Geoscience Reference
In-Depth Information
seawater, dominating autotrophic and hetero-
trophic metabolism in many oceanic systems (Sherr
et al.
2007). The microbial loop is particularly
important in regions with limited 'new' nutrient
supplies where small primary producers have a
distinct advantage in nutrient uptake based on
their high surface area to volume ratio. In addition
to the well-known photosynthetic bacteria
Synechococcus
and
Prochlorococcus
, recent work has
documented the apparent abundance of picoeu-
karyotic phytoplankton (Sherr
et al.
2007 ). It has
been shown that this group can contribute signii -
cantly to primary productivity and biogeochemical
cycles in marine waters (Worden
et al.
2004 ), but the
taxonomic identity of these picoeukaryotes has
only been examined in a few locations and their
physiological capabilities remain poorly known.
Microbial food webs are characterized by a very
tight coupling between organic carbon production
and consumption. While some controversy exists
about the extent to which these microbially domi-
nated ecosystems are net autotrophic or net hetero-
trophic ( Williams 1998 ; Duarte and Prairie 2005 ), it
is clear that small primary producers are grazed
voraciously by a variety of taxonomically diverse
single-celled microzooplankton (Sherr and Sherr
1994), including many different ciliated and l agel-
lated species (this latter group includes mixotrophic
dinol agellates). Unlike copepods and other larger
zooplankton, micrograzers have metabolic rates
that are similar to those of their prey, and they are
thus able to graze small phytoplankton at rates
close to those at which the phytoplankton grow
(Calbet and Landry 2004). Moreover, predator and
prey in the microbial loop are not subject to seasonal
timing 'mismatches', which can occur in the classic
food web. The close coupling between production
and grazing results in high rates of nutrient reminer-
alization, with inorganic nitrogen recycled in the
form of NH
4
+
during grazer excretion (Glibert 1982).
In the absence of strong external nutrient inputs,
primary production in these systems is mainly
fuelled by such 'regenerated' nutrients, in a tight
cycle of production and grazing which limits bio-
mass accumulation.
In both the classic and microbial food webs, a sig-
nii cant fraction of phytoplankton-derived organic
carbon accumulates in a large pool of dissolved
organic carbon (DOC), much of which is refractory
and long-lived, with residence times of the order of
thousands of years (Hansell and Carlson 2002). This
DOC can be released directly by phytoplankton
through exudation and from zooplankton as a result
of egestion and excretion. Marine viruses also play
a role in DOC production through the lysis (i.e.
infection and subsequent rupture) of phytoplank-
ton and heterotrophic bacteria, which releases
cellular constituents into surface waters (see
Chapter 5). Some fraction of the DOC pool is highly
labile and is rapidly taken up by heterotrophic bac-
teria. Bacterial growth efi ciency determines the
fraction of the assimilated DOC that can be reincor-
porated into the food web through microzooplank-
ton grazing versus the fraction that is remineralized
back to CO
2
and inorganic nutrients. The latter proc-
ess contributes to the recycling of nutrients that
fuels regenerated production. Given the relatively
low growth efi ciencies of many natural bacterial
and microzooplankton assemblages (del Giorgio
and Cole 1998), a large fraction of the organic car-
bon cycled through the microbial loop will eventu-
ally be 'lost' as CO
2
, with only relatively small
amounts recycled back into the classic food chain.
6.2.2
The marine carbon cycle
The marine carbon cycle is driven by two independ-
ent processes, the 'solubility pump' and the 'bio-
logical carbon pump' (see also Chapter 12). As the
solubility of gases increases with decreasing seawa-
ter temperature, the cold waters sinking to depths
during deep-water formation at high latitudes are
CO
2
-rich relative to average oceanic surface waters.
As the newly formed deep waters l ow towards
lower latitudes, they carry a high CO
2
load, spread-
ing it throughout the deep ocean. About one-third
of the surface-to-depth gradient of dissolved inor-
ganic carbon (
C
T
) is generated by this solubility
pump. The other two-thirds of the vertical carbon
gradient is caused by the biological carbon pump;
the sinking of biogenic material from the sunlit
surface layer to the deep ocean. Integrated over
the global ocean, the biotically mediated oceanic
surface-to-depth
C
T
gradient corresponds to a car-
bon pool 3.5 times larger than the total amount of
atmospheric CO
2
( Gruber
and
Sarmiento
2002 ).
