Environmental Engineering Reference
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characteristics have been sought at high altitudes, in Arctic regions, glacial cores,
ice accretions and in Antarctic lakes. Indeed, Antarctic expeditions have been
organized with the purpose to identify bacteria with AFPs, which bind to ice
crystals and arrest their growth [7].
To survive subzero temperatures, certain microbes protect themselves against
growing ice crystals which could damage membranes, critically increase their cell
volume and oppose the osmotic gradient produced by the increasing extracellular
solute concentration [8]. They survive these stresses by the production of cryopro-
tectants [7] that can lower ice nucleation temperatures and stabilize membranes and
cell fluids. Shorter acyl chains [8] and unsaturated fatty acids [9] may become more
abundant in the membrane. Low temperatures also influence the production of
chaperones, cold stress proteins and carotenoids that may confer protection from
UV irradiation, oxidative stress and the maintenance of membrane fluidity [10].
Among these, low temperature adaptive proteins are the AFPs and ice nucleation
proteins.
AFPs and antifreeze glycoproteins were first characterized and cloned from
cold-water fish, and later from other metazoans that overwinter in temperate
latitudes [11]. Ironically, it was years after their first discovery that they were
first noted and subsequently cloned in a bacterial species [12]. AFPs inhibit
freezing in a non-colligative manner by binding to ice and making the addition
of water molecules unfavorable, which results in a change in the equilibrium
freezing point [13, 14]. Since melting is affected in a colligative manner, the
interaction between AFPs and the ice surface results in a separation of the
freezing point and the melting point, a phenomenon termed thermal hysteresis
[15]. Ice crystal growth can be perturbed even by small quantities of AFP since
local ice curvature results from the adsorption of the proteins to the surface of
the ice crystal (the adsorption-inhibition hypothesis [15, 16, 17]). Adsorption to
certain ice faces by AFPs seems to be by a surface-to-surface complementarity
of fit, made possible by regularly spaced residues on the regularly spaced crystal
lattice. Once adsorbed to ice, AFPs sit 'snugly', assisted by van der Waals and
hydrophobic interactions [18].
AFPs also impede water mobility at the ice crystal surfaces thereby decreas-
ing the probability of recrystallization. It is assumed that some freeze-tolerant
organisms, such as certain insects and plants, produce AFPs to prevent the
growth of large, damaging ice crystals that form at temperatures close to
melting [19, 20, 21], a property known as ice recrystallization (IR). IR inhibition
activities, and putative AFPs have been reported in a few bacteria including
Moraxella sp. [22], Pseudomonas putida [12, 23, 24], Micrococcus cryophilus and
Rhodococcus erythropolis [25] as well as cyanobacterial mats [26], Chryseobacterium
sp. C14 [27] and in several Antarctic Proteobacteria, including Marinomonas protea
[7, 28].
In contrast to AFPs, ice nucleating proteins (INPs) prevent extensive super-
cooling and allow ice to form at temperatures close to freezing. These proteins
are presently known and cloned from only three genera of bacteria, including
Pseudomonas, Xanthomonas and Erwinia/Pantoea [12, 22, 23, 24, 29, 30]. INPs
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