environmental toxicology: chemical aspects …...environmental toxicology, master sc. in industrial...

Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross

Environmental toxicology: chemical aspects

Aquatic chemistry (6)

Aquatic microbial biochemistry




Microorganisms (bacteria, fungi, protozoa, and algae) are living catalysts* that enable a vast

number of chemical processes to occur in water and soil.

- Degradation and formation of biomass

- Formation of sediments and deposit

- Secondary waste treatment (see dedicated slide hereafter)

- Redox reaction

- Enzymatic reactions

Pathogenic microorganism should be eliminated (typhoid, cholera, water-borne disease)

Virus (0.025-0.1 micron) survive chlorination and water treatment

Microorganism in water

*Catalyst: a substance that increases the rate of a reaction without modifying the overall standard Gibbs energy change in the

reaction; the process is called catalysis. The catalyst is both a reactant and product of the reaction.

https://goldbook.iupac.org/html/C/C00876.html




Primary and secondary wastewater treatmentPrimary treatment

The objective of primary treatment is the removal of settleable organic and inorganic solids by

sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25 to 50% of the

incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil

and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy

metals associated with solids are also removed during primary sedimentation but colloidal and dissolved

constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent.

Secondary treatment

The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove

the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment

and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic

biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by

aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater,

thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O).

Several aerobic biological processes are used for secondary treatment differing primarily in the manner in

which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic

matter.




Primary and secondary wastewater treatment

Source of the picture: britannica.com




Prokaryotes: lack of nuclear membrane, the nuclear genetic material is diffused in the cell.

Eukaryotes: well-defined cell nucleus enclosed by a nuclear membrane.

Differences include: location of cell respiration, means of photosynthesis, means of motility, and

reproductive processes.

All classes of microorganisms produce spores, metabolically inactive bodies that form and survive

under adverse conditions in a “resting” state until favorable growth conditions occur.

Microorganisms in water




Aquatic microorganisms tend to grow at interfaces: on suspended solids at the water/air interface

(aerobic) or are present in sediments at the interface with water (anaerobic).

Light to Chemical energy

Reducers (break down

chemical compounds)Producers (utilize light energy

and store as chemical energy)


Aquatic microorganisms tend to grow at interfaces: on suspended solids at the water/air

interface (aerobic) or are present in sediments at the interface with water (anaerobic).

Aerobic/anaerobic conditions affect metabolism/kind of bacteria



A simple laboratory demonstration - the

Winogradsky column - illustrates how different

microorganisms perform their interdependent roles:

the activities of one organism enable another to

grow, and viceversa.

These columns are complete, self-contained

recycling systems, driven only by energy from light.

The columns are easy to set up with a glass or perspex tube, about 30 cm tall and 5 cm diameter. Mud from the bottom of a lake or

river is supplemented with cellulose (e.g. newspaper), sodium sulphate and calcium carbonate, then added to the lower one-third of the

tube. The rest of the tube is filled with water from the lake or river, and the tube is capped and placed near a window with

supplementary strip lights.




Diagram of a Winogradsky Column illustrating

how microorganisms distribute themselves.

The only organisms that can grow

in anaerobic conditions are those

that ferment organic matter and

those that perform anaerobic

respiration.The sulphur-reducing bacteria

such as Desulfovibrio can utilise

these fermentation products by

anaerobic respiration, using

either sulphate or other partly

oxidised forms of sulphur (e.g.

thiosulphate) as the terminal

electron acceptor (instead of O2),

generating large amounts of H2S

by this process.

Sulphur (or sulphate formed from

it) produced by the photosynthetic

bacteria returns to the sediment

where it can be recycled by

Desulfovibrio - part of the

sulphur cycle in natural waters.

The diffusion of H2S from the

sediment into the water column

enables anaerobic photosynthetic

bacteria to grow.




Diagram of a Winogradsky Column illustrating how microorganisms distribute themselves.




Microbial transformations of carbon

For most microorganisms, the energy-yielding or energy-consuming metabolic

processes involve changes in the oxidation state of carbon.

1. Algae and other plants fix CO2 as carbohydrates ({CH2O}) through photosynthesis:

C(IV)O2 + H2O + hn {C(0)H2O} + O2

2. When algae die, aerobic bacterial decomposition occurs:

{CH2O} + O2 CO2 + H2O + energy (-29.9 kcal/mol)

This energy is used by bacteria to carry our their metabolic processes.

Partial decomposition of organic matter: peat (=torba), lignine, shale-oil, coal, petroleum

bacteria




Degradation of biomass from dead algae may occur also under anaerobic conditions

(fermentation):

{CH2O} CH4 + CO2 + energy

Methane can be then oxidized under aerobic conditions by a number of bacteria:

CH4 {CH3OH, H2CO, HCOOH} CO2

Biodegradation of organic matter in the aquatic and terrestrial environments is a crucial

environmental process, and involves both naturally-occurring substances and organic

pollutants (biocidal).

