Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

Подождите немного. Документ загружается.

Biogeochemical cycles are not maintained by any single species; rather, they repre-

sent the collective effects of diverse, yet inherently coordinated, microbial consortia

as well as interactions of microbes with higher organisms. In the following sections

we examine the speciﬁc eleme nts carbon, nitrogen, sulfur, iron, and manganes e and

their cyclic metabolic transitions, while also discussing oxygen and hydrogen.

In each case, these cycles provide an endless exchange o f oxidation states

through which these atoms are transformed, both oxidatively and reductively (Sec-

tion 5.1.2). The oxidation state assumed by any given atom reﬂects the level and

mode of electrons that it shares with its neighbors. For example, carbon has a range

of oxidation states from 4toþ4, as dictated by its atomic number (AN), 6. This

means that carbon has six protons (positive charge) in the nucleus (equal to its atomic

number) and six electrons (negative charge) outside the nucleus, balancing the atom’s

charge.

The nucleus of an atom (except most hydrogen nuclei) also contains neutra l parti-

cles, called neutrons. The sum of the protons and neutrons is the atomic weight of the

atom. Carbon usually has six neutrons, so that its usual atomic weight is 12 (six pro-

tons þ six neutrons). However, some forms, or isot opes, of carbon have seven or eight

neutrons, so that they have an atomic weight of 13 or 14. These forms are relatively

rare, so that on average, carbon typically has an atomic weight of 12.011.

The electrons surround an atom’s nucleus in orbital shells. The ﬁrst s hell for

any element can only have a maximum of two electrons, while the next can have

a maximum of eight. (The third shell ca n have up to 18, but is also stable with

only eight.) Therefore, after subtracting the two electrons allocated to its ﬁrst

shell, carbon’s second orbital contai ns the four remaining electrons. In this conﬁg-

uration (six electrons along with the six protons in the nucle us), elemental carbon’s

net electrical charge (oxidation state) would then be zero. In fact, in elemental

form, the oxidation state of all elements is zero. This oxidation state will change,

though, as the outer shell either gains or loses electrons while bonding with adja-

cent atoms. For example (Figure 13.1), the ca rbon atom found in a methane mole-

cule, CH

, effectively pulls four additional negatively charged electrons into its

Figure 13.1 Carbon oxidation state extremes.

388

MICROBIAL TRANSFORMATIONS

outer shell (one from each of the four hydrogens), thereby changing its oxidation state

to 4 (10 negatively charged elect rons versus six positively charged protons). Conver-

sely, the carbon found in carbon dioxide (CO

) essentially lends all four of its outer

shell electrons to a pair of surrounding oxygen atoms, thereby shifting its oxidation

state to þ4.

Carbon, nitrogen, and sulfur are the three main elements of biological interest

with a full eight-electron range in oxidation state. As shown in Figure 13.2, the ranges

for these three elements (C, N, and S) are each, respectively, shifted one electron

higher.

Table 13.1 shows the common oxidation states of some of the other elements

of biological importance. Oxygen’s (AN ¼ 8) outer shell has six electrons, pro-

ducing an aggressive tendency to add two additional electrons (thereby ﬁlling its

outer shell) rather than releasing any to other adjacent atoms. This characteristic

qualiﬁes oxygen as having a high degree of electronegativity (Section 3.2). As a

result, oxygen’s metabolic range of oxidation states only includes 2(e.g.,H

O),

besides 0 (diatomic oxygen, O

). Hydrogen (AN ¼1) has p ossible sta tes of 1, 0 ,

and þ1, since its single outer shell has only one electron of the two it would take

to ﬁll it. In living matter it typically donates its electron (thus, oxidation state ¼þ1)

to a mor e electronega tive (e.g., oxygen, carbon, sulfur, nitrogen) receptor. Metabolic

iron (AN ¼26), occurs mainly in the þ2 (ferrous) and þ3 (ferric) oxidation

states, while manganese (AN ¼ 25) is found mainly as the þ2 (manganous) and

þ4 (manganic) forms. Chlorine (AN ¼ 17) as a disinfectant in water (HOCl, hypo-

chlorous acid) has an oxidation state of þ1, but as chloride or in organic compounds

it is 1.

