The role of small heterotrophs (bacteria and protozoa) in ... Reports/Marine... · protozoa in this ecosystem is principally the same as in other parts of the ocean in the temperate

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 183: 144-151 . 1984.

The role of small heterotrophs (bacteria and protozoa) in a shelf ecosystem

Gerhard RheinheimerInstitut für Meereskunde an der Universität KielDüsternbrooker Weg 20, D-2300 Kiel 1, Bundesrepublik Deutschland

During the past decade investigations on the role of bacteria and protozoa in the shelf ecosystem of the western Baltic have been carried out by scientists of the Institut für Meereskunde.

The distribution of small heterotrophs has been studied and bacterial biomass calculated. The turnover times of different organic substances in water and sediment permit statements on microbial degradation, which is dependent on nutrient con­centrations, temperature, salinity, and other variables. Results of investigations on the bacterial hydrogen sulfide production in anoxic zones of Kiel Fjord are reported.

Of special importance are the uptake of organic matter by small heterotrophs and the transfer of microbial biomass to the consumers in the food web. About 35 % of the annual primary production in the western Baltic is transformed to bacterial biomass, which may be consumed to a large extent by protozoa and other members of the zooplankton and zoobenthos. The role of microorganisms as parasites of plants and animals is also discussed.

IntroductionBacteria and protozoa are members of the plankton and benthos communities in the sea. They are free-living in the water or attached to marine organisms and detritus. Their main functions in the ecosystem are remineraliza­tion of organic matter and transfer of energy from the primary producers to the consumers in the food web (Fig. 1). Bacteria and protozoa as well as fungi are parasites of marine plants and animals, thus controlling the development of these organisms.

Already about 90 years ago the first marine micro­biologists (Fischer, 1894; Russel. 1892) saw the im­portant role of small heterotrophs in the marine envi­ronment. In the period before 1914 it could be con­firmed that bacteria and protozoa are distributed throughout the sea and that small heterotrophs are in­volved in the degradation of all organic material of natural origin. It could be shown that under favourable conditions total remineralization of organic matter is possible. However, favourable conditions — that is, suf­ficient oxygen, adequate concentrations of all necessary nutrients, adequate reaction and temperature, and so on — are lacking in many parts of the sea. Especially in the shelf area there are anoxic zones where denitrification, desulfurication, and fermentation are the dominating processes of microbial activity. The production of hy­drogen sulfide leads to the formation of sulfureta where only few microorganisms survive.

During the past two decades new methods have been

developed which have allowed intensive studies to be made of the uptake of organic compounds by microbial populations and single cells. The determination of heterotrophic potential by 14C-labelled substances, autoradiography, epifluorescence microscopy, and scanning electron microscopy have enabled marine microbiologists to study the production of phyto­plankton exudates and lysates and their uptake by bac­

Figure 1. Remineralization of organic matter and energy transfer by bacteria in the pelagic food web.

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30 '11°

55e20 '

F U N E N

55° 55°

v v

8 U C H T4 *♦ A

30'3#EHMARN

Ëckerhfori

Kiel;

30 '11°

Figure 2. Map of Kiel Bight with the stations 1 - 5 and A of the investigations in 1974 and 1975 mentioned in this paper. B indicates the wastewater outlet at Biilk.

teria, which are consumed by protozoans. Thus, the energy transfer through the first links of the food chain could be estimated. However, the present knowledge on the role of commensales and parasitic microorganisms in marine ecosystems remains absolutely insufficient.

In the following pages a view of the role of the small heterotrophs in the shelf ecosystem of the western Bal­tic (Fig. 2) is given, as developed by scientists of the Institut für Meereskunde during the past decade.

Baltic waters are brackish, with very little tidal movement and thus not representative of the shelves of the North Atlantic. However, the role of bacteria and protozoa in this ecosystem is principally the same as in other parts of the ocean in the temperate zone.

Distribution of the small heterotrophs in the shelf area

In the western Baltic - as well as in the North Sea and other coastal waters of the North Atlantic - total bac­

terial counts between some hundreds of thousands and several millions have been found. Total bacterial num­bers determined by Zimmermann (1977) with epi- fluorescence microscopy in 1974 in Kiel Bight where water samples were examined monthly, ranged from 450 000 to 5 240 000 per ml, and the bacterial biomass from 1 • 5 to 25 • 0 mg C m~3.

The total numbers and biomass of small heterotrophs decrease, as a rule, from the shore to the open sea. This holds true for most of the various ecological and physiological groups of bacteria as, e.g., the saprophytic bacteria, which grow on agar plates prepared with yeast extract—peptone media, as well as the proteolytic, lipolytic, and carbon hydrate decomposing forms, cel­lulose digesters, and others. Depending on the quality and quantity of organic matter, the rates of decrease can vary greatly.

