Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 22

Among the different limiting factors, the most popular, until now, are
nitrogen and phosphorus (Table 2.1). It is, however, worth noting, that the
use of the limiting nutrient is more characteristic for growth on noble car-
bon sources, i.e. sugars, or on glycerol, either pure or waste. In cases when
the waste carbon source is used, most of the time, no limiting factor is
applied.
Some studies were specifically oriented towards evaluating the importance
of the limiting factor. Thus, Kim et al. (1994) showed that C. necator
performs better with a delayed nitrogen limitation. 18 More recently,
Mozumber et al. (2013) established that the nitrogen limitation can, of
course, be considered as a sucient condition for the production of PHA by
C. necator, but in no case can this be considered a necessary condition to
trigger such a production. 19
Concerning the phosphorus limitation, Ryu et al. (1997) have shown that
the enhancement of its concentration improves PHA production. 20 The main
argument in favor of the use of limiting factors was the assumption that the
nutrients would be used for the growth and not for the accumulation of PHA
otherwise. However, very recently, Grousseau et al. (2013), while studying the
influence of phosphorus limitation on PHA production by C. necator, have
shown that sustaining residual growth improves PHA production. 21
Furthermore, the loss of nutrient-limiting conditions was also studied by
some teams. Thus, Passanha et al. (2013) showed that different elements
were vital for the initial growth and PHA accumulation, 22 whereas Berezina
(2013) showed that C. necator is able to accumulate PHA up to 34% on reach
medium and even without a specific carbon source for PHA accumulation. 23
Also, Bormann et al. (1998) 24 and Berezina et al. (2008) 25 showed that PHA
content in cells during the accumulation step is not altered by the presence
of nutrient-sucient conditions in the reaction media.
In terms of PHA content in cells, the best results so far (90%) were re-
ported by Passanha et al. (2013) when culturing C. necator on filtered
digestate liquor, 22 whereas the best yield on a carbon source, 0.85 g g 1 , was
attained by Cruz Pradella et al. (2012) on soybean oil. 26 Finally, the best
productivity on a noble carbon source of 3.14 g L 1 h 1 obtained by Ryu et al.
in 1997 remains the highest in the field 20 and was only approached by Shang
et al. in 2003. 27 The best productivity on a waste carbon source, 2.5 g L 1 h 1 ,
was reported by Cruz Pradella et al. (2012) on soybean oil. 26
It is, however, worth noticing that the extensive study of glycerol, mainly
pure but also waste, is mainly due to the assumption that this chemical will
flood the market in the very near future, being a by-product of bio-diesel
production. However, until now, glycerol has been more expensive than
glucose, and thus the economical considerations cannot justify the choice of
this carbon source. Considering the sustainability approach, bio-fuels in
general, and bio-diesel in particular, are no longer considered as promising
ecological replacements of traditional fossil resources, as they bring up
several issues, such as the occupation of arable lands, while the increasing
exploration of shale gases, including the so-called ''wet gazes'' such as

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