13.1 CARBON

Carbon can be converted from inorganic to organic form by a variety of microorganisms

and by plants. It then is transformed, by anabolic and catabolic processes (Section 5.4), to

+4-4

+3+2

-1

-2

-3

+5 +6

Chemical Oxidation States

Carbon

Nitrogen

Sulfur

Iron

Manganese

Hydrogen

Oxygen

Figure 13.2 Redox ranges for some elements of major biochemical importance.

CARBON 389

a tremendous array of biologically important compounds in all living things. Some of

these compounds are released as waste, or eventually the organism itself dies. They are

then broken down, mainly by microorganisms, and eventually the carb on returned to inor-

ganic form.

With carbon’s central importance to all known forms of life, perhaps it is not surprising

that the redox reactions through which these atoms are cycled (Figure 13.3) have such a

high degree of breadth, complexity, and variability. In fact, its role as the dominant

element in living organisms appears to stem from the great diversity of its bonding

options. It typical ly forms fo ur bonds with adjacent at oms. This is mo re than hydrogen

(one bond) or oxygen (two), but not really different from nitrogen (three or four), and

less than phosphorus (ﬁve) or sulfur (two to six). However, its great versatility also

TABLE 13.1 Common Oxidation States of Some Biologically Important Elements

Atomic Atomic

Element Symbol Number Weight Common Oxidation States

Aluminum Al 13 26.98 0, þ3

Arsenic As 33 74.92 þ3, þ5

Bromine Br 35 79.90 1

Cadmium Cd 48 112.40 þ2

Calcium Ca 20 40.08 þ2

Carbon C 6 12.01 4, 3, 2, 1, 0, þ1 þ2, þ3 þ4

Chlorine Cl 17 35.45 1, 0, þ1

Chromium Cr 24 52.00 0, þ3, þ6

Cobalt Co 27 58.93 þ2, þ3

Copper Cu 29 63.54 0, þ1, þ2

Fluorine F 9 19.00 1

Hydrogen H 1 1.01 1, 0, þ1

Iodine I 53 126.90 0, 1

Iron Fe 26 55.85 0, þ2, þ3

Lead Pb 82 207.2 0, þ2, þ4

Magnesium Mg 12 24.31 þ2

Manganese Mn 25 54.94 þ2, þ4, þ7

Mercury Hg 80 200.59 0, þ1, þ2

Molybdenum Mo 42 95.94 þ6

Nickel Ni 28 58.69 þ2, þ3

Nitrogen N 7 14.01 3, 1, 0, þ1, þ2, þ3, þ5

Oxygen O 8 16.00 2, 0

Phosphorus P 15 30.97 þ5

Potassium K 19 39.10 þ1

Selenium Se 34 78.96 2, þ4, þ6

Silica Si 14 28.09 þ4

Sodium Na 11 22.99 þ1

Sulfur S 16 32.06 2, 0, þ2, þ4, þ6

Tin Sn 50 118.69 þ2, þ4

Vanadium V 23 50.94 þ5

Zinc Zn 30 65.38 0, þ2

390

MICROBIAL TRANSFORMATIONS

stems from the fact that it can occur in all of the oxidation states from 4toþ4

(Table 13.2).

Organic Carbon What exactly is meant by organic carbon? The term dates to the time

that such compounds were considered to be produced only by organisms. German Frei-

drich Wo

hler is generally credited with the ﬁrst synt hesis of an organic compound, urea,

in 1928. Now, of course, there are many synthetic organic compounds. So again, what

does organic mean?

Interestingly, most chemistry texts do not provide a real deﬁnition. A dictionary may

state that organic chemistry is the study of carbon compounds, but carbon dioxide is not

considered organic (although it is produced by organisms). Another deﬁnition that has

been used is ‘‘compounds containing a CC bond.’’ This would exclude carbon dioxide,

as desired, but would also exclude methanol, formic acid, and formaldehyde, which are

generally considered organic. (Formic acid, for example, is produced by ants for their

sting, as noted in the scientiﬁc name of the family, Formicidae.) Another deﬁnition is

‘‘compounds containing a CH bond.’’ This would exclude carbon dioxide and include

methanol, formic acid, and formaldehyde. It would also include methane, which some

people consider organic, but not carbon tetrachloride. Interestingly, neither deﬁnition

would include urea!

Perhaps this explains why most texts avoid the question—there is not a simple answer.

