moss in the more ion rich harder water in order to maintain osmotic balance. This is speculative as no analysis of the plant tissues was done.

 

Chlorophyll analysis provided evidence that sulphate is less toxic in water of increasing water hardness. Reduction of chlorophyll content in F. antipyretica suggests that chlorophyll may be a better metric of plant health in response to elevated sulphate exposure than either shoot length or dry weight. The lower total length and chlorophyll levels in the control groups of the well water relative to stream water suggest that water characteristics and/or plant health may have influenced results.

Regardless, the general trend in chlorophyll concentrations per mg DW suggests that F. antipyretica is less sensitive to sulphate in water of increased hardness. However, differences between the stream and soft-water groups may be exaggerated due to the much higher calcium content of the soft well water.

 Another potential explanation for reduced sulphate toxicity with increasing water hardness is the formation of ion pairs between calcium and sulphate. However, calcium concentrations were 0.9 mm/L and sulphate levels were 15.6 mm/L in the medium water at 1500 mg/L sulphate exposure; therefore, a significant reduction in toxicity would be unlikely even if all the calcium paired to sulphate ions.

 

The assessment of sulphate toxicity to F. antipyretica is confounded by the presence of sodium ions even though sodium is considered one of the least toxic cations. The mode of toxicity from sulphate relates to the creation of an unsustainable osmotic imbalance between the plant and its surrounding environment. The presence of confounding ions influence that balance and interact with each other by creating or nullifying osmotic gradients. Therefore, attribution of sodium sulphate toxicity solely to the sulphate ion may give a simplistic assessment of sulphate ion toxicity to F. antipyretica and other plant species.

Applicant's summary and conclusion

Conclusions:

Significant decline in chlorophyll concentration was first detected at 400 mg/L, the first significant difference in dry weight was detected at 600 mg/L and shoots in soft and medium-hardness waters grew less than those in ambient stream water up to 600 mg/L sulphate. The substantial reduction of sulphate toxicity in waters of increasing hardness, which suggests that water chemistry plays a significant role in affecting sulphate toxicity.

Executive summary:

The aquatic moss, Fontinalis antipyretica was exposed to elevated sulphate concentrations for 21-days. Gametophores were sectioned

to 2 cm lengths and exposed to sulphate concentrations up to 1500 mg/L, in waters of different water hardness.

Conclusion

Significant decline in chlorophyll concentration was first detected at 400mg/L, the first significant difference in dry weight was detected at 600 mg/L and shoots in soft and medium-hardness waters grew less than those in ambient stream water up to 600 mg/L sulphate. The substantial reduction of sulphate toxicity in waters of increasing hardness, which suggests that water chemistry plays a significant role in affecting sulphate toxicity.