Environmental Engineering Reference
In-Depth Information
ORGANISMS RESPONSIBLE FOR
DECOMPOSITION
It has been clear for over 40 years (see
Kaushik and Hynes 1971
)thatmicroorganismsare
responsible for the bulk of organic matter decay (i.e., conversion to CO
2
), and recent improve-
ments in microbial biomass and activity measurements have clarified their roles. The two
classes of organisms most relevant for POM decay are bacteria and fungi. In general, their bio-
mass is a few percent of the detrital carbon standing stock but their turnover can be rapid
(1
10 d for bacteria, several days to weeks for fungi) and so they exert a considerable demand
on organic carbon. Their relative biomass varies in a fairly consistent pattern with fungi hav-
ing by far their greatest relative abundance on large particles of litter. For instance, they out-
weigh bacteria by more than 100-fold on standing dead wetland plant material. Bacterial
biomass, relative to fungal biomass, increases as particle sizes decrease due to several
mechanisms and it is often difficult to find fungi on particles smaller than a millimeter or so.
Most fungi in decaying litter have a filamentous morphology composed of hyphae used
to penetrate plant cell walls and reproduction can't occur until enough hyphal mass has
accumulated to support the reproductive structures. The effect of particle size is much
stronger for fungi than bacteria such that even relatively small increases in abundance of
large POM in a system will allow predominance of fungal over bacterial biomass (
Findlay
et al. 2002a
). The difference in biomass across particles or ecosystems is relevant for food
webs—for example, the food web in the surface litter of a pine forest is vastly different from
the finer and more decayed humus layer (
Berg and Bengtsson 2007
;
Figure 4.5
). Moreover,
fungi and bacteria differ in their catabolic and nutrient-acquisition capacities. For instance,
fungi are more able to penetrate and degrade wood and other recalcitrant polymers such as
chitin. On the other hand, bacteria typically have more efficient nutrient acquisition and so
can draw down and assimilate nutrients at lower concentrations than most fungi.
During the decomposition process microbes convert nonliving organic carbon into micro-
bial biomass with some concurrent loss of CO
2
to respiration. The net growth efficiency
(
NGE
respiration, describes the proportion of assimi-
lated carbon ultimately available for consumption by other heterotrophs (see Chapter 3) as
opposed to lost from the food web. Assimilation of this organic carbon into microbial biomass
often requires some immobilization of inorganic nutrients from external sources (see
Chapter 7). Therefore, low growth efficiencies suggest large net losses of organic carbon from
the system and relatively little retention of inorganic nitrogen, phosphorus, and other ele-
ments in decomposer biomass. Given the importance of the NGE there has been considerable
interest in determining its values during decomposition of diverse materials under natural
conditions. Until measurement of microbial secondary production became routine (e.g.,
Gulis
and Suberkropp 2006
) most estimates were derived from lab estimates of growth on defined
(often simple) compounds. These estimates tended toward the higher end of the range (as
high as 75%), leading to overestimates of conversion of natural detrital carbon into microbial
biomass. More recently, growth efficiencies have been determined for a range of aquatic eco-
systems and values span 10% to 50% (
del Giorgio and Cole 2000
; Chapter 3) with most values
below 30%. These NGEs mean that the vast majority of assimilated detrital carbon is lost
as CO
2
and nutrient retention in microbial biomass will be smaller and more transient than
previously believed.
G
/(
G
R
)) where
G
growth and
R
5
1
5
5