Once again, nature does not easily ﬁt into human classiﬁcation systems. For our purposes,

we will consider carbon-containing compounds (not the elemental forms graphite and dia-

mond) to be organics, with the exception of carbon dioxide, carbonic acid, bicarbonate,

carbonate, and carbon monoxide. Methane we consider both organic, since it is the carbon

and energy source for methanotrophs (Section 10.5.6), and inorganic, since it is a major

end product of anaerobic mineralization and also the product of carbon dioxide respiration

(Section 13.1.1).

13.1.1 Carbon Reduction

The reductive side of the carbon cycle is what makes continued life on Earth possible.

Without this transformation, and speciﬁcally the biological ﬁxation of inorganic carb on

-4

+3+2

-1

-2

-3 +4

-4

+3+2

-1

-2

-3 +4

-4

+3+2

-1

-2

-3 +4

Methane

Carbon

Dioxide

Photosynthetic carbon fixation

Chemolithoautotrophic

Organic carbon oxidations

Carbon Oxidation States

carbon fixation

Methanogenesis

Fixed

Cellular

Carbon

Fixed carbon oxidations

Fixed carbon reductions

Fixed

Cellular

Carbon

Fixed carbon oxidations

Fixed carbon reductions

Anaerobic

Fermentations

Figure 13.3 Biochemical carbon transformations.

CARBON 391

TABLE 13.2 Simple Carbon Compounds and the Oxidation State

of the Carbon

Oxidation State

of Carbon Chemical Name Formula Structure

þ4 Carbon dioxide CO

OCO

Carbonic acid H

Bicarbonate HCO



−

Carbonate CO

2

−

Carbon CCl

tetrachloride

þ3 Acetic acid

OOH

þ2 Formic acid HCOOH

þ1 Acetaldehyde

0 Formaldehyde HCHO

1 Ethanol

2 Methanol CH

HOH

3 Ethanol

HCOH

Acetic acid

COOH

4 Methane CH

For the two-carbon compound s, the oxidation state given is for the C marked

with an asterisk.

392 MICROBIAL TRANSFORMATIONS

(carbon dioxide, carbonic acid, bicarbonate, and carbonate) to organic forms, life would

be dependent on chemically ﬁxed organic matter, and thus extremely limited.

Organisms that can grow on inorganic carbon, thereby converting it to ﬁxed, organic

form, are referred to as autotrophs (Section 10.3.2). Carbon reduction requires energy, a

portion of which is stored as chemical energy in the ﬁxed carbon that is then available to

other life-forms. The primary source of energy utilized for this purpose, providing most of

the new organic carbon that is metabolically synthesized on our planet, is light. Organisms

able to utilize light as their energy source are phototrophs and are referred to as photo-

synthetic. This includes almost all plants and algae, and some bacteria. Some autotrophic

bacteria and archaea, referred to as chemolithotrophs (Section 10.3.1), are able to use

inorganic chemical energy to ﬁx carbon.

Once carbon has been ﬁxed, it can be further reduced by organisms to generate the

cellular constituents that they need for growth. However, this reduction also requires

energy, either directly or to regenerate intermediates.

Many organisms (including humans) also reduce carbon during a process known as

fermentation. This is an anaerobic energy-yielding process during which organic carbon

is both oxidized and reduced, either in different molecules or in different parts of the same

molecule. Because it is the oxidation that provides energy, fermentation is discussed in

Section 13.1.2.

Photosynthesis As a new day starts and light intensity builds, photosynthetic reactions

metabolically infuse new organic carbon back into the cycle of life. The net production of

glucose and oxygen generated through this process by plants and algae can be summar-

ized as

6CO

þ 6H

O þ light energy ! C

þ 6O

ð13:1Þ

Cyanobacteria carry out the same reaction, but other photosynthetic bacteria utilize hydro-

gen, hydrogen sulﬁde, or elemental sulfur instead of water as their electron donor, and

thus do not produce oxygen (see Table 10.4).

Almost all phototrophs use the same reaction sequence, referred to as the Calvin cycle

after its discoverer, to invest energy drawn from sunlight into ﬁxing inorganic carbon

(Section 5.4.5). Exceptions are some green sulfur bacteria, which can run the Krebs

cycle (Section 5.4.3) in reverse to ﬁx carbon, and some green nonsulfur bacteria, which

utilize a unique hydroxypropionate cycle to form the two carbon compound glyoxylate

(CHOCOOH) from two carbon dioxide molecules.

