In Dissolved organic carbon available for uptake

In most streams and rivers, a huge
percentage of the reduced carbons are found in dissolved organic matter (DOM) (Volk, Volk, & Kaplan, 1997). According to Petersen , Cummins, & Ward, (1989), biotic
and abiotic means help the reduced carbon can go into association or dissociation
with water. On deposition, transported organic matters are fed upon by biofilm
microorganisms (Kuserk, Kaplan, & Bott, 1984). “Lability”
determines how rapid organic matter is taken up, so that biofilm microbes are
more likely to use up most labile molecules (Cherrier, Bauer, Druffel, Coffin, & Chanton, 1999). The use of
dissolved organic matter by biofilm microorganisms largely depends on the size
of the organic matter (Amon & Benner, 1996).
In contrast, for epilithic bacteria that feed on dissolved organic carbon
(DOC), the DOC usage is influenced by the organic compounds composition (Ford & Lock, 1985). Dissolved organic
carbon available for uptake by bacteria in streams and rivers also depends on
the ratio of H/C:O/C within the organic carbon (Meyer,
1994). Although most of the dissolved organic carbon that are transported
into these water bodies contains mainly polymeric molecules in addition to
humic materials 48. For humic substances and polymeric molecules to be taken
up by heterotrophs, the materials must undergo some form of extracellular enzyme
activities (Chrost, 1990). Research has
it that in streams and rivers, labile compound recycling tends to occur a
little faster when compared to highly refractory materials which often need
further transport before they are acted upon (Kaplan
& Newbold, 2002).

Naiman, Mellilo, Lock,
Ford, & Reice, (1987) explained
that organic matter is generally acted upon by benthic community particularly
within rivers. Hence, the portion of the surfaces beneath the water is often
covered by assemblages of structured autotrophic–heterotrophic buried in
polysaccharidic substances, thus resulting in biofilm (Lock,1993). It is understood that biofilms serve as points where
dissolved organic carbon within rivers and streams are found (Battin, Batturini, & Sabater,1999), hence,
biofilms have a tremendous effect on how carbon is recycled in these water
bodies. There is also a possibility of carbon cycling within biofilms (Wetzel, 1993). This is because of the extreme
closeness between algae community as well as the heterotrophic community in
biofilms that are merged together. This closeness causes the algae materials to
be used up by microbes within the biofilm.

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Furthermore, Romani & Sabater, (1999) opined that the accumulation of algae
aids organic matter usage by heterotrophic communities. This occurs when there
is an increment in bacteria-available substrate (Romani
& Sabater, 1999). For algae (epilithic), there is a stimulation of
bacteria growth on advanced phases. However, epilithic bacteria are controlled
mainly by labile DOC (Sobczak, 1996). Recent
research in the field of hydrology shows that organic carbon produced when
plants manufacture their food and subsequent usage of this carbon by bacteria
all ends up in river/stream sediments (Goto,
Mitamura, & Hisayoshi, 2001). Further studies have also been carried
out on how DOC present in biofilms are taken up by organisms within the biofilm
(Kaplan & Bott, 1983), however,
research is yet to reach an advanced stage with respect to how biofilm
structure impacts DOC uptake. As a result of this development, this study tries
to look critically into this impact and to verify whether dissolved organic
carbon in rivers and streams have their roots in biofilms or whether biofilms
are merely parts of the fluvial system that sink dissolved organic carbon (Romani, et al., 2004). It should however be
clear that the structure of biofilm and the level to which they accumulate biomass
are factors that should be borne in mind (Romani,
et al., 2004). Current research in line with this question as put
forward by Romani, et al., (2004) has
shown that biofilms that develop under sufficient light are likely to possess
minute usage of dissolved organic carbon which is a factor of improved level of
dissolved organic carbon produced by algae for uptake by biofilm heterotroph),
in contrast, biofilms developed under poor light availability are capable of
using up all the available dissolved organic carbon (Romani, et al., 2004). Hence, the level of presence of carbon and
nitrogen which serve as biofilm structural features as well as the various
biofilm microbial organisms all have an impact of the amount of dissolved
organic carbon in any fluvial system. This explains why these factors also
contribute to how dissolved organic carbon is recycled (Romani, et al., 2004).

In a bid to prove or even disprove
their hypotheses, (Romani, et al., 2004) carried
out an experiment in an open channel as well as in a dark pipe gotten
from a river. The open channel was made to pass through rays of light, so that
autotrophic organisms begin to grow. Only heterotrophs develop within the dark
pipe as there is no light penetration. According to (Romani, et al., 2004), the two differing channels gave room for the study
of movement of dissolved organic transport and structure of the biofilm.
The result shall be discussed in details under the methodology section of this