Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 165

ability to photosynthesise and the heterotrophe's to respire. While a deeper cell
permits greater resident biomass, thus elevating the numbers of micro-organisms
available to work on the effluent, beyond a certain limit, the law of diminish-
ing returns applies in respect of light available to algae in the lower reaches.
Warmer temperatures increase metabolic activity, at least within reason, and the
rate of straightforward chemical reactions doubles per 10 ◦ C rise, but at the same
time, elevated water temperatures have a reduced oxygen carrying capacity which
affects the bacterial side of the equilibrium mentioned earlier. As with so much
of environmental biotechnology, a delicate balancing act is required.
After a suitable retention period, which again depends on the character of the
effluent, the design and efficacy of the treatment pond and the level of clean-up
required, the water is discharged for use or returned to watercourses. Obviously,
after a number of cycles, algal and bacterial growth in a functionally eutrophic
environment would, as discussed earlier in the section, begin to inhibit, and then
eventually arrest, the bio-treatment process. By harvesting the algal biomass, not
only are the contaminants, which to this point have been merely biologically iso-
lated, physically removed from the system, but also a local population depression
is created, triggering renewed growth and thus optimised pollutant uptake. The
biomass recovered in this way has a variety of possible uses, of which compost-
ing for ultimate nutrient reclamation is without doubt the most popular, though
various attempts have also been made to turn the algal crop into a number of
different products, including animal feed and insulating material.
Carbon sequestration
Their use as a carbon sink is a simpler process, only requiring the algae them-
selves. However, even as a functional algal monoculture, just as with the joint
algal/bacterial bio-processing for effluents, without external intervention to limit
the standing burden of biomass within the bioreactor, reduced efficiency and,
ultimately, system collapse is inevitable.
In nature, huge amounts of many elements are held in global reservoirs, reg-
ulated by biogeochemical cycles, driven by various interrelated biological and
chemical systems. For carbon, a considerable mass is held in organic and inor-
ganic oceanic stores, with the seas themselves being dynamic and important
component parts of the planetary carbon cycle. Marine phytoplankton utilise car-
bon dissolved in the water during photosynthesis, incorporating it into biomass
and simultaneously increasing the inflow gradient from the atmosphere. When
these organisms die, they sink, locking up this transient carbon and taking it
out of the upper oceanic 'fast' cycle into the 'slow' cycle, which is bounded by
long-term activities within the deep ocean sediments. In this respect, the sys-
tem may be likened to a biological sequestration pump, effectively removing
atmospheric CO 2 from circulation within the biosphere on an extended basis.
The number, mass and extent of phytoplankton throughout the world's seas thus
provide a carbon-buffering capacity on a truly enormous scale, the full size of
which has only really become apparent within the last 20 years, with the benefit