Agriculture Reference
In-Depth Information
et al.
2008
). The
reductive dissolution theory
affirms that the reduction of hydrous
ferric oxides under anaerobic conditions may release occluded P and ferrous iron
(Fe
2+
) to the soil solution (Stemmler and Berthelin
2003
). Finally, the
inorganic
acid production theory
is based on the fact that many acidophilic bacteria
(extremophiles), able to oxidize reduced sulfur compounds (such as pyrite) to
sulfuric acid (H
2
SO
4
), may participate in the solubilization of P in soils (Muchovej
et al.
1989
).
5.6 Cold-Tolerant Microorganisms
The ability of some microorganisms to tolerate (and even proliferate) at low
temperatures was first reported by Forster in
1887
. This ability is related to the
strategies these organisms have evolved to face the challenges imposed by perma-
nently cold environments. Many of these strategies, dependent on mechanisms
which are far from universal (Casanueva et al.
2010
), are quite well known; others
still wait to be elucidated. Some of these are regulation of membrane fluidity, the
synthesis of specialized molecules (e.g., cold-shock proteins, cryoprotectors, and
antifreeze molecules), the regulation of ion channels permeability (osmoregula-
tion), seasonal dormancy, and perhaps the most important adaptation to freezing
temperatures, the modification of enzyme kinetics (Georlette et al.
2004
; D'Amico
et al.
2006
). Although they are both able to grow in cold places, there is an
important difference between cold-tolerant (
¼
psychrotrophs or psychrotolerant)
and cold-loving (
¼
psychrophilic) microorganisms: psychrotrophs are able to resist
20
C) without being seriously
compromised; on the contrary, psychrophiles are adapted to grow efficiently at
low temperatures and strongly depend on this abiotic factor to successfully colonize
cold habitats (Margesin and Miteva
2011
). However, distinguishing between these
two kinds of microorganisms is not always easy, and there has been a debate
concerning such distinction. The classical definition of a psychrophile is related
to its growth rate, which is said to be maximal at the so-called optimal temperature.
Indeed, Morita (
1975
) defined psychrophiles as organisms having an optimal
temperature for growth at about 15
C or lower, a maximal temperature for growth
at about 20
C, and a minimal temperature for growth at 0
C or below. However,
some authors have contradicted this definition in the past few years mainly because
when considering efficient colonization of cold environments, the growth rate may
not be as relevant as growth yield (Bakermans and Nealson
2004
). In a paramount
work in the field, Margesin (
2009
) clearly demonstrated that slow growth rates of
psychrophiles at the so-called suboptimal, lower temperatures are compensated by
high growth yields and maximized cellular fitness. Nevertheless, it is important to
consider here that psychrotolerant microorganisms (able to grow between 20 and
40
C, but also at lower temperatures with much lower rates) are the organisms
most frequently found in cold environments (Hoover and Pikuta
2010
).
suboptimal
temperatures of growth (
<
