Chapter 19 - Transport

Objectives

• Be able to explain the possible interactions of a bacterium with soil pores

• Be able to explain the relationship between bacterial size and soil pore size and effective transport

• Be able to list and understand the four factors that affect microbial transport

• For a given a set of conditions (bacterial shape and size, ionic strength, soil texture) be able to provide an educated prediction of whether microbial transport will occur

Transport of microorganisms in soil

Distribution of microorganisms in nature

• preference is shown for attachment mountain streams sediments

subsurface environments

• microorganisms tend to be found in “patches” or small colonies rather than evenly distributed on soil surfaces

• soil often “filters out” microorganisms as they move with water flow

Importance of understanding transport of microorganisms

• To determine the fate of added micoorganisms (either selected or GEM)

life vs. deathproliferation vs. maintenanceadhesion vs. transport

• To determine the facilitated transport of pollutants

Pore spaces in microaggregates with neck diameters less than 6 um have more activity than pore spaces with larger diameters. Bacteria within the former are protected from protozoal predation.

200 um

microaggregate root

hypha

aggregrates or soil particles

2000 um

macroaggregate solidpore

Assume that 50% of the aggregate is pore space and the pores are 15 um in diameter, there will be 1,000,000 pores

Assume the pores are 30um in diameter, there will be 150,000 pores

Pore Radius

Sand

%

Hayhook

%

Vinton Mixture

%

< 1 um

1 – 10 um

10 - 60 um

> 60 um

0.025

0.35

17.13

82.5

6.51

16.05

30.93

46.51

11.70

17.68

49.9

20.83

Hayhook: 10% clay, 5% silt, 85% sand

Vinton Mix: 5% clay, 10% silt, 85% sand

Pore size distribution for three porous media

Bacteria can be no more than 5% of the average pore diameter to get effective transport.

2 um 40 um

0.05 – 0.5 um

0.5 – 3 um

Convective flux velocity

Q = K DH A t where: zQ = volume of water moving through the column (l3)K = hydraulic conductivity (l/t)DH = hydraulic head difference between inlet and outlet (l)A = cross sectional area of column (l2)t = time (t)z = length of column (l)

103

101

10 -1

10 -3

10 -5

10 -7

Matric potential head (cm)

Hyd

raul

ic c

ondu

ctiv

ity (

cm/h

r)

-10 5-103-101

Sandy soil

Clay soil

large flow

limited flow

Factors affecting microbial transport in soil

Advection - movement with bulk fluid

Dispersion

• mechanical mixing – path tortuosity creates velocity differences depending on pore sizes

• molecular diffusion – random movement of very small particles in a fluid generally due to a concentration gradient. Usually not important for bacteria but might affect virus transport

Faster

Poresize

Longer path

Shorter path

Pathlength

Slower

Slower

Slower

SlowerFaster

Faster

Frictionin pore

A

B

C

Factors that cause mechanical mixing

Adsorption

loss of cells from the solution phase due to interaction with surfaces (ranges from reversible to irreversible)

There are several ways a cell can approach a surface.

• Active movement (chemotaxis) is in response to a chemical gradient

• Diffusion – brownian motion allows random interactions with a surface

• Convective transport - due to water movement, usually several orders of magnitude > than diffusion

Diffusion layer

Active movement

Convection

Diffusion

Surface

Once at the surface several different forces govern the interaction

Electrostatic interactions – repulsive forces

Hydrophobic interactions – attractive forces

Van der Waals forces – attractive forces

Electrostatic interactions

Coulomb’s Law: F = k q1 . q2 where: e . r2

F = force between the particlesq1, q2 = charged particlesk = constante = dielectric constant (depends on ionic strength and type)

Is F expected to be positive or negative between a bacterial cell and the soil?

?

Electrostatic forces are repulsive

What is the effect of increasing the ionic strength of the medium?

Electrostatic repulsion is reduced.

