16
Chapter 1: Toward an Ecological Geography of the Sea
surprises yet in store for us. Archaeobacteria have very recently been found abundantly
in the Southern Ocean at relatively shallow depths.
The most important novelty of recent decades has been the discovery of two groups
of prokaryotic autotrophs among the oceanic picoplankton: the gram-negative cyanobacterial Synechococcus (Johnson and Sieburth, 1979) and the slightly larger prochlorophyte
Prochlorococcus (Chisholm et al., 1988). The latter is furnished with divinyl chlorophylls-a
and -b, has no phycoerythrin, and has an accessory chlorophyll-c-like pigment. These
cyanobacteria are of order 10 m in diameter, and occur at densities of <10
6 ml
−1 form
30–40% of bacterial numbers in most parts of the surface oceans. These are now known
to be the most abundant phytoplankters in the ocean and are the clearest indication that
the concept of the classical “microbial loop” must be replaced.
Exploration of the ecology of Synechococcus and Prochlorococcus has proceeded very
rapidly, and already many solid generalizations may be made. Both are warm-water forms
and, at <14
C, numbers of cells of both genera scale with temperature; in warm seas,
nutrients determine abundance. Prochlorococcus is the more abundant, has a greater depth
range, and may perform 10–80% of water column primary production, the higher values
being in oligotrophic waters between the 40
parallels (Partensky et al., 1999). Thus, in
the mid–North Atlantic in summer the two genera form about 88% of all phytoplankton
numerically, about 39% by its carbon biomass, and generate about 30% of all primary
production (Li, 1995). Prochlorococcus can grow over an irradiance range of four orders of
magnitude and distinct populations are adjusted to each light environment and depth. In
the Arabian Sea and the eastern tropical Pacific, clades of Prochlorococcus occur at <2%
surface illumination, below the oxygen minimum layer, and almost as monocultures.
Synechococcus, on the other hand, is most abundant around the margins of the subtropical
gyres rather than toward the more central regions (Li, in press).
We should not forget that eukaryotic cells also contribute to the pico fraction of
the autotrophic cell population. Green flagellates <2 m, such as Micromonas and
Pedinomonas and very small, scaly prasinophytes 05–10 m from the North Atlantic,
are examples of these organisms. They are certainly more diverse than was revealed even
by the devoted exploration in the mid-20th century by Mary Parke and Irene Manton.
Oceanic 18S rDNA sequences from 75 m in mid-Pacific have very recently been used
to demonstrate unexpectedly high diversity among eukaryotic cells of <3 m in the
pico fraction of the cell population. Most are assignable to prasinophytes, haptophytes,
dinoflagellates, choanoflagellates, and acantharians, but some appear to belong to novel
lineages (Moon-van der Staay et al., 2001).
The borderland between the viscous and inertial realms is occupied by ultraplankton
2–5 m and nanoplankton 5–20 m, some of which exert the primary control over
numbers of <20-m picoplankton, whether these are heterotrophic bacteria or Synechococcus and Prochlorococcus. These heterotrophic organisms represent, in general, the
smallest eukaryotes in the sea and comprise a great range of mastigophoran flagellates
and aplastidic dinoflagellates, ciliates (both tintinnids and oligotrichs), and sarcodines
(both radiolarians and heliozoans). They are responsible for the removal of up to 35%
of cyanobacteria and prochlorophytes daily in ocean water and a greater proportion in
coastal water, and are capable of controlling blooms of single-celled phytoflagellates like
Phaeocystis.
These size fractions of the microplankton also include a photosynthetic capability
because symbiosis by plastid retention, more or less permanently, occurs in a wide
variety of mixotrophic genera, especially in late summer. Ciliate biomass in coastal
water may equal that of larger zooplankton, while 40% of the ciliate cells may contain
active, photosynthesizing plastids (Stoecker, 1987). Wholly photoautotrophic genera of
microplankton range in size from -flagellates to the larger dinoflagellates that border
on the dimensions of large diatoms.
