Physiological ecology: the physical environment

Topics for this class: What is physiological ecology? Organisms interact with physical environment Exchange of materials, energy with environment

studied as fluxes Organisms maintain themselves in improbable

steady-state relative to physical environment Major constraints of physical environment

include properties of water, air, soils, temperature, and light

Introduction to physiological ecology

Level of organization = individual organism Define physiological ecology: the study of the

interaction of the organism with its physical environment, and its adaptations to that environment

Physical environment not only necessitates adaptations for organisms to exist in particular environments, but it also constrains the distribution of organisms e.g., vampire bat, desmodus rotundus Its northern distribution = 10º C Jan. minimum isotherm

(next slide)

Biological systems (e.g., organisms) interact with physical environment

All organisms are constrained by physical, chemical environment (i.e., according to physical laws)

Physical world provides window of conditions within which life can exist (e.g., temperatures at which water is in liquid state)

Physical world also provides life-giving conditions: E.g., portions of electromagnetic spectrum -->

ultimate form of energy for most life (all but deep sea, hot vent organisms)

Soils and water contain various elements needed by living organisms (e.g., C, O, N, P, Ca, Na, Fe, Mg)

Interaction with physical environment also implies alteration of it by organism

E.g., Life first arose in a reducing atmosphere (<1% O2), but living things created an oxidizing atmosphere (ca. 21% O2) within Proterozoic Era

Another example: Organisms continually create soils by weathering rocks: E.g., Negev Desert snails weather rocks at rate of 0.7-1 metric tons/ha/yr, by eating endolithic lichens (growing within rocks)

Exchanges of energy, materials between environment & organisms depend on Flux

In general, Physiological (biological) Flux = Surface area X Gradient X Conductance Units are material (or energy) per time per area (e.g.,

radiant energy flux expressed as Watts m-2, and Watts = Joules of energy per second)

Gradient is the difference in material concentration or energy across the boundary (between organism and external environment)

Conductance = ease of material (or energy) crossing barrier

Flux can also refer to exchanges of energy, nutrients among pools within an ecosystem

Distinguish physical vs. biological flux Physical (= passive) flux is the natural

thermodynamic tendency of materials or energy to move from areas of high concentration to areas of low concentration (effect: increase entropy in system). This flux is proportional to gradient.

Biological (= active) flux is the property of living systems to accumulate substances (and energy) against a physical gradient, and thus to remain in improbable state (distinct from surrounding environment)

Flux example: gas exchange in bird eggs Wedge-tailed shearwater has same egg size, vapor

pressure in nest as chicken. OK, so?

Lessons from shearwater example:

Constraints of physical laws concerning gas exchange (flux)

Tradeoffs

What are some of major constraints of physical environment? First, Water Solvent

Salts dissociate in water, some readily, --> anion availability to organisms (e.g., Cl-, S04

-, HCO3-)

Transport - nutrients, hormones, organic substances, toxins

Leaches - elements and nutrients from foliage, detritus, rocks (--> weathering), and soil

Water bodies differ in ionic composition (next slide) Seawater, accumulating ions over eons, dominated

by Na+, Cl-, and significant amounts Mg2+, SO42-

Water over limestone dominated by Ca2+, HCO3-

(hard water) Water over granite contains lower ion concentrations

Living systems require particular nutrients, elements

Typically organisms require O2, N, P, K, Ca, Na, Mg, S, Fe

How are these different elements used by organisms?

Why are N, P often limiting to growth of organisms?

Concentration of hydrogen ions in water is particularly important biologically

Concentration of hydrogen ions = acidity Measured as pH (logarithmic scale) At low pH (3-4), compounds that are highly toxic to

living organisms become soluble (e.g., aluminum compounds), causing biologically “dead” lakes and rivers

Hydrogen ions also bind tightly to clay particles in soil, “flushing” other cations like Na, Ca, Mg from soil, thereby decreasing soil fertility

Physical properties of water, cont. Surface tension of water is basis for transpiration Define transpiration = evaporation of water from

leaves, via stomata (specially adapted holes, typically on leaf undersides, designed for gas exchange) Evaporation of water generates negative pressure

within leaves, which pulls more water up from ground Up to 50% or more of water that falls can be transpired

by plants Plants pull water from ground in transport vessels

(xylem tissue, formed from hollow cells, end-to-end) Water moves in these tubes via capillary action This transport of water moves nutrients to leaves, stems

