Environmental Engineering Reference
In-Depth Information
reduced S, Fe
2
þ
,Mn
2
þ
, and H
2
and CH
4
. Des Marais
(2000) estimated that microbes dependent on hydro-
thermal energy could sustain annual fixation of less than
25 Mt C, whereas oxygenic photosynthesizers, able to
tap a virtually unlimited supply of hydrogen from water,
could eventually fix more than 100 Gt C/a. Descendants
of the earliest cyanobacteria continue to fill almost every
aquatic and terrestrial niche. Unicellular Prochlorococcus,
the smallest and the most abundant photosynthesizer
in the ocean, discovered only in 1988 (Chisholm et al.
1988), contributes 30%-80% of total primary production
in oligotrophic waters.
Grypania, the first fossil alga, dates to 2.1 Ga bp,
which means that eukaryotic phototrophs had to evolve
at some time during the preceding 500 Ma. Their
phototrophic ability was not an original evolutionary
achievement but an import (Nitschke et al. 1998): algal
chloroplasts were derived directly from cyanobacteria
through a primary endosymbiosis. The fossil record
shows microbial mats declining, and green and red
algae increasing in abundance only about 1-0.9 Ga
bp. Both bryophytes (nonvascular liverworts, hornworts,
and mosses without distinctive water-conducting
tissues) and tracheophytes (vascular lycopods, horsetails,
ferns, and seed plants) evolved from charophytes, fresh-
water green algae whose fossils are documented from
more than 600 Ma bp. Liverworts, the earliest land
plants, may have appeared during the mid-Ordovician
period, after 450 Ma bp, and fungus-plant symbioses
were
port, structural tissues that enabled them to reach un-
precedented heights, roots and leaves with stomata for
respiratory exchange of gases, and specialized sexual and
spore-bearing organs and seeds; cellulose, a microfibrillar
polysaccharide, emerged as the dominant structural com-
pound that now accounts for about half of all phytomass.
Angiosperms (flowering plants) have been dominant
since the mid-Cretaceous period, about 90 Ma bp. Pho-
tosynthesis of complex organic compounds is energized
by photons absorbed by a small group of plant pigments.
The synthetic sequence always shares the core of an intri-
cate multistep reductive pentose phosphate (RPP) cycle;
its rates are determined by the same limiting factors in
all terrestrial environments; and the bulk of the newly
formed phytomass has a highly uniform energy density.
After describing the fundamentals of the photosynthetic
process I appraise the productivities of major biomes
and the magnitudes of standing phytomass in principal
ecosystems.
3.1 Photosynthetic Pathways
The standard scientific description of photosynthesis is as
a process of CO
2
fixation and O
2
evolution. As Tolbert
(1997) noted, it is not easy to overcome this dogma,
which goes back to the mid-nineteenth century. Photo-
synthesis is actually a complex process of O
2
and CO
2
exchange that is energized by the absorption of specific
wavelengths of solar radiation (D. O. Hall et al. 1993;
Raghavendra 1998; Hall and Rao 1999; Lawlor 2001;
K
ˆ
2001). Every photosynthetic sequence starts with
light absorption by pigments in disk-like thylakoid mem-
branes inside chloroplasts in specialized leaf cells (in some
species also stem cells). Excitation of pigments (chloro-
phylls a and b, bacteriochlorophyll, and carotenoids)
takes place in two different reaction centers: photosystem
essential
in
colonizing
nutrient-poor
and
desiccation-prone environments (Blackwell 2000).
Following rapid diversification and diffusion, land
plants became the dominant photosynthesizers by the
end of the Devonian period, 360 Ma bp (Bateman et al.
1998). They acquired stems with complex fluid trans-
