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Introduction
Bacteria are found everywhere in the ocean.
However, they are not distributed uniformly over depth, region or time. In a well-known
textbook, Charles J. Krebs (1972) has succinctly stated
his view of ecological research: "Ecology is the
scientific study of the interactions that determine
the distribution and abundance of organisms. We
are interested in where organisms are found, how many occur there, and why".
Since the last century, it has been known
that bacteria are part of the marine plankton. For
most of this period, the study of marine bacteria
has followed the tradition of the microbiological
giants Pasteur and Koch wherein cells are first
isolated from nature and then cultured in the
laboratory on artificial media. This is the species
approach. In recent decades, a different approach has developed which emphasizes the role of
microbes in their natural habitats. This is the
process approach. In marine studies, this
approach led to a new tradition pioneered in 1962
from Nanaimo by the oceanographers Parsons and Strickland wherein the biological activity of
cells are assayed in situ. A flourish of new ideas
and results have followed which clearly point to
the vital importance of bacteria in the ocean.
In the pelagic realm, bacteria are
indispensable for two major reasons: they are eaten by
other organisms, and they degrade organic matter. In
a loose sense, bacteria are at both the start and
end of the food chain: they contribute to the first
production of particulate food-stuff (by
conversion of dissolved organic substrates), and they are
responsible for the ultimate breakdown of organic matter leading to the return of nutrients to the sea.
Bacteria may be the crucial link or sink between detritus, dissolved organic matter and
higher trophic levels. For these reasons, bacteria
occupy a central role in two inter-connected
environmental issues of global concern, namely the
sustenance of harvestable living resources and the
mitigation of climate change by sequestration of carbon
into the deep ocean.
Figure 1: Depth
distribution of bacteria in the Labrador Sea in May, 1997
Counting Bacteria
The most basic information we need is
the number of bacteria at a given place and time. Without this, we cannot even offer a
descriptive geography, much less any assessment of
microbial impact on marine production. Not all
bacteria are the same. Some are metabolically
active while others are not. Further, in a given
water parcel, it is possible to find genetically
distinct populations which, though adapted for
optimal growth under different conditions, cooccur in
the given permissible environment. Yet, the
first-order task of counting bacteria, assuming
(incorrectly) that they are all the same, is not as
straightforward as perhaps might be imagined.
Unlike other important oceanographic variables such
as temperature, salinity, irradiance, oxygen and
colour which can all be monitored in an automated and continuous fashion, or in the case of some
be sensed remotely by airborne platforms, bacteria need to be counted manually in discrete
samples of seawater.
Remarkably, it has only been in the last 20
or so years that oceanographers have had accurate counts of the total number of bacteria. Before
this time, marine bacterial populations were
severely underestimated because of inadequate methods.
The breakthrough that ushered in the modern view of planktonic bacteria was a
methodological one. With the availability of chemical
stains that specifically bind to biological
macromolecules in general, and DNA in particular, it became
possible to visualize all the bacteria mounted on
a microscope slide and made to fluoresce by light of a suitable wavelength. As researchers
everywhere adopted this method of epifluorescence microscopy, data slowly began to
accumulate, contributing to the development of the
contemporary idea known as the "microbial loop"
in which the role of free-living bacteria in
marine food webs is paramount.
At the Bedford Institute of
Oceanography (BIO), we have semi-automated the procedure
for counting DNA-stained bacteria (Li et al. 1995).
Microscopic examination is an extremely timeconsuming and labourintensive task.
By using a flow cytometer to perform electronic detection of cells based on optical characteristics, the
counting procedure is greatly improved by
enhanced precision, speed, and ease of operation. Thus, whereas it requires 10 minutes to count
500 bacteria by microscopy, it only takes 10
seconds to count 5,000 bacteria by flow cytometry.
Recently, researchers in France (Jacquet et al.
