The role of competing anions on the uptake and ...

The role of competing anions on the uptake and biotransformation of two contaminants, As(V) and Se(VI) Joshua Baker Nautilus Environmental Burnaby, BC

Contaminants of Concern - Arsenic  Speciation  Inorganic forms: Arsenate and arsenite  Organic Forms: Methylarsenicals  Seasonal/Algal Blooms

 Toxicity  Inorganic forms more toxic than

organic counterparts.

Algae and Arsenic 1. 2.

Uptake -How much gets in to the algae? Biotransformation - How is it converted?  

Reduction

As(V)  As(III)

Methylation As(III)  MMAs(V)  DMAs(V)

3.

Excretion - What comes out of the algae?

As(V) As(V) As(III)

DMAs(V)

MMAs(V)

Algal Arsenic Uptake 

Phosphorus uptake pathway ○ Nutrient Mimicry ○ Competitive interaction



Arsenate As(V)

Research question ○ Do the phosphorus dynamics control the uptake of

arsenic by a freshwater algae and consequently the biotransformation (speciation produced) of arsenic

Phosphate PO4

Laboratory Setup 

Freshwater alga Chlorella vulgaris grown for 7 days with 10 µg/L As(V) ○ Exponential phase, 16:8 light schedule, 110 rpm and 25°C ○ Multiple phosphorus concentrations, maintained daily



Anion exchange chromatography with detection by ICP-MS used for As speciation determination

High Phosphorus Concentrations 

Arsenate still predominant species in media after 7 days of 400 µg P/L daily  Competitive inhibition for As-uptake from phosphorus



Low levels of DMAs(V) found

As(V) As(III)

MMAs(V)

 Only accounted for 1% of original arsenic  Final metabolite of cellular arsenic transformation pathway.

Further methylation in marine and larger freshwater/terrestrial organisms  DMAs(V) not only produced at low phosphorus concentrations

DMAs(V)

Middle Phosphorus Conditions 

Biotransformation increased ○ Presence of As(III), MMAs(V), DMAs(V) and Other Species in culture

media ○ DMAs(V) still predominant excreted species. ○ As phosphorus concentration lowered, all metabolites increased in similar manner

As(V) As(III)

DMAs(V)

MMAs(V)

Arsenic speciation in Mid-P cultures. Average and σ from triplicate cultures

Daily P µg/L

As(V) µg/L

As(III) µg/L

MMAs µg/L

DMAs µg/L

OS

110a

9.11 ± 0.79

0.09 ± 0.04

0.04 ± 0.02

0.25 ± 0.07

0.13 ± 0.04

a- One daily addition at 110 µg/L

µg/L

Low Phosphorus Conditions 

As(V)

High As-uptake

As(III)

○ Low competition from phosphorus

MMAs(V)

○ Concentrations of metabolites increased



DMAs(V)

New predominant mechanism of transformation ○ Uptake, reduction and excretion

Arsenic speciation in Low-P cultures. Average and σ from triplicate cultures

Daily P µg/L

As(V) µg/L

As(III) µg/L

MMAs µg/L

DMAs µg/L

106a

4.58 ± 0.73

3.54 ± 0.70

0.06 ± 0.01

0.29 ± 0.01

0.13 ± 0.01

53 40

1.95 ± 1.13

7.72 ± 3.12

0.06 ± 0.01

0.34 ±0.04

0.18 ± 0.04

0.90 ± 0.89

7.29 ± 1.54

0.08 ± 0.01

0.31 ± 0.02

0.18 ± 0.02

b- Two daily additions of 53 µg/L

OS

µg/L

As uptake (µmol g-1d.w.)

Rates- Uptake, Methylation and Reduction 4 3.5

y = -0.0098x + 4.6 R² = 0.9603

3 2.5 2 1.5 1 0.5 0 0

100

200

300

400

P uptake (µmol g-1 d.w.)

500

600

Rates- Uptake, Methylation and Reduction 0.7

DMA

?