The Calvin cycle itself was summarized in Figure 5.13 and is shown in more

detail in Figure 13.4. It can be considered to consist of three parts. The ﬁrst step com-

bines the carbon dioxide with a ﬁve-carbon sugar containing a phosphate on each end

[ribulose 1,5-diphosphate; also called ribulose 1,5-bisphosphate (RuBP)] . The unstable

product breaks down to produce two molecules of a phosphorylated, three-carbon com-

pound cal led 3-phosphoglycerate. This overall reaction is catalyzed by ribulose-1,5-

bisphosphate carboxylase/oxygenase (RubisCO), which may be the most abundant

enzyme on Earth.

During the secon d part of the cycle, the newly ﬁxed carbon is reduced further,

using additional energy, to glyceraldehyde 3-phosphate. In the ﬁnal part of the cycle, a

six-carbon sugar is produced, and the ribulose 1,5-diphosphate regenerated in preparation

for subsequent cycles.

CARBON 393

Photosynthetic organisms utilize part of the carbon they ﬁx as an energy source, par-

ticularly in the dark (Section 13.1.2). However, the rest is used to produce the organic

compounds they need for growth. It is this net ﬁxation of carbon that sustains the ecosys-

tems dependent on these primary producers, directly or indirectly, for their carbon and

energy supply.

Of course, there are considerable variations in the rates of organic carbon assimilation

around the world. Tropical rain forests, for example, have some of the highest rates of

carbon uptake on Earth, with annual net ﬁxation of up to 1:5kg C=m

of land surface

area. Freshwater wetlands are almost as productive. Forests, grasslands, and cultivated

plants in the middl e latitudes typically assimilate 50% as much carbon, although intensive

agriculture can more than double the production of the rain forest. The open ocean,

deserts, and the Arctic tundra ﬁx much less carb on (typic ally 0.05, 0.1, and 0:2kg=m

respectively, per year). Sunlight intensity, temperature, nutrient supply, and water avail-

ability are all factors (see Chapters 14 and 15).

The use of sunlight for photosynthesis is not very efﬁcient. On average, only

about 0.1% of the energy available is utilized. Even with intensive agriculture, this

value is unlikely to exceed 1%. Each gram of carbon ﬁxed represents about 10 kJ of

stored energy.

It is estimated that photosynthesis provides our planet with a cumulative carbon assim-

ilation rate of approximately 10

metric tons per year. Based on the stoichiometry in

equation (13.1), this means that at the same time, almost 3 10

metric tons per year

of oxygen is produced. The long-term maintenance of this essential activity, however,

Carbon Sugar (1)

NADH

ATP

Carbon dioxide (6)*

3-Phospho-

glycerate (12)

1,3-

glycerate (12)

(6)

Ribulose 1,5-

(6)

OPO

HOH

H O

C OH

Carbon dioxide (6)*

3-Phospho-

glycerate (12)

1,3-

glycerate (12)

1,5-

OPO

OHOH

HOH

H O

C OH

ATP

RubisCO

HOH

HOHOH

phosphate

Ribulose 5-

Glyceraldehyde

phosphate (12)

Diphospho-

diphosphate

Figure 13.4 Calvin cycle carbon ﬁxation process.

(No.) refers to number of molecules.

394

MICROBIAL TRANSFORMATIONS

is sensitive to disruption. Whether measured in acreage of rain forest lost to deforestation,

wetlands drainage, or accidental tanker spills and resultant oil slicks, humans certainly

have the potential to cause severe environmental stress.

Chemolithoautotrophic Carbon Fixation Although they utilize chemical energy stored

in inorganic molecules rather than light, chemolithoautotrophs also utilize the Calvin

cycle to ﬁx inorganic carbon. Worldwide, a substantial amount of carbon is ﬁxed

through their action. In fact, in som e environments in which sunlight is absent, they

may be the major or only primary producers. Surrounding thermal vents in the deep

ocean, for example, the release of hydrogen sulﬁde serves as an energy source for sul-

ﬁde-oxidizing bacteria and archaea, which then form the base of an extensive ecosys-

tem, including giant worms and clams. It also appears that the abiotic release of

hydrogen gas in deep deposits of basalt (a volcanic rock) in the Paciﬁc northwestern

region of the United States may support specialized autotrophic bacteria that serve as

the base of a microbial ecosystem.

