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Distribution and Abundance of Bacteria in the Ocean

W.K.W. Li and P.M. Dickie


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.

Depth distribution of bacteria

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.

Abundance of bacteria

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).

Seasonal cycle of surface temperature and bacteria

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.

Locations sampled by BIO for bacteria

Figure 4. Locations sampled by BIO for bacteria, 1987-1997


The biomass dominance of bacteria

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.

Locations of datasets
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.

Relationship between bacterial abundance and temperature

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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Last Modified : 2003-01-30