OS As(III)

MMA

As (µg/L)

0.6 0.5 0.4 0.3

As(III) media (µg/L)

10.00 y = 0.9269x - 1.011 R² = 0.9618

8.00 6.00 4.00 2.00 0.00

0.2

0.00

2.00

4.00

6.00

8.00

10.00

0.1

Loss of arsenate from media (µg/L)

0 0

200

400

P/As molar ratio

600

Algae and Arsenic - Conclusions 

Phosphorus ○ Controls arsenic speciation in biologically productive systems



Low Arsenic uptake - Methylation ○ Saturation of algal methylation (1 ag/cell of As)



High Arsenic uptake - Reduction ○ Strong relation at realistic phosphorus concentrations



Freshwater observations ○ Spring (Log) and Summer (Stat) related to our two scenarios of

biotransformation

Contaminants of Concern - Selenium  Speciation  Inorganic forms: Selenate and selenite  Organic Forms: SeMet, SeCys

 Toxicity  Selenomethionine  Food chain bioaccumulation  Fish-based guideline

Biotransformation and Risk Assessment 1.

Uptake

2.

Accumulation, biotransformation and transport

3.

Threshold

Se

7.91 µg/g Se ?

Algae and Selenium 

Sulfate

○ Nutrient Mimicry ○ Competitive interaction



Selenate Se(VI)

Research question

○ Do the sulfate dynamics control the uptake of selenium by a

freshwater algae and consequently the amount of selenium bioaccumulation



Laboratory Setup

○ Freshwater alga Pseudokirchneriella subcapitata grown for 7

days with 10 to 100 µg/L Se(VI) ○ Exponential phase, 16:8 light schedule, 25°C and aerated ○ Multiple initial S:Se ratios and algal Se concentration determined at termination

Sulfate SO4

Total Algal Selenium (mg/kg wet weight)

Selenium Uptake – Background Sulfate 0.900 0.800 0.700 0.600 0.500 0.400 0.300 0.200 0.100 0.000

• 5 mg/L SO4 • 0.1 to 0.7 mg/kg (wet weight)

0

20

40

60

80

Selenium concentration ( µg/L)

100

• Increase in exposure Se concentration leads to increase in algal Se concentration

Selenium Uptake – Single Sulfate Tests 0.080

Total Algal Selenium (mg/kg wet weight)

Total Algal Selenium (mg/kg wet weight)

560 mg/L Sulfate 0.070 0.060 0.050 0.040 0.030 0.020 0.010 0.000 0

50

100

Selenium Concentration

0.01 to 0.07 mg/kg (wet weight)

180 mg/L Sulfate

0.200 0.180 0.160 0.140 0.120 0.100 0.080 0.060 0.040 0.020 0.000 0

50

100

Selenium Concentration

0.02 to 0.15 mg/kg (wet weight)

Increase in exposure Se concentration leads to increase in algal Se concentration

Total Algal Selenium (mg/kg wet weight)

Selenium Uptake – Constant Ratio 0.080 0.070 0.060 0.050 0.040 0.030 0.020 0.010 0.000

Increase in exposure Se concentration did not lead to increase in algal Se concentration if sulfate:selenate ratio was maintained

100-10

180-18

320-32

560-56

S04 (mg⁄L) -Se (µg⁄L)

1000-100

Algae and Selenium – Conclusions The concentrations of selenium in algae were an order of magnitude higher in background sulfate concentrations (5 mg/L) in comparison to higher sulfate concentrations (>100 mg/L)



The concentrations of selenium in algae had insignificant differences when the ratio of sulfate:selenate was maintained, regardless of increasing selenate concentrations 1.000 Combined Data





The concentrations of selenium in algae increased with increasing ambient selenate concentrations, when the sulfate concentration was unchanged Future work

Total Algal Selenium (mg/kg wet weight)



0.800 0.600 0.400 0.200 0.000 0.01

0.1

1

10

Log (Sulfate:Selenate Ratio x103)

100

Thank you...

Dr. Dirk Wallschläger

Trent University and EnLS/WEGP

Ministry of the Environment

Nautilus Team