AQUATIC ORIGINS OF LIFE

Introduction Aqueous Conditions Required for Life
Drake Equation Biogeochemical Properties of Water
Aqueous Origins of Life Oxygen as a Toxicant
Organismal Adaptations Gaia Hypothesis
Humans Water Intoxication
References Water as an Environmental Pollutant

 

Introduction

The course title, Aquatic Toxicology, approaches that of an oxymoron. Life, as we know it, can not exist without water, as evidenced by reports that discoveries of water on planets and moons raise the possibility of extraterrestrial life. The intent of this title, therefore, is simply to emphasize aspects of toxicity that are primarily associated with water. These include both the degradation of aquatic ecosystems and the exposure of other organisms, including humans, to water borne toxins. Therefore, this chapter concludes with a case study on malformed and declining amphibian populations, which illustrates the complexity of aquatic toxicology.

Aqueous Conditions Required for Life

"The appearance of life on this Earth was not so much a matter of change of substance, in fact, life made use of all substances already occurring on Earth, particularly the universal medium of water. It was, rather, an enormous increase in the information and complexity with which these elements combined both in spatial management, forming the molecules of which life is built, and in dynamics, enabling them to reproduce the patterns through endless and varied generation."

(Bernal, 1967)

 

The conditions for extraterrestrial life have been calculated by astronomers, based on the presence of liquid water that is required for all life on Earth. The planetary body, or moon, must be located far enough away from its star to allow its crust to solidify and to prevent its carbon, nitrogen, and water from being vaporized. It must be located at a distance where some water temperatures are between the freezing (0 ¯C or 32 ¯F) and boiling (100 ¯C or 212 ¯F) points of water. (Since these temperatures are based on the Earth's atmospheric pressure, they will vary for different planetary masses.) It must have enough gravitational mass to retain an atmosphere. And, it cannot be on a "wacky intersecting orbit", which would result in collisions with other large mass that would cause large melts, raise atmospheric temperatures to thousands of degrees, and boil off the oceans. The importance of the latter condition has been realized within the past two decades as the global extinctions on Earth have been traced to meteoric impacts.


Drake Equation

The preceding criteria are incorporated in the Drake Equation, which estimates the number of technological civilizations that may exist within our galaxy:

N = R*•fp•ne•fl•fi•fc•L

In this equation,

N = the number of communicative civilizations within the Milky Way

 R* = the rate of formation of stars with large enough lifetimes & "habitable zones"

 fp = the fraction of those stars with planetary systems

 ne = the number of planets per planetary system with a "habitable zone"

 fl = the fraction of those planets where life develops

 fi = the fraction of those life sites where intelligence develops

 fc = the fraction of the intelligence life sites where technology develops

 L = the "lifetime" of those communicating civilizations

 Again, the term for a "habitable zone" is based on a planetary orbit that maintains a temperature that allows water to exist in a liquid form.


Biogeochemical Properties of Water

The biogeochemical properties of water are associated with the dipolar structure of its molecule (Figure 1-1). The angled orientation (105ß) of the small (0.3 nm diameter) H-O-H molecule reflects the outer orbitals of the unshared oxygen electrons, which occupy two corners of a tetrahedral. As a consequence of this charge imbalance, the area around the hydrogen nuclei is relatively positive and the area around the oxygen nucleus is relatively negative.

 

Figure 1-1: The dipolar structure of water.

 

The dipolar structure of water molecules accounts for their ability to dissolve other polar molecules, as illustrated in Figure 1-2. It shows the dissolution of solid sodium chloride (NaCl) in water, through the formation of sodium and chloride ions surrounded by clusters of water molecules. NaCl and other ionizable molecules that dissolve in water are termed hydrophilic (water loving).

 

Figure 1-2: The dissolution of sodium chloride (NaCl) in water.

 

Nonpolar molecules that do not readily dissolve in water are termed hydrophobic (water fearing). These include organic molecules, which form oil droplet type structures through hydrophobic interactions within water. Those processes account for the theoretical concentration of organics in the primordial soup from which life may have arisen.

 

In addition, there are molecules with both polar and nonpolar regions, which are termed amphipathic (consisting of two parts), that cluster in water as illustrated in Figure 1-3. These types of molecules are important components of cell membranes and contribute to mechanisms regulating transport processes across those membranes. Other amphipathic molecules, including detergents, solubilize nonpolar molecules in water, as discussed later.