Nitrifying (see more in Section 13.2.2) and sulfur-oxidizing (Section 13.3.2) bacteria

may be important in waste treatment and aquatic and soil environments, and sulfur-and

iron-oxidizing (Section 13.4.2) bacteria play a major role in acid mine drainage. However,

in these cases it is not the carbon ﬁxation that is of greatest environmental signiﬁcance. It

recently has been suggested, however, that in drinking water systems using chloramina-

tion (an ammonium/chlorine combination) for disinfection, the nitriﬁcation that some-

times occurs may actually contribute materially to the otherwise low organic carbon

content of the water.

Methanogenesis Methanogenesis, the production (genesis) of methane, is a form of car-

bon reduction carried out solely by a group of strictly anaerobic archaea referred to col-

lectively as methanogens (Section 10.6.3). One reaction commonly utilized is actually an

anaerobic respiration in which CO

[equation (13.2)], or occasionally, CO [equation

(13.3)], is reduced to methane during oxidation of H

. Many methanogens can ferment

formic acid [equation (13.4)] or methanol [equation (13.5)], with some of the molecules

being oxidized to CO

and others reduced to CH

. A few methanogens can similarly

ferment methyl amines or, less commonly, methyl sulﬁdes. Many can also ferment acetic

acid, oxidizing part of the molecule to CO

while reducing the other part of same mole-

cule to CH

[equation (13.6)]:

þ CO

! CH

þ 2H

O ð13:2Þ

þ CO ! CH

þ H

O ð13:3Þ

4CHOOH ! CH

þ 3CO

þ 2H

O ð13:4Þ

4CH

OH ! 3CH

þ CO

þ 2H

O ð13:5Þ

COOH ! CH

þ CO

ð13:6Þ

Often under methanogenic conditions, such as in landﬁlls and anaerobic digesters, the net

gas production is almost two-thirds methane and one-third carbon dioxide, with small

amounts of other gases. Additional details about methanogenesis are given in Sections

13.1.1 and 16.2.1.

CARBON 395

13.1.2 Carbon Oxidation

The oxidation of organic carbon compounds provides energy to a cell. Even in photosyn-

thetic organisms, it is the oxidation of the ﬁxed carbon produced (with the energy from

sunlight) that provides the cells with most of the energy they use.

The two overall types of energy-yielding metabolism are respiration (Section 5.4.3)

and fermentation (Section 5.4.2). In respiration, an inorganic molecule acts as the term-

inal electron acceptor, whereas in fermentation, an organic molecule serves this function.

Fermentation is an anaerobic process, whereas there are both aerobic and anaerobic forms

of respiration.

Respiration Respiration provides more energy to the cell then fermentation, so is

favored whenever it is possible. Aerobic respiration, in which molecular oxygen (O

)

is the terminal electron acceptor, provides the most energy; thus, aerobic organisms are

likely to dominate wherever oxygen is available.

Since our atmosphere is 21% oxygen, it might be expected that oxygen is readily avail-

able except in sealed systems. However, oxygen is poorly soluble in water, with only 9.1

mg/L of dissolve d oxygen (DO) present at saturation (equilibrium with the atmosphere)

at 20



C. Also, oxygen diffusion through pores in solid matrices (such as soil and compost-

ing material) occurs by a factor of 4 orders of magnitude more slowly in water than in air.

Thus, in aqueous systems (such as wastewater, liquid sludges, surface waters, river and

lake sediments , groundwater, and ﬂooded soils) the oxidation of even small amounts of

organic material can quic kly deplete DO, leading to anaerobic conditions.

In the absence of oxygen, many bacteria can carry out respiration using nitrate as the

terminal electron acceptor (Section 5.4.3). This denitriﬁcation (Section 13.2.1) provides

about 85% of the energy available aerobic ally from oxidation of the same organic com-

pound with oxygen. The nitrate is reduced ﬁrst to nitrite, then eventually to nitrogen gas.

Thus, nitrite can also be utilized, but yields still less energy. Den itrifying bacteria include

a wide diversity of aerobic bacteria that can utilize nitrate and nitrite as alternative elec-

tron acceptors when oxygen is absent. The process itself is discussed in more detail as part

of the nitrogen cycle (Section 13.2).

Ferric iron can also be utilized for respiration by some aerobic bacteria, providing a

similar amount of energy to nitrite. Some other metals may also be utilized by faculta-

tively or strictly anaerobic bacteria.

