Biology Reference
In-Depth Information
200 m of the ocean waters receives sunlight (euphotic zone), the
deep sea is characterized by absence of light, high pressure, and
low temperatures (−1 to 4°C) and covers about 95% of the seabed
(benthos). High organic carbon input and mainly anoxic condi-
tions prevail in sediments at upwelling areas, continental margins,
hydrothermal vents, and cold seeps. In contrast, remote ocean
areas, e.g., in the South Pacific, may have oligotrophic oxic
sediments (
1
).
Microbes thrive in all ocean environments, from the sunlit surface
waters across the deep sea into the seafl oor sediments and the deep
subseafl oor biosphere, as well as from tropic waters to the arctic
regions. Marine microbes represent the “unseen majority.” It is
estimated that 3.6 × 10
28
prokaryotic cells occur in the euphotic
zone, 6.5 × 10
28
cells in the ocean water below 200 m, 1.7 × 10
28
cells in the top 10 cm of the seafl oor sediment, and 2.1 × 10
30
below
10 cm sediment surface. In total, the marine microbial biomass
corresponds to about one-third of the estimated total carbon in
plants (
2
).
In seawater, less than 0.1% of the total cell counts can be covered
by cultured microbes, i.e., for more than 99% of the microbial
diversity in the oceans, only molecular fi ngerprints are available
(
3
). In general, half of the more than 50 presently known bacterial
phyla do not contain a single cultured representative (
4
). For the
deep subseafl oor biosphere, investigations into abundance, diver-
sity, and activity of prokaryotes are at their early beginnings (
5
).
The pioneering large-scale metagenomic analyses of near-surface
seawater samples have revealed a hitherto unknown large number
of novel phylotypes and protein families (
6, 7
), further underscoring
the vastness of unexplored microbial diversity in the oceans and
opening a new avenue for microbial ecology. For instance, com-
parison of 45 distinct microbiomes revealed that metagenomic
differences actually correlated with the biogeochemical conditions
prevailing in the studied environments (
8
). Moreover, the diversity
of marine microbial populations is being cataloged by massive
DNA sequencing (
9
), and large-scale metagenomic/transcriptomic
studies are being conducted across spatial and temporal gradients
of marine habitats (
10
). This trend will be further propelled by
emerging ultrafast next-generation sequencing technologies (
11
).
Besides resolving biogeographical patterns, the functional challenge
will be to extract from the increasing (meta)genome data experi-
mentally validated key redox biocatalysts of biogeochemical pro-
cesses and to assess their distribution within microbial communities
and diversity (Fig.
1
) (
12
).
1.1.2. Abundance and
Diversity of Prokaryotes
in the Oceans
The top 200 m of the water column receive enough sun light for
the phytoplankton to generate about one-half of the global primary
production of organic carbon (
13
). About half of the fi xed carbon
1.1.3. Fate of Organic
Carbon
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