SYMBIOSIS - Cyanobacteria - page 344

from SW N Atlantic (27° N 50° W), the contribution of Richelia as an endosymbiont of the diatom
Hemiaulus has been noted on the average to the tune of 3110±1315 µmol N m -2 d -1 (Carpenter et al .,
1999). The rates of nitrogen fi xation by the DDAs assume signifi cant proportions both in the open
oceans (Fong et al ., 2008; Zeev et al ., 2008; Kitajima et al ., 2009; Foster et al ., 2009, 2011; Turk et al .,
2011; Villareal et al ., 2011) and nearshore systems (White et al ., 2007a,b; Subramanian et al ., 2008).
However, along a Mediterranean transect the dominance of rhizobia has been reported with lower
concentrations of unicellular diazotrophic cyanobacteria in the western Mediterranean Sea and
Richelia in the eastern basin. (Le Moal et al ., 2011). R . intracellularis contributes to the extent of 35%
to 48% of nitrogen demand in the Gulfs of California at Guaymas and Carmen basins (White et
al . 2007a).
Zeev et al . (2008) estimated low rates of nitrogen fixation (~1.1 nmol N L -1 day 1 )
by R . intracellularis from ultraoligotrophic waters of Levantine basin of the eastern Mediterranean Sea.
The microscopic identifi cation has been correlated with the reverse transcribed PCR amplifi cation of
the nifH gene that showed 98.8% identity with the known nifH sequence of R . intracellularis . The wide
spread occurrence of Hemiaulus - R . intracellularis symbiosis in southwest North Atlantic Ocean has been
reported by Villareal(1994). The infl uence of Amazon river plume on the distribution of free-living and
symbiotic cyanobacteria in the western tropical north Atlantic has been reported (Foster et al. , 2007).
By using nifH gene amplifi cation by quantitative PCR method, Foster et al . (2009) showed the gene
abundance and gene expression of diazotrophic populations from the Eastern Equatorial Atlantic.
H . hauckii - R . intracellularis association has been found to be abundant (>10 4 nifH copies L -1 ) in the
north-west of the Congo River plume. Although Calothrix - Chaetoceros association is poorly represented
at the surface, its abundance (3.7 x 10 4 nifH copies L -1 ) at a depth of 40 m in the equatorial upwelling
region has been noted. In case of Rhizosolenia - R . intracellularis association though the number of gene
copies is lowest, the transcript abundance has been found to be high (9.4 x 10 1 to 1.8 x 10 4 ). These
observations are supported by the nanometer scale secondary ion mass spectrometry approach to
measure nitrogen fi xation rates and the release of fi xed products into the natural waters. Richelia
and Calothrix symbionts fi xed 171-420 times more nitrogen than the free cells. The cell specifi c rates
(1.15-7.5 fmol N cell -1 h -1 ) among the symbionts resembled each other and the fi xed nitrogen was
rapidly transferred. On the average Richelia as an endosymbiont fi xed 81-744% more nitrogen than
what is needed for its growth and transferred nearly 97.3% of the fi xed nitrogen to the diatom cell
(Foster et al ., 2011). In the tropical waters of eastern North Atlantic, in the vicinity of Cape Verde
Islands, the nitrogen fi xation rates >6 nmol N L -1 h -1 have been found. The amplifi cation of nifH
transcripts by RT-PCR resulted in 605 nifH transcripts of which 76% belonged to six operational
diazotrophic populations. The contribution of unicellular diazotrophic cyanobacteria appeared to
be signifi cant in both the coastal and open ocean waters.
Studies on the DDAs of oligotrophic waters of North Pacifi c Ocean, specially northwest of
Hawaii have been conducted mostly near station ALOHA (22° 45' N, 158° 00' W), recognized
as one of the Hawaii Ocean Time Series (HOT) programme (Heinbokel, 1986; Scharek et al .,
1999a,b; White et al ., 2007a,b; Fong et al ., 2008). The other stations include the CLIMAX station
(Vanrick, 1974) and some to farther east (Mague et al ., 1974; Wilson et al ., 2008). The distribution
and signifi cance of Rhizosolenia - R . intracellularis association has been for the fi rst time reported by
Vanrick (1974). Nitrogen transport by the vertically migrating diatom mats in the North Pacifi c
Ocean seems to be the regulating feature for the occurrence of the blooms in this oligotrophic
water body (Villareal et al. , 1993). The temporal variation in the abundance of H . hauckii-R .
intracellularis association has been recorded by Scharek et al . (1999a,b). Quantitative RT-PCR
determination of nifH gene copies revealed the abundance of Trichodesmium , Crocosphaera watsonii