In the absence of oxygen, nitrate, nitrite, and ferri c iron, some anaerobic bacteria and

archaea can utilize sulfate as an electron acceptor, producing hydrogen sulﬁde. This is

particularly important in salt marshes and other systems where sulfate is abundant (Sec-

tion 13.3). Once the sulfate is also depleted, methanogens can utilize CO

as an electron

acceptor, producing methane [equation (13.2)].

Under anaerobic conditions, particular bacteria, especially some sulfate reducers , may

be able to utilize other ‘‘unusual’’ electron acceptors. Examples include the use of a num-

ber of chlorinated organic compounds, including important environmental contaminants

such as polychlorinated biphenyls (PCBs), trichloroethylene (TCE), tetrachloroethylene

[commonly called perchloroethylene (PCE)], and vinyl chloride, during which a hydrogen

is substituted for a chlorine atom. This represe nts both a chemical reduction and a dechlo-

rination of the organic molecule and results in the release of a chloride ion (Cl



). A simi-

lar reaction may occur with brominated and perhaps other halogenated compounds. Thus,

the more general term reductive dehalogenation is often used to describe this process.

396 MICROBIAL TRANSFORMATIONS

Fermentation Although it is usually bacteria, archaea, some fungi, and a few protozoa

that are thought of as being anaerobic, higher organisms also carry out fermentations.

During vigorous exercise, for example, not enough oxygen can be delivered to the mus-

cles, leading to buildup of the fermentation product lactic acid. This is wha t makes your

arms and legs feel heavy and tired. Gradually, as you recover, the lactic acid is transported

to the liver and converted there for other uses.

Because an organic compound serves as the electron acceptor in fermentation, some

of the organic material is reduced while oxidizing other organic molecules. In the

examples above for methanogenesis, one reaction involves four molecules of formic

acid [equation (13.4)], one of which is reduced to CH

while the other three are oxi-

dized to CO

. For the fermentation of the more reduced compound methanol [equation

(13.5)], the ratio of these two products is reversed. With acetic acid [equation (13.6)],

on the other hand, of the two carbons of a single molecule, one is oxidized while the

other is reduced.

13.1.3 Carbon in Environmental Engineering and Science

It traditionally has been the organic carbon (along with pathogens) that was of the greatest

concern in water pollution (Section 15.2.7), leading to the construction of wastewater

treatment plants (Chapter 16) that focus on its removal. Management of wastewater treat-

ment sludges often has stabilization of the organic material as a major objective. (Stabi-

lization involves conversion of readily degradable materials to those that change only

slowly; see later in this subsection). Municipal solid waste management also must stabi-

lize the organic material (e.g., by incineration or composting), or else d eal with the con-

sequences (e.g., attraction of vermin, settling, and leachate and gas production during

landﬁlling). Similarly, with soil and groundwater contamination, it is often organic carbon

that is the target of remediation. Undesirable tastes and odors in drinking water, and the

formation of cancer-causing compounds during disinfection, are traceable to organic com-

pounds present in the water supply. Even air pollution control may involve organics, such

as volatile organic com pounds (VOCs) and soot (which includes organic particles), as

important contaminants. Most individual toxic compounds of concern in water, soil,

and air are also organics.

Thus, much of environmental engineering and science is directed at control of organic

carbon or an understanding o f its fate and effects in the environment. In particular cases

the emphasis may be on a single compound, a particular class of compounds (such as pet-

roleum hydrocarbons or chlorinated solvents), some broad fraction (such as oil and

grease, or oxygen demanding biodegradable compounds), or the total organics. One con-

cern might be rapid biodegradation, leading to depletion of oxygen, while another mate-

rial might pose a hazard because it is very resistant to degradation. Slow-to-degrade

compounds may be bioaccumulated (Section 18.7.2) in organisms, perhaps leading to

toxic effects even if they are present only at low concentrations in the environment.

Other slowly degrading compounds, such as many plastics, may pose aesthetic problems,

or perhaps injure wildlife that eat (discarded plastic bags that are mistaken for jellyﬁsh) or

become entangled (abandoned ﬁshing nets or six-pack rings) in them.

In some water bodies, contamination with excess levels of nitrogen and/or phosphorus

may lead to excessive growth of aquatic plants, algae, or cyanobacteria (eutrophication;

Section 15.2.6). In this case organic carbon is not added directly, but instead, becomes

problematic after it is formed through photosynthesis.

CARBON 397