Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 849

Assimilation

Plant
Lignocellulosics
Carbohydrates
Proteins, Fats

Animal
Carbohydrates
Proteins, Fats

Death

Death

Nonliving
Organic matter

Excretion

Fossilization

Oxygen gas
O 2

Sequestered carbon
Peat, coal, oil, gas

Urea
H 2 NCONH 2

Carbon dioxide
CO 2

with Oceans
or Soil

Carbonate CO 2-
Bicarbonate HCO 3

3

-

FIGURE 16.26 Simplified schematic of the carbon and major oxygen cycles.

The simplistic root for bioinstability is the difference in specific growth rate and mutation.
Biostability is of concern for genetically engineered cells. Genetically engineered cells for
industrial applications are commonly exploiting the cells to produce protein or products
that are not generic to the host cells. As a result, the growth characteristics are altered as
compared with the host cells. Genetic evolution back to the generic cell function looses the
desired traits imposed to the cells. The genetic evolution can be modeled quite accurately
with kinetic considerations.
Populations containing multiple species are important in natural ecosystems, well-
defined processes, wastewater treatment, and systems using genetically modified cells.
Some examples of interactions among these species are competition, neutralism, mutualism,
protocooperation, commensalism, amensalism, predation, and parasitism. In real systems,
several modes of interaction may be present. Mathematical analyses can be used to show
that neither pure competition nor pure mutualism gives a stable steady state in a chemostat.
Predator and prey model shows that while there are no stable steady states, the bioprocess
can be sustainable and the final prey and predator populations oscillate in a confined cycle.
Continuous culture can be employed to screen organisms of certain traits based on the
growth instability. Spatial heterogeneity, dynamic fluctuations, and the addition of other
interactions can lead to the sustained coexistence of species with competitive or mutualistic
interactions.
One of the major process uses of mixed cultures is wastewater treatment. The activated-
sludge system is commonly employed in treating wastewaters. Such a system can be consid-
ered a chemostat with cell recycle under aerobic conditions.