Phytotechnology and Photosynthesis 157in nature. Moreover, much of the biological pollutant abatement potential of the
system exists through the synergistic activity of the entire community and, in
purely direct terms, this largely means the indigenous microbes. Functionally,
there are strong parallels between this and the processes of enhanced rhizo-
spheric biodegradation described for terrestrial applications. While exactly the
same mechanisms are available within the root zone in an aquatic setting, in
addition, and particularly in the case of submerged vegetation, the surface of
the plants themselves becomes a large extra substrate for the attached growth of
closely associated bacteria and other microbial species. The combined rhizo- and
circum-phyllo- spheres support a large total microbial biomass, with a distinctly
different compositional character, which exhibits a high level of bioactivity, rel-
ative to other microbial communities. As with rhizodegradation on dry land,
part of the reason is the increased localised oxygenation in their vicinity and
the corresponding presence of significant quantities of plant metabolic exudates,
which, as was mentioned in the relevant earlier section, represents a major pro-
portion of the yearly photosynthetic output. In this way, the main role of the
macrophytes themselves clearly is more of an indirect one, bringing about local
environmental enhancement and optimisation for remediative microbes, rather
than being directly implicated in activities of primary biodegradation. In addi-
tion, physico-chemical mechanisms are also at work. The iron plaques which
form on the plant roots trap certain metals, notably arsenic (Otte, Kearns and
Doyle 1995), while direct adsorption and chemical/biochemical reactions play a
role in the removal of metals from the wastewater and their subsequent retention
in sediments.
The ability of emergent macrophytes to transfer oxygen to their submerged
portions is a well-appreciated phenomenon, which in nature enables them to
cope with effective waterlogging and functional anoxia. As much as 60% of the
oxygen transported to these parts of the plant can pass out into the rhizosphere,
creating aerobic conditions for the thriving microbial community associated with
the root zone, the leaf surfaces and the surrounding substrate. This accounts for
a significant increase in the dissolved oxygen levels within the water generally
and, most particularly, immediately adjacent to the macrophytes themselves.
The aerobic breakdown of carbon sources is facilitated by this oxygen transfer,
for obvious reasons, and consequently it can be seen to have a major bearing
on the rate of organic carbon biodegradation within the treatment system, since
its adequate removal requires a minimum oxygen flux of one and a half times
the input BOD loading. Importantly, this also makes possible the direct oxida-
tion of hydrogen sulphide (H 2 S) within the root zone and, in some cases, iron
and manganese.
While from the earlier investigations mentioned on plant/metal interactions
(Hutchinson 1975) their direct contribution to metal removal is small, fast-
growing macrophytes have a high potential uptake rate of some commonly
encountered effluent components. Some kinds of water hyacinth,Eichhornia