 

Figure 1-3: The clustering of amphipathic molecules in water.

 

Water's ability to solubilize so many different chemicals, often in relatively large amounts, is unique. This has resulted in its recognition as the "universal solvent". That ability, partially, accounts for the essential role of water in the origin and maintenance of life.

 

In summary, water is essential for life because organic molecules require an aqueous medium for biologic activity. Water also provides a stabilizing effect for organisms in a variable climate through its high heat capacity, high heat of vaporization, relatively wide range between freezing and boiling points, solubilizing ability, high dielectric constant, ionizing ability, and high surface tension. Each of those terms will be addressed, in detail, throughout the text.

 

 

Aqueous Origins of Life

"Now, when ultra-violet light acts on a mixture of water, carbon dioxide, and ammonia, a vast array of organic substances are made, including sugars and apparently some of the materials from which proteins are built up. This fact has been demonstrated in the laboratory of Baly in Liverpool and his colleagues. In this present world, such substances, if left about decay - that is to say, they are destroyed by micro-organisms. But before the origin of life they must have accumulated til the primitive oceans reached the consistency of hot dilute soup."

(Haldane, 1929)

 

All theories on the origins of life on Earth are based on the presence of water (Ehrlich, 1996). The two primary theories on of how life may have arisen de novo on Earth are the "organic soup scenario" and the 'surface metabolism scenario". In the former scenario, abiotic chemical reactions formed organic molecules in a dilute broth (an amino acid stew) and those simple molecules eventually formed self-replicating polymeric molecules (RNA and DNA), whose ability to reproduce constituted the beginning of true life. In the latter, more recent scenario, the molecular building blocks of life were formed on the surfaces of minerals in aqueous solutions. Even the alternative theory of panspermia, which is that preformed life arrived as spores from another world, invokes the presence of interstellar gases, including H2O, to shield the organisms from ultraviolet (UV) irradiation during their intergallatic travel.

Figure 1-4a: Abiotic formation of complex organic compounds. The abiotic formation of complex organic compounds that are required for life is illustrated by these cylinders of phospholipids created by David Deamer using a hydration/dehydration process.

 

Figure 1-4b: Organic material on earth. A thin layer of egg phosphatidylcholine (PC) stained with rhodamine to reveal lipid nilayers forming both vesicles and multi-lamellar structures (from Dave Deamer and Charles Apel).

Figure 1-4c.Extraterrestrial organic material. Extracts of the Murchison meteorite processed in the same manner as the preceding egg PC and showing a similar range of organic compounds as in the egg PC, including bilayer vesicle-forming fatty acids (octanic and nonanoic acid) and polycyclic compounds (from Dave Deamer and Charles Apel).

 It is now believed that those first organisms appeared within 500 million to 1 billion years after the formation of the Earth, which is now 4.6 billion years old (Bengston, 1994; Deamer and Fleischaker, 1994) They were anaerobic prokaryotes, who were either heterotrophic (organic soup scenario) or autotrophic (surface metabolism scenario). Aerobic prokaryotes subsequently evolved with the formation of an atmosphere with free oxygen generated by photosynthesis. Presumably, eukaryotic forms then appeared after the atmosphere became fully oxidizing, around 1 billion years ago.

 

 Figure 1-5: Proposed Origins of Life on Earth

 (insert)

 

This followed the evolution of photosynthetic prokaryotes, which evolved around 3 billion years ago and released molecular oxygen (O2) as a by-product of photosynthesis. Since that free oxygen is a highly reactive gas, it is toxic to all forms of life. Consequently, all aerobic organisms, including modern eukaryotes that require oxygen in their metabolic pathways, developed mechanisms to shield them from oxidation; and physiologically archaic, anaerobic organisms are now restricted to anaerobic environments, including reducing sediments, hydrothermal vents, decaying organic matter, and the interiors of some animals.

 


 Oxygen as a Toxicant

 The role of oxygen as a toxicant is of note for several reasons. First, aeration is a common method of treating contaminants in waste water and contaminated soils. Second, many other toxins act as oxidants. Third, many pathogenic organisms flourish in anaerobic environments and become toxic when they infect internal tissues. Finally, it illustrates the adage in toxicology that:

 "the dose defines the poison"

 For, while low doses of oxygen are essential for higher organisms, higher doses of oxygen are poisonous to those organisms.

