Biology Reference
In-Depth Information
Nostoc
sp. over
Microchate
sp.,
Gloeocapsa
sp
.
and
Asterocapsa
sp. has been noted with other fungal
mycelia and heterotrophic bacteria (Spaink
et al
., 2004). The association of
Nostoc
sp.,
Calothrix
sp.
and
Stigonema
sp. with
P
.
schreberi
in contributing towards nitrogen fi xation in boreal forests of
northern Sweden has been recognized (Gentili
et al
., 2005).
Nostoc
sp. and
Calothrix
sp. have been
successfully isolated and reconstitution experiments with feather moss shoots yielded potential shoots
that effectively fi xed nitrogen. The temperature optima for nitrogen fi xation by
Nostoc
sp. (5ºC) and
Calothrix
sp. (above 30ºC) suggests that during winter
Nostoc
sp. fi xes high levels of nitrogen while
Calothrix
sp. did not. At 30ºC, nitrogen fi xation by
Calothrix
sp. was three-fold greater than
Nostoc
sp. The ecological implications are that these two genera are active nitrogen fi xers depending on
the season.
Nostoc
sp. with low temperature optimum for nitrogen fi xation is active during winter
months whereas
Calothrix
sp. fi xes nitrogen in mid-summer. Although colonization of
P
.
schereberi
with
Nostoc
sp. and
Stigonema
sp. has been predominantly observed,
Stigonema
sp. could not be
isolated into pure cultures.
VIII.
AZOLLA
The genus
Azolla
is a heterosporous fl oating aquatic fern (of the family Salviniaceae) that is
distributed in various types of aquatic bodies such as freshwater ponds, lakes or streams. Species of
Azolla
are native to Asia (e.g. China and Vietnam), Africa (e.g. Senegal, Zaire, Sierra Leone), N. and
S. America and the two poles. It is widely distributed in tropical, sub-tropical and temperate regions
of the world.
Azolla
-
Anabaena
symbiotic association is distinctive for the fact that
Anabaena azollae
is
known to persist throughout the reproductive cycle of the plant. It is transmitted from generation
to generation (i.e. vertically) through the megasporophyte (sporocarp) of
Azolla
.
The classifi cation of
Azolla
is based on morphological and reproductive features and accordingly
seven species of
Azolla
have been recognized (Svenson, 1944). These are
A.
caroliniana
,
A.
fi liculoides
,
A.
mexicana
,
A.
microphylla
,
A
.
rubra
,
A
.
pinnata
and
A.
nilotica
. Based on the presence of number of fl oats
on megaspores, these seven species have been distributed into two sections (or subgenera).
Azolla
(or
Euazolla
) is characteristic in possessing three fl oats on the megaspores to which the fi rst fi ve species
belong and
Rhizosperma
, possesses nine fl oats on the megaspore to which the last two mentioned
species, i.e.
A.
pinnata
(with two varieties
A
.
pinnata
var.
imbricata
and
A
.
pinnata
var.
pinnata
) and
A
.
nilotica
belong. Of these species, the neotropical species
A.
caroliniana
,
A
.
mexicana
and
A.
microphylla
exhibit closer resemblances in vegetative, ecophysiological and genetical features and are distinct from
other taxa (Lumpkin
et al
., 1991). In addition, physiological and biochemical features have also been
taken into account to confi rm the traditional classifi cation. However, except for the taxonomic status
of
A.
rubra
in
Euazolla
(some consider it as a subspecies of
A
.
fi liculoides
) the rest of the species have
closer affi nities. This has been confi rmed by DNA:DNA hybridization studies and the generation of
hybrids between
A.
mexicana
-
A.
caroliniana
and
A.
microphylla
(Zimmerman
et al
., 1991).
Unlike other symbiotic associations of cyanobacteria,
Azolla
-
Anabaena
association has been
effectively put to use as a biofertilizer in rice cultivation in South and Southeast Asia. The biomass
of
Azolla
under optimal conditions gets doubled in less than 48 h with the addition of nitrogen by
its potential to fi x nitrogen.
Azolla
thus can accumulate 5-6.5% N on dry weight basis. Apart from
serving as manure, fertilization of rice fi elds with
Azolla
has two other advantages. It can be used
as a fodder and a weed suppressor (Moore, 1969).
In
Azolla
-
Anabaena
symbiosis the cyanobiont is transferred from one generation to another
through megasporophyte. The development of megasporocarp and microsporocarp is identical
and
Anabaena
is packaged into both the structures (Perkins and Peters, 1992). Due to rupture of