Surface tension of water also influences water availability in different soil types Consider good soil for plants (loam) = mixture of fine

clay, intermediate sized silt, coarse sand Soil properties determine curve of soil water potential

versus soil water percent (next slide) Water potential measured in atmospheres (= strength of

force needed to pull water from soil) Wilting point (15 atmospheres) determined by strong

surface tension of water, close to particles Field capacity (ca. 0.1 atmospheres) = force of gravity Saturation capacity determined by size interstices in soil Available water to plants determined by field capacity -

wilting point 7-32% in loam; ca. 3-10% in sand

From Ricklefs, R.E. Ecology, 3rd Ed., W.H. Freeman

loam

Water availability to plants depends on surface tension, soil structure Different soil types,

with different particle sizes (and size distributions) have different soil water availability

Sandy soils (e.g., North Shore) hold less water than silts or clays (e.g., MS delta)

Available water ca. 3-10% in sand, much less than in silt, loam, or clay (25-55%)

Some other properties of water

Most dense at 4º C Thus ice at 0º C is less dense, and floats This allows organisms to be active under water

even when surface frozen High specific heat of water, high latent heat -->

resistance to changes in state (liquid to gas, liquid to solid) Water thus tends to buffer temperatures, and large

bodies of water tend to ameliorate climate

Biologically relevant properties of light Plants absorb light only in limited range of

frequencies (400-700 nm = Photosynthetically Active Region, i.e., PAR)

Shorter wavelengths (ultraviolet) have higher energy, potentially damaging to organisms Ozone in upper atmosphere absorbs ultraviolet

radiation, protecting living things Ozone hole results from loss of ozone due to

chlorofluorocarbons (CFC’s), produced by humans Longer wavelengths (infrared) have too little

energy to power biochemical reactions necessary to life

Different plant pigments absorb light in different parts of electromagnetic spectrum--and reflect colors that they don’t absorb: chlorophylls green, carotenoids yellow-red

Water tends to absorb longer wavelengths, scatter shorter ones; thus greens penetrate deepest

Surface plant such as green alga (Ulva) thus has pigments like terrestrial plants; deeper water red alga (e.g., Porphyra) absorbs most efficiently in the green wavelengths

Biologically relevant properties of air Composed of different substances: 78% N2, 21% O2,

0.03% CO2, traces of CH4, N2O, etc. Air less viscous, less buoyant than water (organisms

move easily thru it, but need more support) Diffusion of gases much more rapid in air than water

O2 diffuses rapidly in air (solubility 0.21 cm3/cm3 air); slowly in water (solubility 0.01 cm3 O2

/cm3 water)

O2 often limits organisms in water-saturated environments, especially where decay organisms (heterotrophs like bacteria) take up O2

This leads to anoxic conditions (like sulfur-stink of mucks in Lafitte Park)

CO2, by contrast, is rare, often limiting, in air (0.03%); dissolves readily in water (carbonic acid, bicarbonate)

Many plants tend to have great difficulty getting enough CO2, when stomata are open enough to transpire water; this is particular problem in desert environments (see next lecture)

Chemistry of life

Living systems based largely on chemical reactions of C, O2, H

Plants use photosynthesis to convert carbon from oxidized form (CO2) to reduced form of organic carbon, e.g., sugars, starches

•Reduction reaction less favorable energetically than oxidation--former requires energy from sun via chlorophyll molecules as energy-absorbers;

•energy of living things stored in reduced carbon bonds, e.g., carbohydrates

Respiration is reverse of photosynthesis

Respiration involves coupled oxidation & reduction (redox) half reactions, the reverse of those in photosynthesis O2 + 4e- + C4+ = CO2; Reduction half-reaction (oxygen is

reduced by gain of electrons) CH2O = C4+ + H2O + 4e-; Oxidation half-reaction (carbon is

oxidized) Coupled together: CH2O + O2 = CO2 + H2O

Overall reaction is favorable (net release of energy) because reduction of oxygen (top step) releases more energy than reduction of carbon; and oxidation of carbon (bottom step) releases more energy than reduction of oxygen requires

Temperatures of living things Temperatures of living things determined by range of

temperatures at which water is in liquid phase Few organisms can survive temperatures > 45ºC,

because of protein denaturation at high temperatures Some organisms can exist at higher temperatures due

to particularly heat-stable proteins Most organisms cannot tolerate body (cell)

temperatures below freezing, because of damage to cells from ice crystals Some organisms can exist at slightly lower

temperatures using antifreezes such as salts, glycerol Increased temperature sets higher rate of chemical

reactions (2-4 times increase in rate per 10ºC)

Conclusions: We can learn much about the distribution and

properties of organisms by understanding the physical environment in which they live

Organisms cannot escape the physical-chemical laws of the universe, but to a limited extent they can evolve modifications that take advantage of peculiar conditions--we’ll examine some adaptations of organisms next class