1998) have made flow cytometric counts of cells
collected at high frequency (minutes) by an autonomous sampler consisting of a microprocessor
controlled system of fraction collector,
peristaltic pump and a set of tube-pinching electrovalves.
It is therefore now feasible to map the distribution of bacteria at a resolution approaching
that of chlorophyll when both are monitored at sea.
This ability to match the scales of variation for microorganisms of different nutritional modes
is important as we seek to understand how ocean food webs function.
Depth Distribution
Typically,
bacteria are most abundant in the sunlit upper layer, and their numbers
decrease with depth. For example, in the Labrador Sea (Figure 1), bacteria
are present at concentrations on order 105 to 106
per millilitre in the top 100 metres, and on order 104 to
105 per
millilitre at greater depths. Bacteria are sustained by the flux of
dissolved organic matter from phytoplankton and zooplankton. Therefore the
restriction of primary production to the sunlit layer is an obvious
determinant in the vertical distribution of bacteria. Bacteria persist
deep into the aphotic zone where phytoplankton are absent; there they are
a dominant metabolic agent mediating the dynamics of organic
material.
Figure 2. Abundance of bacteria
in surface waters in July, 1995 (upper panel) and the mean distribution of
chlorophyll in July from the Coastal Zone Colour Scanner (lower
panel)
Regional Distribution
In general, the distribution of bacteria at
the regional scale is not well-studied. Broadly,
typical bacterial abundances (cellsml-1) in
eutrophic lagoons and estuaries (107), in coastal zones
(106), and in the open ocean
(105) are set by the magnitude of the flux of dissolved organic matter: a
manifestation of the dominance of "bottom-up"
(resource limitation) over "top-down" (grazing
pressure) control factors at large time and space
scales (Ducklow 1992). An example of the coherence
of bacteria and phytoplankton can be seen in summer from Nova Scotia to the Labrador Sea
(Figure 2).
Figure 3.
Seasonal cycle of surface temperature and bacteria in Bedford
Basin
Seasonal Distribution
In temperate waters, the annual cycle of
bacterial abundance may be quite regular.
Generally, cells are most abundant in summer and least so
in winter. Long term monitoring of plankton at
fixed locations offer the opportunity to detect
annual and decadal variabilities. In the Bedford Basin,
a time series was initiated in October, 1991 to
establish an uninterrupted record of important
physical, chemical and plankton variables,
including bacterial abundance. Weekly observations
(Figure 3) show that short term responses of bacteria
to presumed shifts in controlling factors do not
obscure the underlying annual cycles.
At the seasonal scale, temperature emerges
as a dominant influence. Monthly averages of bacteria and temperature are tightly correlated
from January to June. The monthly average temperature of
14oC in June marks
the start of bacterial decline, presumably due to the dominating effects
of another factor. An earlier study (Taguchi and Platt 1978) had shown
that microzooplankton biomass in Bedford Basin was low through the winter
and increased from May to a peak in September, suggesting significant
grazing pressure in summer. The role of substrate supply to bacteria has
not been investigated here but it can be assumed to be increasingly
important in the summer when metabolic rates increase with temperature.
Biomass Pyramid
One of our most persistent views of food
webs is the pyramid of biomass. In this construct,
we visualize a pyramid whose broad base represents the large biomass of plants supporting
successively smaller strata of consumer organisms up
to the apex predator. Bacteria, being dependent (di
rectly and indirectly) on photosynthetic
production, are at a stratum higher than that of
the phytoplankton. Yet it is not uncommon to find bacterial biomass exceeding
phytoplankton biomass; in other words, an "inverted pyramid".
In the past 10 years, the Bedford Institute
of Oceanography has collected plankton bacteria on more than 1000 occasions (Figure 4) from
both near (e.g. Bedford Basin) and far (e.g. coastal
waters of Greenland, northern Africa, etc). We,
like other researchers (Gasol et al. 1997 and
references cited therein), have found that as
phytoplankton biomass decreases from nutrient-rich waters
to nutrient-poor waters, so too does bacterial biomass. However, the relative decrease in
bacteria is less than that of phytoplankton. In
other words, the decline of bacteria from
nutrient-rich to poor waters is less marked than the decline
of phytoplankton. A plot of the ratio of
biomasses indicates 2 domains: in chlorophyll-rich
waters, phytoplankton exceed bacteria; conversely in
chlorophyll-poor waters, bacteria exceed phytoplankton (Figure 5).