 Additionally, the toxic actions of many chemicals and radiation involve the formation of activated oxygen species. These include the free radicals (O2 .- , .OH) and related species (H2O 2, 1O2), which can cause cancer and disrupt cellular structures. Consequently, much of the class will focus on the oxidative damage induced by toxins.

 

* Some of that oxygen reacted to form the ozone (O3) layer in the stratosphere, where it now provides protection from ultraviolet (UV) irradiation. Since a few milliliters (mL) of water provides the same protection, most aquatic organisms are not directly threatened by degradation of the ozone layer.


Organismal Adaptations from Aquatic to Terrestrial Environments

 

"Dessication, the loss of body water to surrounding air, is the foremost obstacle to terrestrial life."

 (Levine and Miller, 1994)

 Terrestrial Plants

 The extension of higher life forms onto land occurred in the Ordovician when plants developed mechanisms to survive out of water. Recent genetic analyses indicate that ancient relatives of modern liverworts (tiny, rootless plants that, like green algae, lack all three introns) emerged from aquatic environments around 470 million years ago (Palmer et al., 1998). Their surfaces evolved into waterproof evaporation barriers, with minute pores to allow gas exchange in photosynthesis and respiration. This was followed by the development of structures to obtain water and nutrients from the soil (roots) and to support photosynthetic (leaves) and reproductive (flowers) tissues above the ground. Vascular tissues then formed to transport water and nutrients to the different tissues; and diverse mechanisms for the dispersion of reproductive gametes, seeds, and spores in the absence of water were developed.

 As each of those mechanisms improved, there was an adaptive radiation of plants into more arid environments until virtually all terrestrial habitats were populated. That radiation was also sequential. Presumably, it was initiated by unicellular green algae. These were sequentially succeeded by liverworts, hornworts, mosses, and vascular plants. The current dominance of the later species in most terrestrial environment attests to their competitive success in various adaptations to water limitations.

 

 Terrestrial Animals

 The subsequent terrestrial colonization by animals involved the same types of adaptations. These included the development of impermeable surface membranes to prevent dessication, the formation of internal systems to cycle water and nutrients throughout the body, support structures to survive in a non-buoyant world, and mechanisms for reproduction and dispersion in the absence of water. Consequently, all animal life began in an aquatic environment and their emergence onto land was, again, dependent upon the development of mechanisms to compensate for the relative absence of water.

 In summary, all life is essentially constituted of individual or clusters of cells of self-replicating polymeric molecules within a dilute organic soup. The cells are surrounded by a semi-permeable membrane which allows the transport of chemicals needed to maintain the dilute chemical broth required for the replication of polymeric molecules. Therefore, toxic reactions involve (1) the destruction of the cell wall, (2) alterations in the internal broth, or (3) degradation of the self-replicating polymeric molecules, as discussed throughout this course.

 


 Gaia Hypothesis

 The systematic evolution of life on Earth with concurrent changes in its physical and chemical composition has been synthesized into the Gaia hypothesis (Lovelock, 1988). It is named after the ancient Greek goddess of Earth, and proposes that organisms have interacted with their abiotic environment to establish relatively stable conditions on Earth. This controversial proposal of a global homeostasis suggests the planet now functions as a superorganism, with complex feedback mechanisms that enable life to persist and evolve. Although the proposal is controversial, it illustrates the importance of biogeochemical cycles on life. These include the cycling of water, nutrients, and gases that are essential to life. Similarly, it illustrates how disruptions of those processes, by either natural or anthropogenic processes, may be toxic to life.

 


 Humans

 "The cells that make up the bodies of all but the simplest multicellular animals, both aquatic and terrestrial, exist in an "internal sea" of extracellular fluid (ECF) enclosed within the integument of the animal. From this fluid, the cells take up O2 and nutrients; into it they discharge metabolic waste products. The ECF is more dilute than present-day seawater, but its composition resembles that of the primordial oceans in which, presumably, all life originated."

(Ganong, 1997)

 

Water is the most important of the approximately 50 essential nutrients for humans. As previously indicated, water is both an essential component of all individual cells and an essential for the physiologic functions of multicellular organisms. These include the osmotic regulation of electrolytes, respiration, the excretion of metabolic wastes, and the regulation of body temperature.

Consequently, water is the most common constituent in humans. Ninety-nine (99) out of every 100 molecules in the human body are water, its concentrations in most cells (other than fat cells) ranges from 70 to 85%, and it accounts for approximately 60% of the total body weight in humans. That body water is distributed among different intracellular and extracellular fluids (Table 1-1).