Pore volume

0 1 2 3 4 5 6 7

C/C

o

0.0

0.1

0.2

0.3

0.4

C0 = 5 x 107 cells/ml

CEC = 0.03%

Transport of Pseudomonas aeruginosa 9027 through sand

deionized water

2 mM NaCl

Bai et al., 1997. Appl. Environ. Microbiol. 63:1266-1273

Model prediction

Experimental data

Modeling was performed using a one-dimensional advection-dispersion model that includes combined instantaneous and rate-limited sorption and two first-order irreversible retention terms.

where: C = bacterial concentration (M V-1)S = sorbed phase bacterial concentration (M V-1)R = retardation factorT = time= fraction of instantaneous retardationP = Peclet number, = dimensionless first order cell sticking rate constants

****1*

)1(*

2

2

SCX

C

XP

C

T

SR

T

CR

retardation first-orderretention terms

advectiondispersion

Hydrophobic interactions

Nonpolar molecules attract each other

Electrostatic repulsion is reduced and hydrophobic interactions can increase

What is the effect of increasing ionic strength?

van der Waals Forces

Occurs between neutral molecules. Electron motion is such as to produce net electrostatic attraction at every instant.

van der Waals forces are attractive

Polystyrene

Glass

70

70

60

60

50

50

40

40

30

30

20

2020

0

0

-1.0

-1.0

-2.0

-2 .0

-3.0

-3.0

100

100

75

75

50

50

25

25

0

0

Contact angle ( )o

Contact angle ( )o

Electro

phoretic

mobilit

y

(10

mete

r / V

- se

c )

-8

Electrophore

tic

mobilit

y

(10

mete

r / V

- se

c )

-8

Ad

hesi

on(%

cov

era

ge)

Adh

esio

n(%

co

vera

ge)

A.

B .

Reversible“secondary m inimum”

Irreversible“primary m inimum”

G E

G tot

G A

H

P r i m a r y m i n i m u m

S e c o n d a r ym i n i m u m

= electrostatic interaction

= separation distance

= van der Walls interaction

When a cell is right next to the surface the attraction is very strong due to attractive forces creating a primary minimum (H-bonding and dipole interactions).

As the two surfaces separate slightly (several nm) repulsive forces grow quickly.

At slightly longer distances another, smaller minimum exists. At the secondary minimum the cell is not in actual contact with the surface and so the cells can be removed by increasing water velocity or by changing the chemistry of the system.

DLVO theory – Gibbs free energy between a sphere and a flat surface

FibrilsPolymers

Reversible vs. irreversible attachment

Factors affecting microbial transport in soil

Advection - movement with bulk fluid

Dispersion• mechanical mixing – path tortuosity creates velocity differences

depending on pore sizes• molecular diffusion – random movement of very small particles in a fluid

generally due to a concentration gradient. Usually not important for bacteria but might affect virus transport

Adsorption – loss of cells from the solution phase due to interaction with surfaces (ranges from reversible to irreversible)

Decay – loss of cells from the solution phase due to death (irreversible)

1.0

0.5

0.0

Distance (m)

Con

cent

ratio

n (C

/C) o

Advection only

Advection, dispersion

Advection, dispersion, adsorption

Advection, dispersion, adsorption, decay

A short pulse of cells have been added to a column and this is a snapshot of the distribution of cells along the length of the column at some time later.

Organism Ionic strength Mineral grain size % Recovery

W6

W6

W6

W6

W8

W8

W8W8

low

low

high

high

low

low

high

high

fine

coarse

fine

coarse

fine

coarse

fine

coarse

W6 a coccus with radius 0.75 um

W8 a bacillus with dimensions 0.75 x 1.8 um

Summary and Homework (predict relative recoveries)

14.5%

80.4%

2.8%

49.3%

3.9%

43.6

0.3%

4.3%

100

60

40

20

0

Pore volumes

% In

itia

l per

me

abili

ty

Cells starved for 4 weeks

Cells starved for 2 weeks

Vegetative cells

A series of experiments were performed in glass bead columns to determine the impact of injected cells on the permeability of the column.

500 PV of Klebsiella pneumoniae (108 CFU/mL) were injected. The cells were either vegetative, starved for 2 weeks, or starved for 4 weeks.

MacLeod et al., 1988. Appl. Environ. Microbiol. 54:1365-1372.

1000

100

10

Core depth (cm)

10 C

ells

/gra

m

Cells starved for 2 weeks

Vegetative cells

8

Differences in the DNA-derived cell distribution in glass bead cores injected with vegetative or starved cells.

MacLeod et al., 1988. Appl. Environ. Microbiol. 54:1365-1372.