Microbial transformations of carbon




http://goldbook.iupac.org/E02159.html

Enzyme Nomenclature

http://www.chem.qmul.ac.uk/iubmb/enzyme/

Macromolecules, mostly of protein nature, that function as (bio)catalysts by increasing the reaction

rates. In general, an enzyme catalyses only one reaction type (reaction specificity) and operates

on only one type of substrate (substrate specificity). Substrate molecules are attacked at the same

site (regiospecificity) and only one or preferentially one of the enantiomers of chiral substrates or

of racemic mixtures is attacked (stereospecificity).

Enzymes

http://goldbook.iupac.org/E02159.html

http://goldbook.iupac.org/B00652.html

http://goldbook.iupac.org/S06083.html

http://goldbook.iupac.org/C01057.html

http://goldbook.iupac.org/R05026.html




Aerobic oxidation (stepwise reactions catalyzed by oxygenase enzymes): primary

degradation way of petroleum wastes. Hydrocarbons biodegradability varies

significantly: strong preference of

microorgaanims for straight-chain

hydrocarbons (branching inhibits beta-

oxidation)

Microbial oxidation of hydrocarbons




Despite their chemical stability, aromatic (aryl) rings are susceptible to microbial

oxidation.

Hydroxilation

Enzyme-mediated




Reductions are carried out by reductase enzymes




Microbial transformations of nitrogenMicroorganisms-mediated chemical reactions involving nitrogen compounds are

summarized in the nitrogen cycle, describing the dynamic processes through which nitrogen

is interchanged among the atmosphere, organic matter, and inorganic compounds.




Nitrogen fixation: conversion of atmospheric nitrogen in a chemically-combined

form. Essential for plant growth in absence of fertilizers (mediated by Rhizobium

bacteria, most important nitrogen-fixing bacteria, which use photosynthetically

produced energy by plants to break N triple bond)

3 {CH2O} + 2 N2 + 3 H2O + 4 H+ 3 CO2 + 4 NH4+

Nitrification: conversion of N(-III) to N(V). Very common in water and soil

When plants die

2 O2 + NH4+ NO3

- + 2 H+ + H2O (Keq = 107.6)

3 O2 + 2 NH3 2 NO2- + 2 H+ + 2 H2O (Nitrosomonas)

2 NO2- + O2 2 NO3

- (Nitrobacter)

Microbial transformations of nitrogen

Adsorbed

by plants




Nitrate reduction: nitrogen in chemical compounds is reduced to lower oxidation

states under anaerobic conditions. N(-III) essential component of proteins (-NH2)

{CH2O} + 2 NO3- 2 NO2

- + CO2 + H2O (intermediate reaction)

Denitrification: mechanism by which fixed nitrogen is returned to the

atmosphere.

{CH2O} + NO3- + H+ N2 + CO2 + H2O

Ammonification: urea conversion to ammonia.

(NH2)2CO + H2O 2 NH3 + CO2





Microbial transformations of phosphorous

Biodegradation of phosphorus compounds is important in the environment for two reasons:

it provides a source of algal nutrient orthophosphate from the hydrolysis of

polyphosphates (involved also in eutrophication).

biodegradation deactivates highly toxic organophosphate compounds, such as the

organophosphate insecticides.




Degradation of organic sulfur compounds of natural and pollutant origin is an important

microbial process having a strong effect upon water quality.

Among all, the most common sulfur-containing organic compounds are thiols (–SH),

disulfide (–SS–), sulfides (–S–), sulfoxides (S=O), sulfonic acids (–SO2OH), thioketones

(C=S), and thiazoles (S-heterocycles).

Biodegradation of sulfur-containing aminoacids (cysteine, methionine) results in the

production of volatile organic sulfur compounds, such as methane thiol/mercaptane

(CH3SH), dimethyl disulfide (CH3SSCH3), and H2S.

Microbial transformations of sulfur




Microbial transformations of sulfur

The most common source of sulfur in water is the sulfate ion SO42-.

Reduction of sulfates (Desulfovibrio bacteria):

2 {CH2O} + SO42- + 2 H+ H2S + 2 CO2 + 2 H2O

3 {CH2O} + 2 CaSO4 2 CaCO3 + 2 S + CO2 + 3 H2O

Oxidation of sulfides:

2 H2S + O2 2 S + 2 H2O

2 S + 2 H2O + 3 O2 2 SO42- + 4 H+




Other microbial transformations

Dehalogenation: reactions involving the conversion of organohalides into the

corresponding alcohols or the “simple” replacement of the halogen atom with OH.

Microbial transformations of metals: bacteria-catalyzed oxidation of Fe(II) to Fe(III)

under aerobic conditions.

Microbial transformations of metalloids: reductive processes under anaerobic

conditions can reduce both SeO32- and SeO4

2- ions to elemental selenium.

Microbial corrosion: corrosion is a redox phenomenon and much corrosion in nature is

bacterial.

environmental toxicology: chemical aspects …...environmental toxicology, master sc. in industrial...

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