Chapter 1: Toward an Ecological Geography of the Sea
surprises yet in store for us. Archaeobacteria have very recently been found abundantly
in the Southern Ocean at relatively shallow depths.
The most important novelty of recent decades has been the discovery of two groups
of prokaryotic autotrophs among the oceanic picoplankton: the gram-negative cyanobacterial Synechococcus (Johnson and Sieburth, 1979) and the slightly larger prochlorophyte
Prochlorococcus (Chisholm et al., 1988). The latter is furnished with divinyl chlorophylls-a
and -b, has no phycoerythrin, and has an accessory chlorophyll-c-like pigment. These
cyanobacteria are of order 10 m in diameter, and occur at densities of <10
6 ml
−1 form
30–40% of bacterial numbers in most parts of the surface oceans. These are now known
to be the most abundant phytoplankters in the ocean and are the clearest indication that
the concept of the classical “microbial loop” must be replaced.
Exploration of the ecology of Synechococcus and Prochlorococcus has proceeded very
rapidly, and already many solid generalizations may be made. Both are warm-water forms
and, at <14
C, numbers of cells of both genera scale with temperature; in warm seas,
nutrients determine abundance. Prochlorococcus is the more abundant, has a greater depth
range, and may perform 10–80% of water column primary production, the higher values
being in oligotrophic waters between the 40
parallels (Partensky et al., 1999). Thus, in
the mid–North Atlantic in summer the two genera form about 88% of all phytoplankton
numerically, about 39% by its carbon biomass, and generate about 30% of all primary
production (Li, 1995). Prochlorococcus can grow over an irradiance range of four orders of
magnitude and distinct populations are adjusted to each light environment and depth. In
the Arabian Sea and the eastern tropical Pacific, clades of Prochlorococcus occur at <2%
surface illumination, below the oxygen minimum layer, and almost as monocultures.
Synechococcus, on the other hand, is most abundant around the margins of the subtropical
gyres rather than toward the more central regions (Li, in press).
We should not forget that eukaryotic cells also contribute to the pico fraction of
the autotrophic cell population. Green flagellates <2 m, such as Micromonas and
Pedinomonas and very small, scaly prasinophytes 05–10 m from the North Atlantic,
are examples of these organisms. They are certainly more diverse than was revealed even
by the devoted exploration in the mid-20th century by Mary Parke and Irene Manton.
Oceanic 18S rDNA sequences from 75 m in mid-Pacific have very recently been used
to demonstrate unexpectedly high diversity among eukaryotic cells of <3 m in the
pico fraction of the cell population. Most are assignable to prasinophytes, haptophytes,
dinoflagellates, choanoflagellates, and acantharians, but some appear to belong to novel
lineages (Moon-van der Staay et al., 2001).
The borderland between the viscous and inertial realms is occupied by ultraplankton
2–5 m and nanoplankton 5–20 m, some of which exert the primary control over
numbers of <20-m picoplankton, whether these are heterotrophic bacteria or Synechococcus and Prochlorococcus. These heterotrophic organisms represent, in general, the
smallest eukaryotes in the sea and comprise a great range of mastigophoran flagellates
and aplastidic dinoflagellates, ciliates (both tintinnids and oligotrichs), and sarcodines
(both radiolarians and heliozoans). They are responsible for the removal of up to 35%
of cyanobacteria and prochlorophytes daily in ocean water and a greater proportion in
coastal water, and are capable of controlling blooms of single-celled phytoflagellates like
Phaeocystis.
These size fractions of the microplankton also include a photosynthetic capability
because symbiosis by plastid retention, more or less permanently, occurs in a wide
variety of mixotrophic genera, especially in late summer. Ciliate biomass in coastal
water may equal that of larger zooplankton, while 40% of the ciliate cells may contain
active, photosynthesizing plastids (Stoecker, 1987). Wholly photoautotrophic genera of
microplankton range in size from -flagellates to the larger dinoflagellates that border
on the dimensions of large diatoms.