When the other heterotrophic (i.e.
dependent on organic substances) members of the food web (i.e. animals)
are included with the bacteria, the demand for phytoplankton production is
even greater. Apparently, this demand can be satisfied by a high turnover
of phytoplankton, a high level of detritus, or an import of organic matter
from outside the system.
Global Perspective
In temperate waters, bacterial abundance can evidently
track water temperature throughout the year (Figure 3). A broader issue,
however, is whether temperature exerts a significant influence on
abundance across a biogeographical range. In other words, is there a
relationship between climatological averages of abundance and temperature
in diverse marine habitats? If so, the large scale distribution of these
cells might conceivably be mapped by temperature.
Figure 4. Locations sampled by
BIO for bacteria, 1987-1997
Figure 5. The
biomass dominance of bacteria over phytoplankton in chlorophyll-poor
waters.
Depth profile of autumn Labrador Sea bacteria
shown as flow cytograms of SYTO-13induced fluorescence versus orthogonal
light scatter.
Figure 6A. Locations of datasets that
document seasonal variation of bacterial
abundance.
A global annual climatology of
bacterial abundance was developed based on 45
separate studies (Figure 6A) covering the full range of pre
vailing annual temperatures. Below
14oC, annual average abundance of bacteria is directly
related to annual average temperature; however,
above 14oC, no relationship is discernible (Figure 6B).
In other words, on an annual worldwide basis, the
abundance of bacteria is predicted with
reasonable accuracy by temperature in regions where
the average is less than 14oC.
Figure 6B. Relationship between
bacterial abundance and temperature at the annual scale from datasets in
Fig. 6A
In the world oceans, 49% of all one-degree latitude-longitude grid
boxes, representing a global ocean area of 32%, exhibits an average
surface temperature below 14oC. For these regions which extend
towards the north and south poles from approximately 40o
latitude in each hemisphere, it appears that marine bacteria may have a
surface distribution that is largely a reflection of the poleward decrease
in temperature. This pattern in observable cell abundance is the net
manifestation of the underlying processes of growth and loss upon which
the direct physiological effects of temperature are exerted. Further
progress in the ecological geography of marine bacteria can be expected
when climatological averages are compiled for the rates of growth and loss
and when due consideration is given to the entire population in the upper
mixed layer
References
DUCKLOW, H. W. 1992. Factors regulating bottom-up control of
bacteria biomass in open ocean plankton communities. Archiv für
Hydrobiologie Beihefte Ergebnisse der Limnologie 37:
207-217.
GASOL, J.M., P.A. del GIORGIO, and C.M.
DUARTE. 1997. Biomass distribution in marine
planktonic communities. Limnology and Oceanography 42:
1353-1363.
JACQUET, S., J.F. LENNON, and D. VAULOT.
1998. Application of a compact automatic sea water
sampler to high frequency picoplankton studies.
Aquatic Microbial Ecology 14: 309-314.
KREBS, C.J. 1972. Ecology: the experimental analysis
of distribution and abundance. Harper & Row
Publishers, New York, 694pp.
LI, W.K.W., J.F. JELLETT, and P.M. DICKIE. 1995.
DNA distributions in planktonic bacteria stained with
TOTO or TO-PRO. Limnology and Oceanography 40:1485-1495.
TAGUCHI, T. and T. PLATT. 1978. Size
distribution and chemical composition of particulate matter
in Bedford Basin, 1973 and 1974. Fisheries &
Marine Service Data Report No. 56, Canadian Department
of Fisheries & Environment, p 1-370.
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