Chapter 3: Adaptation to Aquatic and Terrestrial Environments

Chapter 3: Adaptation to Aquatic and Terrestrial Environments Robert E. Ricklefs The Economy of Nature, Fifth Edition

Chapter Overview - Basics • The physical world both provides the context for life and constrains its existence. • A world of environmental factors... • resources: water, minerals and food items • conditions: temperature and relative humidity • Most factors have extremely wide ranges: • each type of organism is typically adapted to a narrow range of each factor

Chapter Overview - Regulation • Organisms typically contrast with their external environments: • internal conditions are maintained +/- constant • fluxes of heat and substances must be regulated • but organisms are open systems... • resources must be acquired • wastes must be eliminated • How do organisms accomplish this?

Chapter Overview - Bottom Line • It is important for us to understand the mechanisms organisms use to interact with their environment. • This understanding may lead to insights: • why organisms are specialized • why organisms have specific geographic distributions • why certain adaptations are associated with certain environments

What’s next? • This chapter examines adaptations by considering various challenges facing organisms, for example: • how do plants acquire water and nutrients from soils and transport these? • how do plants carry out photosynthesis under varied environmental conditions? • how do plants and animals cope with extremes of temperature, water stress, and salinity?

Availability of Soil Water • Water molecules are attracted to: • each other (causes surface tension) • surfaces (causes capillary action) • When a soil is saturated and excess (gravitational) water drains: • remaining water exists as thin films around soil particles (mineral and organic) • the greater the area of such particles (as in clayey soils), the more water the soil retains

All soil water molecules are not equal. • It’s all a matter of physical attraction... • the closer a water molecule is to a soil particle, the greater the force with which it is attracted • this force is the matric potential of the soil, contributing to the overall water potential • matric potentials (units are MPa or atm) are considered increasingly negative as they represent greater attractive forces

It’s all a matter of potential... • Soil water potential is: • usually dominated by matric forces • determined as the force required to remove the most loosely bound water molecules • Typical “benchmark” values are: • -0.1 atm (field capacity) • -15 atm (wilting point) • -100 atm (exceedingly dry soil)

Plants obtain water from the soil. • How do water molecules move? • in the direction of more negative potential • across most biological membranes • Why does water move from the soil into plant roots? • water potential in cells of the root hairs is more negative than that in the soil • negative potential in root cells is generated mostly by solutes -- osmotic potential

Membranes are selectively leaky. • Can solutes exit root cells as readily as water enters? • no, internal and external concentrations would equilibrate and osmotic potential gradient would disappear • cell membranes are semipermeable; large molecular weight solutes (carbohydrates and proteins) cannot readily leave the cell

So why does water move into roots? • Internal (cellular) osmotic potential is more negative than external (soil) matric potential, up to a point: • root hair cells with 0.7 molar concentration of solutes maintain inward flux of water against a soil matric potential as low as -15 atm: • as soil becomes drier, water flux ceases and may reverse, leading to wilting and death • desert plants may obtain water to soil matric potentials as low as -60 atm (high solute conc.)

Moving Water from Roots to Leaves • Once water is in root cells, then what? • water moving to the top of any plant must overcome tremendous forces caused by gravity and friction in conducting elements (xylem): • opposing force is generated by evaporation of water from leaf cells to atmosphere (transpiration) • water potential of air is typically highly negative (potential of dry air at 20 oC is -1,332 atm) • force generated in leaves is transmitted to roots -- water is drawn to the top of the plant (tension-cohesion theory)

Adaptations to Arid Environments 1 • Most water exits the plant as water vapor through leaf openings called stomates: • plants of arid regions must conserve limited water while still acquiring CO2 from the atmosphere (also via stomates) - a dilemma! • potential gradient for CO2 entering plant is substantially less than that for water exiting the plant • heat increases the differential between internal and external water potentials, making matters worse

Adaptations to Arid Environments 2 • Numerous structural adaptations address challenges facing plants of arid regions by: • reducing heat loading: • increase surface area for convective heat dissipation • increase reflectivity and boundary layer effect with dense hairs and spines • reducing evaporative losses: • protect surfaces with thick, waxy cuticle • recess stomates in pits, sometimes also hair-filled

Plants obtain mineral nutrients from soil water. • Nutrients must move from the soil solution into cells of root hairs… • a nutrient element moves passively (via diffusion) into root when its concentration in soil water exceeds that of root cells • when nutrient concentration in soil water is lower than that in roots, active uptake (energy-demanding) is essential

Other Plant Strategies for Obtaining Nutrients • Enlist partners! • many plants have intimate associations (symbioses) with fungi -- fungal partners enhance mineral absorption • Regulate growth! • plants of nutrient-poor soils typically: • grow slowly, maintain leaves for multiple growing seasons (evergreenness), and store surplus • shift growth toward more root and less shoot

