Environmental Engineering Reference
In-Depth Information
Life's great dichotomy is between
autotrophs,
organisms that can nourish them-
selves, and
heterotrophs,
or life forms that must feed on other organisms. And for
comparisons among different organisms, their biomasses must be adjusted for
widely differing water content and expressed in absolutely dry terms or, alterna-
tively, their carbon content may be calculated using specii c conversion rates.
Autotrophs (whose sizes span nearly ten orders of magnitude, from the smallest
bacteria at 0.01
m to the tallest trees at more than 100 m) can transform inorganic
carbon (drawn overwhelmingly from atmospheric CO
2
), hydrogen (mostly from
H
2
O), and smaller amounts of nitrogen and mineral nutrients into complex organic
compounds.
Most autotrophs are
phototrophs
, organisms that rely on solar radiation to
power complex photosynthetic processes, but some bacteria are
chemoautotrophs,
which derive the needed energy by oxidation of sulfur or H
2
S, and an even smaller
group of facultative autotrophs (most notably methanogenic bacteria) can switch
to a heterotrophic existence in the presence of suitable organic substrates.
Auto-
trophic biomass
is the correct term for all tissues produced by photosynthesizers
and chemotrophs, but I will use a slightly narrower term,
phytomass,
which refers
to both planktonic organisms (cyanobacteria, coccolithophorids, silicol agellates,
diatoms) and macroscopic aquatic autotrophs (algae, sea grasses), the two categories
whose direct harvests by humans are rather limited, as well as to all terrestrial
autotrophs (that is, mostly to herbs, trees, and shrubs), whose harvests dominate
the photosynthate that is used by humans.
Because of substantial differences in the moisture content of fresh biomass—
phytoplankton cells or young plant shoots are more than 95% water, freshly cut
wood trunks contain about 50%, mature cereal grain and dry straw have only about
15% water, and some dry seeds have less than 5% moisture—a comparative account-
ing should be done only in terms of
absolutely dry biomass
. Its values are obtained
after desiccating the fresh tissues at 104°C-105°C to constant weight. Dry biomass
has the density range of 0.2-0.9 g/cm
3
, and about 95% of it is organic matter
(although minerals can make up as much as 70% of dry phytomass in some phy-
toplankton species). Conversions to absolutely dry biomass introduce two possibly
signii cant errors.
First, no conversions done on a scale larger than that of a small and fairly homo-
geneous plant community can use a single representative i gure for the water content
of fresh-weight biomass: simplifying assumptions will always be needed to choose
a single approximate value, which might easily err by 10%-15%. When I take a
closer look at crops, crop residues, and wood harvests I will explain that difi culties
μ