Normally the saprophytes and other specialists among the small heterotrophs react much more rapidly to changes in nutrient concentrations caused by eutro­phication than do the total bacterial numbers. Thus, in a transect from the rather polluted inner Kiel Fjord to the

10 Rapports et Procès-Verbaux 145

B a c t . /m l

Station 5 10m

2.0

Figure 3. Total numbers of saprophytic bacteria in central Kielbacteriaof saprophyticTotal numbersBight (station 5, 10 m) from January 1974 to March 1975.

relatively clean central Kiel Bight the total bacterial number decreased by a factor of 2 and the saprophyte number, in contrast, by a factor of 50.

The quantitative distribution of the bacteria in the Baltic Sea shows seasonal fluctuations that are more distinct in the near-shore areas than in the open sea. The yearly cycles of bacterial numbers and bacterial biomass demonstrate a connection with phytoplankton development. Maxima occur in spring and autumn, whereby a time lag in bacteria counts can be recognized. The curves of the saprophyte counts in particular show these two peaks clearly (Fig. 3).

The saprophytic bacterial flora of the western Baltic Sea is dominated by members of the genera Micrococ­cus, Achromobacter, Flavobacterium, Pseudomonas, Vibrio, Agrobacterium, Bacillus, and Corynebacterium (Bolter, 1977).

A few specialized groups of microorganisms have only one maximum in late summer or autumn. This is, for example, the case with chitinoclastic and cellulolytic bacteria (Lehnberg, 1972). A distinct correlation exists here between the substrate availability (chitin, cel­lulose) and the increase in those microorganisms that are able to utilize these compounds as food. However, this relationship between substrate availability and amount of corresponding bacteria can be disturbed or masked in unstable water.

Investigations by Zimmermann (1977) showed that in the western Baltic only a small amount of the bacteria is attached to detritus particles. The main values are below 10 % of the total bacterial number. Only 20 % of the particles were colonized by microorganisms.

Most of the bacterial strains isolated from the Baltic are halophilic. Besides proper marine bacteria with sa­

linity optima between 25 to 40 %o there are also brack­ish water forms with salinity optima between 10 and 25 %o. In addition to these two ecological groups there are freshwater forms with relatively greater salinity tol­erance. Their proportion in the saprophytic flora, even in coastal regions, remains relatively small since they cannot compete with the halophilic bacteria. A certain role is also played by the osmophilic forms, which do not require NaCl for their development, yet need a corresponding osmotic pressure. Such osmophilic bac­teria enter the sea with waste water and have relatively high nutritional demands. Therefore they can be found mainly in the more polluted areas of the shelf, such as harbours and bays. In the comparatively nutrient-poor waters of the open Baltic, they are unable to compete in the long run and are suppressed by halophilic forms, which are able to utilize very small concentrations of nutrients. In the more nutrient-rich sediments the pro­portion of non-halophilic bacteria is as a rule greater. This is also the case with the “aufwuchs” (attached) bacteria (Rheinheimer, 1971; 1977; 1980).

However, statements in the literature differ in respect to the composition and amount of attached bacteria. This probably results from the fact that with young cells “aufwuchs” is lacking among many algae but starts gradually during the development of a phytoplankton population or the aging of thalli of higher benthic algae. Moreover, such changes do not occur in the same man­ner among different algae. Thus, algae with cells sur­rounded by mucus have a different microflora from those without a mucus envelope. Investigations by Rieper (1976) in Schlei Fjord in the western Baltic showed that a specific and correspondingly species-poor bacterial population lives in the slime layer of M icro­cystis aeruginosa, which serves as a nutrient. These mucus layers consist of polysaccharides or their sulfuric esters. The microflora of the mucus is probably more or less independent of any exudates. On the other hand, the genuine “aufwuchs” bacteria that live directly on the algal cells feed mainly on the nutrients released by exudation and lysis. According to the availability of different easily degradable organic compounds such as proteins, amino acids, sugars, etc., a more species-rich bacterial flora can develop, which represents a good nutrient basis for grazers. This is particularly noticeable on the older thallus parts of benthic algae. Here also the more solid parts can be utilized, which leads to a further diversification of the “aufwuchs” flora, and often ciliates and rotifers that feed on bacteria are present.

