Our New Understanding of the Role of Very Small Organisms
15
dominate. What has been learned about the ecosystem of the viscous realm in the last
20–25 years has turned our understanding of ecological processes in the ocean upside
down. But the new knowledge has not yet completely penetrated our collective consciousness, even though we know that the classical model of a “microbial loop” inhabiting this
realm no longer suffices.
In the viscous realm water is a viscous medium and gravity a negligible force; it is largely
populated by extremely small, very ancient forms of life that are ubiquitous and abundant
in all seas, and at all seasons. In the inertial realm water is a fluid, gravity is a significant
force, and inertia is an important consequence of motion; this realm is inhabited by the
marine organisms that most people eat or study, such as fish or zooplankton, of linear
dimensions one to several hundred millimeters. According to Stokes’ law, the relative
significance of inertial and viscous forces for a moving particle is a function of its size and
velocity, together with appropriate values for the density and viscosity of the medium.
From this relationship is derived a simple ratio, Reynolds’ number, which takes small
values when viscous forces dominate. As Purcell (1977) put it, a swimming man takes
values around R = 10
4 , a small fish about 10
2 , and a motile bacterium about 10
−4 or
10
−5 . The last can move relatively very much more rapidly than you can swim, at about
>10 m sec
−1 , using a very small fraction of its metabolic energy. But when it stops, it
stops. And does not sink.
At the dimensions of bacteria, diffusion of nutrient molecules toward and waste
molecules away from the organism occurs very rapidly and indeed, as Purcell pointed
out, local “stirring” of the medium cannot increase its nutrient supply. At nutrient
concentrations typical of oligotrophic water, NO 3 molecules are about 10 m apart so that
a bacterium-sized cell would have only a few such molecules adjacent to its surface at any
one time. Motion is therefore critical to diffusion-limited nutrient uptake, and small cells
are favored in low-nutrient water. Most biota of the viscous realm are either rod-shaped
or approximately spherical, unlike those of the inertial realm where form has evolved so
as to reduce sinking rates, to sense food particles, and to serve various other functions.
But what kinds of organisms inhabit the curious viscous realm? Certainly, very many
more kinds than there were thought to be only 20 years ago although, before getting
into that, I note that there is also an abundant population of nonliving particles to be
thought about. In clear coastal seawater, particles of dimension 05–10 m ESD occur
at abundances of about 1 × 10
7 ml
−1 , together with about three orders of magnitude
higher numbers of colloidal particles of around 001 m. Such numbers suggest a blurring
of the concept of dissolved and particulate organic material (Longhurst et al., 1992).
Viral particles are also very numerous everywhere in seawater, at 10
7 –10
9 ml
−1 , are of
equivalent dimension to the small nonliving particles, and are more abundant by an
order of magnitude than the picoplankters having dimension <20 m. Not much larger
than viral particles is the bacterial clade SAR11 in coastal seawater, which is the smallest
living cell yet cultured (Rappé et al., 2002). The lunate cells of these bacteria are only
∼01 m long.
Also among the picoplankton < 20 m, we now know that many oceanic bacteria are aerobic, anoxygenic photoheterotrophs, containing bacteriochlorophyll. These
-proteobacteria, such as Erythrobacter, obtain energy from sunlight and use it not only
to metabolize DOM molecules obtained from seawater, but also to enable them to
function as photoautotrophs. Such cells are globally distributed in the oceanic photic
zone, preferentially in oceanic gyres where DOM concentrations are low and where they
may form <10% of the total oceanic microbial community (Kolber et al., 2000, 2001).
Finally, in the picoplankton we should note the newly found Archaeobacteria, which are
an independently evolved domain differing in its ribosomal DNA structure from both
Prokaryotes and Eukaryotes. Though these are very abundant below the oceanic photic
zone, forming 40% of the microbial biomass at 1000 m in mid-Pacific, there may be
15
dominate. What has been learned about the ecosystem of the viscous realm in the last
20–25 years has turned our understanding of ecological processes in the ocean upside
down. But the new knowledge has not yet completely penetrated our collective consciousness, even though we know that the classical model of a “microbial loop” inhabiting this
realm no longer suffices.
In the viscous realm water is a viscous medium and gravity a negligible force; it is largely
populated by extremely small, very ancient forms of life that are ubiquitous and abundant
in all seas, and at all seasons. In the inertial realm water is a fluid, gravity is a significant
force, and inertia is an important consequence of motion; this realm is inhabited by the
marine organisms that most people eat or study, such as fish or zooplankton, of linear
dimensions one to several hundred millimeters. According to Stokes’ law, the relative
significance of inertial and viscous forces for a moving particle is a function of its size and
velocity, together with appropriate values for the density and viscosity of the medium.
From this relationship is derived a simple ratio, Reynolds’ number, which takes small
values when viscous forces dominate. As Purcell (1977) put it, a swimming man takes
values around R = 10
4 , a small fish about 10
2 , and a motile bacterium about 10
−4 or
10
−5 . The last can move relatively very much more rapidly than you can swim, at about
>10 m sec
−1 , using a very small fraction of its metabolic energy. But when it stops, it
stops. And does not sink.
At the dimensions of bacteria, diffusion of nutrient molecules toward and waste
molecules away from the organism occurs very rapidly and indeed, as Purcell pointed
out, local “stirring” of the medium cannot increase its nutrient supply. At nutrient
concentrations typical of oligotrophic water, NO 3 molecules are about 10 m apart so that
a bacterium-sized cell would have only a few such molecules adjacent to its surface at any
one time. Motion is therefore critical to diffusion-limited nutrient uptake, and small cells
are favored in low-nutrient water. Most biota of the viscous realm are either rod-shaped
or approximately spherical, unlike those of the inertial realm where form has evolved so
as to reduce sinking rates, to sense food particles, and to serve various other functions.
But what kinds of organisms inhabit the curious viscous realm? Certainly, very many
more kinds than there were thought to be only 20 years ago although, before getting
into that, I note that there is also an abundant population of nonliving particles to be
thought about. In clear coastal seawater, particles of dimension 05–10 m ESD occur
at abundances of about 1 × 10
7 ml
−1 , together with about three orders of magnitude
higher numbers of colloidal particles of around 001 m. Such numbers suggest a blurring
of the concept of dissolved and particulate organic material (Longhurst et al., 1992).
Viral particles are also very numerous everywhere in seawater, at 10
7 –10
9 ml
−1 , are of
equivalent dimension to the small nonliving particles, and are more abundant by an
order of magnitude than the picoplankters having dimension <20 m. Not much larger
than viral particles is the bacterial clade SAR11 in coastal seawater, which is the smallest
living cell yet cultured (Rappé et al., 2002). The lunate cells of these bacteria are only
∼01 m long.
Also among the picoplankton < 20 m, we now know that many oceanic bacteria are aerobic, anoxygenic photoheterotrophs, containing bacteriochlorophyll. These
-proteobacteria, such as Erythrobacter, obtain energy from sunlight and use it not only
to metabolize DOM molecules obtained from seawater, but also to enable them to
function as photoautotrophs. Such cells are globally distributed in the oceanic photic
zone, preferentially in oceanic gyres where DOM concentrations are low and where they
may form <10% of the total oceanic microbial community (Kolber et al., 2000, 2001).
Finally, in the picoplankton we should note the newly found Archaeobacteria, which are
an independently evolved domain differing in its ribosomal DNA structure from both
Prokaryotes and Eukaryotes. Though these are very abundant below the oceanic photic
zone, forming 40% of the microbial biomass at 1000 m in mid-Pacific, there may be