Animals and Oxygen

All multicellular animal life is dependent on the uptake of oxygen during respiration. Thus, any change in the level of O2 might be expected to affect animal physiology. Physiology includes such things as thermal balance, respiratory gas exchange, and animal flight. Animals with lungs have an advantage over those without lungs because they have the ability to change breathing rates and to evolve bigger or smaller lungs according to the availability of O2. However, in animals that metabolize by passive diffusion of O2 into their bodies, respiration should be readily affected by changes in atmospheric O2. This is especially true of insects (Vogel, 1994; Dudley, 2000). Larger insects, with a lower surface area-to-mass ratio, should respire more slowly at a fixed level of O2. However, if O2 concentration were to rise, for a constant body size and shape, this would allow increased respiration for the larger sizes. Proof that the metabolism of dragonflies is O2-limited during flight has been demonstrated by Harrison and Lighton (1998). This suggests that an elevated atmospheric O2 concentration could allow for an increase in the size of this insect.

In the fossil record, giant insects, including dragonflies (Graham et al., 1995; Dudley, 1998; Lane, 2002), are confined to the Permo-Carboniferous time of hypothesized high O2 level. The dragonfly fossils reach wing spans up to 80 cm, and their thoraxes are so large that they could not obtain enough O2, under present-day levels of 21% O2, to be able to fly (Lane, 2002). Along with dragonflies, there are unusually large amphibians, mayflies, millipedes, hexapods, and arachnids confined to this same time span (Dudley, 1998), and these organisms also metabolize by passive diffusion. Thus, animal fossils provide further evidence for the hypothesized high O2 concentrations during the Permo-Carboniferous.

Further proof that insects will adapt to an increase in O2 concentration by increasing in size is shown by the experiments of R. Dudley (see Berner et al., 2003). Dudley exposed five generations of Drosophila fruit flies to hyperbaric (1.1 atm) conditions such that the partial pressure of O2 was 0.23 atm (equivalent to 23% O2 at one atmosphere total pressure). Each successive adult generation was found to become bigger, and the mean size eventually leveled off at a constant value, indicating acclimitization to elevated O2. Restoring the sixth generation to ambient conditions allowed for the exclusion of intragenerational phenotypic plasticity. Results shown in table 6.1 indicate a statistically significant increase in mean size after the six generations for both males and females. This is in comparison to a normobaric (21% O2, 1.0 atm total pressure) control run simultaneously. It is unfortunate that the experi-

Table 6.1. Results of experiments after subjecting five generations of Drosophila melanogaster to PO2 = 0.23 atm at a total pressure of 1.1 atm.

Body mass (mg)

Po2 (atm)

Gender

No. of samples

(±1 SD)

0.21

Males

30

0.64 ± 0.07

0.23

Males

36

0.73 ± 0.12

0.21

Females

32

1.07 ± 0.19

0.23

Females

31

1.18 ± 0.24

Experiments of R. Dudley from Berner et al. (2003).

Experiments of R. Dudley from Berner et al. (2003).

ment was not continued and the fifth generation of fruit flies subjected to further increases of O2 partial pressure to see if additional increases in mean size would ensue. In a separate experiment, subjecting a single generation of Drosophila to a sudden large increase to 40% O2 did not result in a consistent increase in mean size (Frazier et al., 2001), probably due to the inability of the organisms to adjust this quickly to the adverse effects of hyperoxia such as excessive OH radical production.

These exploratory studies suggest a need for further experimental work on the growth and metabolism of insects and other diffusion-respiring animals under elevated O2. For this purpose an initial study on amphibian physiology and growth under elevated O2 is underway at Yale University. Results may help explain the presence of giant amphibians at the time of hypothesized maximal atmospheric oxygen.

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

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