Among the protozoa ciliates are widely distributed in the shelf seas together with certain flagellates and rhizopodes. Some of the free-living species like Uronema marinum and Euplotes spec, are cosmopoli­tans with a rather wide salinity tolerance. The same is true of Vorticella octava, which could be found among the “aufwuchs” of the blue-green algae Microcystis aeruginosa in Schlei Fjord and Nodularia spumigena in the western and central Baltic.

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Degradation of organic materialAmong the small heterotrophs bacteria play a major role in the degradation of organic material from differ­ent sources. The organic matter is converted by micro­organisms into compounds with a smaller energy con­tent and finally, under appropriate conditions, into the original mineral substances. This remineralization of organic substrate is the main function of bacteria and fungi in the marine ecosystem, but in this process pro­tozoa are equally involved. The limiting plant nutrients are recycled, allowing new plant growth. Complete mi­neralization can, as a rule, be attained only in the pre­sence of oxygen, and under anoxic conditions the break­down often remains incomplete.

Easily degradable substances, such as protein, sugar, and the like, are decomposed to a large extent by end- oxidations; but more resistant substances, such as fats, cellulose, and lignin, accumulate and, together with breakdown products, may contribute to the formation of the so-called sea humus.

Decomposition of organic substance varies according to its constituents and the environmental conditions. The extent of the breakdown of dissolved organic nu­trients is expressed in the turnover time, during which the total amount of a substance (e.g. glucose, acetate, amino acids) in water or sediment would be broken down at in situ temperature by the heterotrophic mi­croorganisms present. This turnover time is measured by regression analysis using the method described by Gocke (1977) in which, after the addition of 14C- labelled glucose, acetate, or amino acids and after 3 hours’ incubation, the substance uptake and the re ­leased amounts of C 0 2 are determined radiochemically.

Gocke (1977) studied the annual cycle of turnover time at different stations in Kiel Fjord and Kiel Bight. As an example Figure 4 shows the seasonal distribution

300 S t a t i o n 5

10mT,[h]

200

100

0

Figure 4. Seasonal changes of the turnover time of glucose in central Kiel Bight (station 5, 10 m) from February 1974 to March 1975 (after Gocke, 1977).

240

Tt I hiS t a t i o n 5

10m180

120

60

Figure 5. Seasonal changes of the turnover time of an amino acid mixture in central Kiel Bight (station 5, 10 m) from Feb­ruary 1974 to March 1975 (after Gocke, 1977).

of the turnover time of glucose and Figure 5 that of an amino acid mixture in central Kiel Bight. As a rule, turnover times for all compounds mentioned were dis­tinctly higher in winter than in summer.

The annual averages of the turnover time of glucose in the rather polluted inner Kiel Fjord were 15 hours at 2 m and 17 • 7 hours at 10 m depth, and in the relatively clean central Kiel Bight, they were 73 -3 hours at 2 m, 72 9 hours at 10 m, and 96-5 hours at 18 m depth.

Meyer-Reil et al. (1978) calculated the turnover time for glucose in sandy sediments of the western Baltic. An example from Falckenstein Beach at the outer Kiel Fjord also shows seasonal differences roughly related to temperature:

Temperature(°C)

Turnover time(h)

10 Nov 1976 ............. ............. 7 0 ■ 7018 Jan 1977 ............. ......... 1 2-1314 Apr 1977 ............. ............. 4 1 -69

2 May 1977 ........... ............. 9 0-974 Jul 1977 ............... ............. 19 0-32

Turnover times in similar sediments seem to show only relatively minor site differences, as revealed by inves­tigations on the Schleswig-Holstein and southern Swedish coasts. Thus, in the shelf areas, especially dur­ing summer, the degradation of organic compounds can proceed rather rapidly, while, according to Jannasch et al. (1971), organic material in the deep sea is degraded at an extremely slow rate.

10* 147

The decomposition process of organic substances produced by plants or animals is accompanied by per­manent changes in the microflora, which may be recog­nized by microscopic observation.

Particularly numerous in the sea are proteolytic bac­teria, of which many are also able to degrade sugars. On the other hand rather few specialists among the bacteria (e.g. pseudomonads, cytophaga, actinomycetes) and the fungi can utilize high molecular compounds like chitin, cellulose, and lignin. The substances are hydrolyzed by exoenzymes.

Anaerobic breakdown of cellulose is particularly im­portant in marine mud. It is carried out mainly by a few species of Clostridium. During anaerobic cellulose fer­mentation, ethanol, formic acid, acetic acid, lactic acid, hydrogen, and carbon dioxide are produced.