 

Table 1-1: Distribution of water in a "normal" 70 kg person (male).

 

Intracellular fluid..........................

28 L

Extracellular fluid

interstitial fluid

11 L

plasma

3 L

Total

42 L

Maintenance of that body water requires a relatively sensitive balance between intake and loss (Table 1-2). The former occurs through the direct ingestion of water and the oxidation of organic nutrients. The latter occurs through the skin, respiratory passages, the gastrointestinal tract, the urinary tract, and menstrual flows in women. Losses through the skin and respiration are called insensible water loss, because individuals are not generally aware of their occurrences.

 

Table 1-2: Average daily intakes and losses of water in adults

 
Intakes

ingestion

1100 mL

food

1000 mL

metabolic production

200 mL

___________________

_______

total

2300 mL

Losses

insensible (skin)

350 mL

insensible (lungs)

350 mL

sweat

100 mL

gastrointestinal tract (feces)

100 mL

urinary tract (urine)

1400 mL

________________________

_______

total

2300 mL

Those amounts may be markedly different among individuals and under different physiological conditions. During prolonged exercise, the amount of ingested water may need to be tripled to balance increased water losses from the lungs (650 mL/day) and sweat (5,000 mL/day), in spite of reduced losses from urine (500 mL/day). This imbalance may be exacerbated by the use of diuretics, including alcohol, caffeine, glucose, and creatine.

 Similarly, increased inputs of water are required to compensate for excess losses from vomiting and diarrhea. These are often caused by water borne pathogens, as discussed later. In fact, many forms of toxicity in humans, and other organisms, involve perturbations in their water balance.

 

 Water Intoxication

 Further substantiating the adage that "the dose defines the poison", are cases of water intoxication in humans. These occur when water is ingested at greater rates than exceed the maximum urine flow during water diuresis ( ˜ 16 mL/min.) for a protracted period. The increased amount of water in the ECF causes a swelling of cells, including brain cells. This can lead to convulsions, coma, and death. Water intoxication is also caused by excess vasopressin, either administered exogenously or secreted endogenously in response to trauma.

 

 

Water as an Environmental Pollutant

 Even water, the most essential constituent for life, may be an environmental pollutant. This is illustrated by the toxicity caused by large amounts of fresh water discharged into San Francisco Bay from municipal and industrial wastewater outfalls (137 million gallons a day) that are destroying estuarine habitats. Since these are the homes of several endangered species, including the California clapper rail and the salt marsh harvest mouse, extensive efforts are now being made to reduce those discharges to levels where their salt marsh habitat can be sustained (Apendix 1-1).

 

 

Figure 1-5: California clapper rail

 (insert)

 However, many more problems in aquatic toxicology are being caused by losses or contamination of freshwater habitats, due to increasing anthropogenic rates of water consumption discussed in the following section. The complexity of these problems is indicated by recent observations of malformed amphibians and associated reports of declining amphibian populations that have often been attributed to aquatic toxins. Therefore, this introduction concludes with a case study showing the complex interactions that must be addressed in identifying and resolving problems in aquatic toxicology.

 

References:

Bengston, S. (Editor). 1994. Early Life on Earth: Nobel Symposium No. 84., Columbia University Press, New York, NY, 630 p.

Bernal, J.D. 1967. The Origin of Life. World, New York, NY,

Deamer, D.W. and G.R. Fleischaker. 1994. Origins of Life" The Central Precepts. Jones and Bartlett Publishers, Boston, MA, 431 p.

Ehrlich, H.L. 1996. Geomicrobiology (Third Edition, Revised and Expanded), Marcel Dekker, Inc., New York, New York, 719 p.

Ganong, W.F. 1997. Review of Medical Physiology, 18th edition. Appleton & Lange, Stamford, CN, 829 p.

Haldane, J.B.S. 1929. The origin of life. Reprinted in: The Origin of Life (J.D. Bernal, 1967). Weidenfeld and Nicolson, London.

Levine, J.S. and K.R. Miller. 1994. Biology: Discovering Life, Second Edition. D.C. Heath and Company, Lexington, MA 988 p.

Lovelock,J. 1988. The Ages of Gaia. Norton. New York, NY.

Palmer et al., 1998. Nature (in press).

 

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