Plant Mineral Nutrition - a Case Study in Patchiness • Distributions of nutrients in soils is highly patchy (heterogeneous) - how does such patchiness affect plant mineral nutrition? • ragweed and pokeweed plants, when grown in monoculture, performed best when soil nutrients were patchy instead of homogeneous • when these plants were grown together, advantage of patchy nutrients disappeared

Photosynthesis varies with levels of light. • Photosynthetic rate is a function of light intensity (proportional to light intensity at low light levels, leveling off at high levels): • in dim light, plants fail to offset respiratory losses with photosynthetic gains • as light intensity increases, a break-even point (losses offset by gains) is reached, called compensation point • at saturation point, further increase in light level does not stimulate further photosynthesis

Plants modify photosynthesis in stressful environments. • Fixation of atmospheric carbon into glucose (dark reactions of photosynthesis) is accomplished by Calvin cycle: • first step involves synthesis of two 3-carbon molecules (PGA) from RuBP and CO2: • CO2 + RuBP  2PGA • enzyme accomplishing this is RuBP carboxylase...

C3 Photosynthesis • C3 plants depend solely on Calvin Cycle for photosynthetic CO2 fixation. • C3 plants have certain disadvantages: • RuBP carboxylase has low affinity for its substrate, CO2 • RuBP carboxylase also catalyzes the oxidation of PGA when leaf [CO2] low and [O2] high, especially at high temperatures

C4 Photosynthesis • C4 plants add an additional carboxylation step to the Calvin cycle: • CO2 + PEP  OAA • carbon is fixed to OAA in mesophyll cells, then shuttled to bundle sheath cells where CO2 is unloaded for use in Calvin cycle • PEP regenerated in bundle sheath cells is reused (shuttled back to mesophyll)

Advantages of C4 Photosynthesis • Biochemical and anatomical features lead to photosynthetic advantages: • Calvin cycle isolated from high O2 levels while supplied with high levels of CO2 - leads to much more efficient operation • PEP carboxylase has high affinity for CO2, thus permitting plant to obtain CO2 while increasing stomatal resistance to water loss • these advantages come at an energy cost, but are especially helpful under conditions of high light, high temperature and water stress

Photosynthesis in Hot/Arid Environments • C4 photosynthesis favored as environmental conditions become increasingly hot/arid: • latitudinal gradients quite conspicuous: C4 plants become much more common in transect from polar regions toward equatorial regions • but, C3 species are favored in cooler, moister habitats because: • disadvantages of C3 photosynthesis are lessened • C3 approach is biochemically more energy-efficient

Carbon Assimilation in CAM Plants • Some plants (succulents in several families) add a temporal “twist” to C4 process... • CO2 is acquired at night when evaporative demand is lowest • carbon from CO2 is stored in 4-C organic acids (such as OAA) • stored carbon is used by Calvin cycle during daylight hours when energy is available for dark reactions

Balancing Salt and Water • Osmotic regulation is not just a problem for plants • Aquatic animals are rarely in equilibrium with their surroundings: • fresh-water fish are hyperosmotic (internal salt concentration higher than that of medium) • marine fish are hypo-osmotic (internal salt concentration lower than that of medium)

Ion retention is critical to freshwater organisms. • Freshwater fish must eliminate excess water and selectively retain dissolved ions: • they gain water by osmosis • they eliminate excess water in their urine • their kidneys selectively retain dissolved ions • active uptake of ions via gills is also important

Water retention is critical to marine organisms. • Saltwater fish must retain water and excrete excess ions: • they tend to lose water to surrounding sea water and must drink to replace this • excess salt must be excreted from gills and kidneys • some fish (sharks and rays) raise osmotic potential of blood by retaining waste nitrogen as urea -- their high internal osmotic potential matches that of seawater

Water and Salt Balance in Terrestrial Plants • Plants take up excessive salts along with water, especially in saline soils. • plants must actively pump salts back into soil • In coastal mudflats, mangroves must acquire water while excluding salts. They: • establish high root osmotic concentrations to maintain water movement into root • exclude salts at the roots and also excrete excessive salts from specialized leaf glands

Water and Salt Balance in Terrestrial Animals • Terrestrial animals must eliminate excess salts acquired in diet: • copious amounts of water can serve to flush excess salts in more humid climates • where water is scarce, other options exist: • desert mammals produce highly concentrated urine • birds and reptiles eliminate excess salts via salt glands

Animals excrete excess nitrogen. • Carnivorous animals acquire excess nitrogen from their high-protein diet: • excess nitrogen must be eliminated: • aquatic animals eliminate nitrogen as ammonia • terrestrial animals cannot afford copious amounts of water necessary for elimination of ammonia • mammals excrete urea • birds and reptiles excrete uric acid, which can be eliminated with very little water

