Phytoplankton Carbon Stock - Blue Carbon Reservoir of the Blue Planet - page 149

oceans (Bishop and Wood 2009 ). Of the captured
carbon dioxide, an estimated 0.5 GtC year − 1

due to minimum precipitation and maximum
evaporation rate. Under this situation, the volume
of phytoplankton and subsequent stored carbon
was lowest (Tables 5.2 and 5.3 ).
The regulatory role of salinity on stored car-
bon in phytoplankton is highlighted in Table 5.4 .
Basically, we infer that the highest carbon in the
phytoplankton during monsoon is due to their
maximum volume acquired during this season as
a result of high dilution factor of the ambient
aquatic phase. The direct relationships between
cell volume and cell carbon in different types of
phytoplankton are illustrated clearly in Figs. 5.5 ,
5.6 , 5.7 , 5.8 , 5.9 and 5.10 .
It is extremely important to understand the
enzymatic role in storing carbon dioxide by phy-
toplankton. Carbonic anhydrase converts inor-
ganic carbon from the form it takes in sea water
(bicarbonate) to carbon dioxide that is used in
phytoplankton photosynthesis. The carbonic
anhydrase enzyme usually contains zinc, which is
scarce in surface waters of the open ocean.
Because carbon acquisition is essential to cell
growth, the amount of zinc may limit the rate of
primary production and carbon dioxide

is

stored at the seabed (Seiter et al. 2005 ).
In this chapter, we attempt to project a quan-
titative estimate of carbon-sequestering capacity
of the phytoplankton that are distributed along
the Indian Sundarbans estuaries based on our
observation on shapes and subsequent volumes
of various phytoplankton species during 2011.
Some common shapes of phytoplankton are
highlighted in Fig. 5.4 in the form table. The
readers may consult Baltic Sea Environment
Proceedings No. 106 ( www.helcom. /stc/ les/

Publications/Proceedings/bsep106.pdf ) for fur-

ther study and research in this vertical.
Carbon uptake by phytoplankton and its export
as organic matter to the ocean interior (a mecha-
nism known as the biological pump) lowers the
partial pressure of carbon dioxide in the upper
waters and facilitates the diffusive drawdown of
atmospheric carbon dioxide. However, precipita-
tion of calcium carbonate by coccolithophores
increases the partial pressure of carbon dioxide
and promotes outgassing from the ocean to the
atmosphere (known as the alkalinity pump).
While these carbon

xation.
Thus, carbon cannot be acquired if organisms lack
zinc, or a means of compensating for its absence.
Lane and Morel ( 2000a ) have identi

uxes have been, over the
past 100 million years, modulated by the abun-
dance of diatoms and coccolithophorids, resulting
in biological feedback on atmospheric carbon
dioxide and Earth

fl

ed and
characterized a diatom carbonic anhydrase
enzyme that can substitute cobalt for zinc without
a loss of enzyme activity. In these experiments,
zinc was limited in the diatom growth medium and
cobalt was made available. Carbonic anhydrase
activity was still observed in the

s climate, the processes, which
determine the distribution of these phytoplankton
groups, remain poorly understood. Considering
this gap area, the present chapter deals with the
storage of carbon by the phytoplankton cells,
which is a direct function of their volume. Very
few studies have been conducted in the tropical
ocean and estuaries on the phytoplankton carbon
storage capacity and the inter-relationships of the
same with abiotic parameters. In this chapter, we
have highlighted a case study of variation of
carbon storage by phytoplankton in different
salinity, which is a major seasonal feature of
tropical waters. The highest volume of phyto-
plankton and carbon content was observed in the
monsoon season in the estuaries of the lower
Gangetic region. We observed a completely
contrasting picture in the pre-monsoon season
when the salinity is highest in the estuarine water

'

Thalassiosira

weissflogii culture in the absence of zinc. Protein

puri cation yielded a cobalt-containing carbonic

anhydrase enzyme, which cochromatographed
with the Zn-containing enzyme on an electro-
phoresis gel, indicating that the new enzyme is just
like the old one, except for the metal. Extended X-
ray absorption

ne structure (EXAFS) studies
allowed comparison of the coordination environ-
ment of the two enzymes and provided further
evidence that the zinc- and cobalt-containing
enzymes are otherwise identical.
Lane and Morel ed

another carbonic anhydrase enzyme containing
cadmium in the same diatom

( 2000b ) also identi

Thalassiosira