STRESS RESPONSES - Cyanobacteria - page 609

1) Habitat sensing: The microorganisms sense changes in the surrounding environment with the
help of two-component regulatory systems. Okamoto et al . (2007) established a correlation between
signal transduction domains and habitats in cyanobacteria by applying hmmpfam (Pfam-1s Version
14.0) program to the 14 cyanobacterial genomes [ Anabaena variabilis , Anabaena sp. strain PCC 7120,
Gloeobacter violaceus PCC 7421, Nostoc punctiforme ATCC 29133, Synechocystis sp. strain PCC 6803,
S. elongatus PCC 6301, S. elongatus PCC 7942, Thermosynechococcus elongatus BP-1- all freshwater;
Crocosphaera watsoniii WH8501, Prochlorococcus marinus MED4, P . marinus SS120, P . marinus MIT9313,
Synechococcus sp. strain WH8102, and Trichodesmium erythraeum- all marine]. A positive correlation
between habitats and functional domains of signalling pathways existed as freshwater species
developed more signal transduction domains than the seawater species that had fewer domains.
In view of the existence of the cyanobacteria since Precambrian period, these organisms must have
faced very challenging situations to obtain suffi cient levels of CO 2 from environments with wide
fl uctuations in O 2 levels ranging from anaerobiosis to saturation levels. Thus the cyanobacteria
during the course of evolution have developed a CO 2 concentrating mechanism (CCM) along with
the evolution of carbonic anhydrases (CAs) and the main carboxylating enzyme, RUBP carboxylase/
oxygenase (RuBisCO). Badger et al . (2006) reviewed the current status of the genomic diversity of the
CCM among cyanobacterial species and the evolution of this mechanism to match with their habitat
requirements. A comparison 18 genomes of cyanobacteria (besides the 14 genomes mentioned above,
four others, i.e Synechococcus sp. strain PCC 7002, Synechococcus sp. strain CC9605, Synechococcus
sp. strain CC9902 and Synechococcus sp. strain CC9311 are included in this study) with their habitats
revealed a correlation between the type of carboxysome and the environment. Cyanobacterial species
inhabiting open oceans are equipped with α-type of carboxysomes (so classifi ed as α-cyanobacteria that
possess form 1A type of RuBisCO to which Prochlorococcus and α- Synechococcus spp. belong) whereas
freshwater and other marine species possess β-type of carboxysomes (so called as β-cyanobacteria
that possess form 1B type of RuBisCO). Four CAs have been identifi ed that are responsible for
generation of CO 2 within the carboxysome and three of these (CcaA, CcmM and CsoS3) are related
to carboxysome function. There are limited number of transporters for bicarbonate (SbtA- HCO 3 - ,
BicA- HCO 3 - and NDH1 4 - CO 2 ) and CO 2 uptake in the open ocean species of Prochlorococcus . Species
inhabiting coastal and estuarine environments with fl uctuations in CO 2 and temperature are equipped
with variable number of bicarbonate and CO 2 uptake systems. Likewise, species inhabiting symbiotic
environments, thermal hot springs and calcareous rocks (where fl uctuations in Ci, temperature, O 2
and nutrients exist) showed most number of composite Ci transporters (BCT- HCO 3 - , SbtA- HCO 3 - ,
BicA- HCO 3 - , NDH-1 3 -CO 2 and NDH-1 4 -CO 2 ; the former three being located on the cytoplasmic
membrane and the latter two on the thylakoids).
The physiology of abiotic stress has been understood in plants and cyanobacteria. Mainly two
types of damages are known to be caused by most of the abiotic stresses. The fi rst is the water-defi cit
stress that is caused by a number of environmental conditions including drought, salinity, heat and
cold. The second type of damage results due to the production of reactive oxygen species (ROS)
that damage cellular constituents including proteins and nucleic acids. This is called as oxidative
stress.
2) Water-defi cit stress: The water status of a cell (or tissue) is defi ned by two parameters, i.e. water
potential and relative water content. Water potential of a system is represented by Ψ and is measured
in units of pressure (M Pascal or MPa). This is similar to electric potential, just as the current fl ows
from a compartment of high potential to a compartment of low potential when a connection is made
between them. Thus water is driven from a high potential to a low potential. When suffi cient water