Conserving Water in Hot Environments 1 • Animals of deserts may experience environmental temperatures in excess of body temperature: • evaporative cooling is an option, but water is scarce • animals may also avoid high temperatures by: • reducing activity • seeking cool microclimates • migrating seasonally to cooler climates

Conserving Water in Hot Environments 2 • Desert plants reduce heat loading in several ways already discussed. Plants may, in addition: • orient leaves to minimize solar gain • shed leaves and become inactive during stressful periods

The Kangaroo Rat - a Desert Specialist • These small desert rodents perform well in a nearly waterless and extremely hot setting. • kangaroo rats conserve water by: • producing concentrated urine • producing nearly dry feces • minimizing evaporative losses from lungs • kangaroo rats avoid desert heat by: • venturing above ground only at night • remaining in cool, humid burrow by day

Organisms maintain a constant internal environment. • An organism’s ability to maintain constant internal conditions in the face of a varying environment is called homeostasis: • homeostatic systems consist of sensors, effectors, and a condition maintained constant • all homeostatic systems employ negative feedback -- when the system deviates from set point, various responses are activated to return system to set point

Temperature Regulation: an Example of Homeostasis • Principal classes of regulation: • homeotherms (warm-blooded animals) - maintain relatively constant internal temperatures • poikilotherms (cold-blooded animals) - tend to conform to external temperatures • some poikilotherms can regulate internal temperatures behaviorally, and are thus considered ectotherms, while homeotherms are endotherms

Homeostasis is costly. • As the difference between internal and external conditions increases, the cost of maintaining constant internal conditions increases dramatically: • in homeotherms, the metabolic rate required to maintain temperature is directly proportional to the difference between ambient and internal temperatures

Limits to Homeothermy • Homeotherms are limited in the extent to which they can maintain conditions different from those in their surroundings: • beyond some level of difference between ambient and internal, organism’s capacity to return internal conditions to norm is exceeded • available energy may also be limiting, because regulation requires substantial energy output

Partial Homeostasis • Some animals (and plants!) may only be homeothermic at certain times or in certain tissues… • pythons maintain high temperatures when incubating eggs • large fish may warm muscles or brain • some moths and bees undergo pre-flight warm-up • hummingbirds may reduce body temperature at night (torpor)

Delivering Oxygen to Tissues • Oxidative metabolism releases energy. • Low O2 may thus limit metabolic activity: • animals have arrived at various means of delivering O2 to tissues: • tiny aquatic organisms (<2 mm) may rely on diffusive transport of O2 • insects use tracheae to deliver O2 • other animals have blood circulatory systems that employ proteins (e.g., hemoglobin) to bind oxygen

Countercurrent Circulation • Opposing fluxes of fluids can lead to efficient transfer of heat and substances: • countercurrent circulation offsets tendency for equilibration (and stagnation) • some examples: • in gills of fish, fluxes of blood and water are opposed, ensuring large O2 gradient and thus rapid flux of O2 into blood across entire gill structure • similar arrangement of air and blood flow in the lungs of birds supports high rate of O2 delivery

Conservation and Countercurrents • Countercurrent fluxes can also assist in conservation of heat; here are two examples: • birds of cold regions conserve heat through countercurrent circulation of blood in legs • warm arterial blood moves toward feet • cooler venous blood returns to body core • heat from arterial blood transferred to venous blood returns to core instead of being lost to environment • kangaroo rats use countercurrent process to reduce loss of moisture in exhaled air

Each organism functions best under a restricted range of conditions. • Organisms function best in a relatively narrow range of conditions, the optimum: • optimum is a result of natural selection for biochemical properties of enzymes and lipids, as well as internal structures, body form, etc. • such specialization precludes efficient function across wide ranges of conditions, which would be expensive and compromise optimal function

Compensation is possible. • Many organisms accommodate to predictable environmental changes through their ability to “tailor” various attributes to prevailing conditions: • rainbow trout are capable of producing two forms of the enzyme, acetylcholine esterase: • winter form has highest substrate affinity between 0 and 10oC • summer form has highest substrate affinity between 15 and 20oC

Adaptation is the key to under-standing success of organisms. • Organisms living in different environments function equally well under their constraints: • Antarctic and tropical fish both swim actively! • Acclimatization permits some degree of adjustment to changing conditions: • rainbow trout example • rapid adjustment of O2 transport capabilities to changing partial pressure of O2 with elevation in vertebrates, including humans

Summary • The mechanisms by which organisms interact with their physical environment help us understand why organisms are specialized to narrow ranges of conditions and how adaptations of morphology and physiology are associated with certain conditions.

Chapter 3: Adaptation to Aquatic and Terrestrial Environments