Fatty acids are also produced as a result of the hy­drolysis of fats and waxes and during various fermenta­tion processes. The incomplete breakdown of fatty acids in an anoxic environment is accompanied by the forma­tion of methane and carbon dioxide by a few anaerobic bacterial specialists. In overlying aerobic water bodies methane as well as hydrogen can be oxidized by another group of specialized bacteria.

In this connection it should be mentioned that bac­teria play an important role in the decomposition of mineral oils in the sea. Under suitable conditions almost all hydrocarbons, aliphatic and aromatic, can be broken down by microorganisms. Thus, after an oil spill the bacterial flora in water and sediment changes in com­position, and an increase in number and biomass could be shown by Gunkel et al. (1980) during extensive in­vestigations in the North Sea. In consequence protozoa and other bacteria feeders also grow.

In nutrient-rich environments oxygen may vanish completely by microbial action. Then nitrate can be used as an oxygen source by the many denitrifying bac­teria that are always present in the sea. They reduce nitrate via nitrite to molecular nitrogen or, to a lesser extent, to ammonia. When this oxygen source is de­pleted a few highly specialized obligatory anaerobic bacteria can use sulfate to oxidize organic matter, espe­cially fatty acids. This process is called desulfurication, because during the bacterial sulfate reduction, hydrogen sulfide is produced which may escape into the air and thus be lost from the substrate. The most important sulfate reducer in the Baltic as well as in the North Sea is Desulfovibrio desulfuricans subsp. aestuarii, from which halophilic and halotolerant strains are reported.

In one of the deeper holes of Kiel Fjord, where every summer H 2S can be found, in the uppermost zone of the sediment, strong seasonal fluctuations in the number of Desulfovibrio cells cm-3 were determined by Bansemir and Rheinheimer (1974):

4 Nov 4 Dec 11 Feb 19 Mar 6 May1968 1968 1969 1969 1969

100 000 5 500 20 000 5 500 100 000

In summer the number of Desulfovibrio cells is greater than 10« 000 cm-3. However, at a depth of 10 cm in the sediment the number decreases to a few hundred. In the water column the numbers observed never increased beyond 20 cells cm-3. Thus, nearly all the hydrogen sulfide in the holes of the western Baltic must be pro­duced in the upper 2 to 3 cm of the sediment, and only a very small amount has its origin in the water.

Jørgensen (1977) calculated that only 3 % of the sul­fide in the sediments of the Limfjord in northern Den­mark came from organic sulfur compounds. He deter­mined sulfate reduction rates by means of tracer techniques and parallel determinations of different sul­fur compounds and found very high values in the sur­face zone of the sediment with 25—200 nmol S O ^ c m '3 d~‘. Sulfate reduction showed marked seasonal fluctua­tions with clear peaks in the summer months. Of the hydrogen sulfide formed, 10 % was precipitated by metallic ions in the oxygen-free sediment, while the re ­maining 90 % was oxidized to sulfate again at the sur­face.

For sulfate the turnover times amounted to 4 —5 months and for hydrogen sulfide 1 — 5 days.

In regions bordering on aerobic zones, sulfur-oxi- dizing bacteria, particularly thiobacilli, or, on the sedi­ment surface Beggiatoa and also Thiothrix species, may again contribute to the oxidation of sulfides. A cycling of sulfur often takes place between sediment and the water above it, but, as a rule, it is incomplete since only part of the sulfur reaches the water again. Jørgensen (1977) found in the Limfjord that 9 ’8 mmol S m~2 d_1 entered the sediment in the form of sulfate and organic compounds and only 8 • 8 mmol S returned to the water again.

The appearance of hydrogen sulfide is always fol­lowed by a complete change in the bacterial popula­tions, and an entirely new extreme biological environ­ment comes into being with a living community of only a few kinds of microorganisms, called a sulfuretum.

The role of small heterotrophs in the food webPart of the primary production by phytoplankton and phytobenthos is excreted and serves bacteria as an easily utilizable nutrient. After Iturriaga and Hoppe (1977), 2 - 2 1 % of the phytoplankton primary produc­tion in Kiel Bight is released in the form of exudates. The uptake rates of the bacteria for the released sub­stances vary between 8 and 17 5 % per hour. Accord-

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ing to Wolter (1980) 0 - 1 5 - 6 % of the primary pro­duction or 0 —8 8 '7 % of the exudates were taken up within 24 hours by the bacteria during an investigation of an annual cycle in 1978 in the inner Kiel Fjord. How­ever, the various plankton algae behave differently with regard to the release of exudates. Thus, for example, Chaetoceros spec., Prorocentrum micans, and flagellates emit more exudates than Gymnodinium spec, and Peridinium spec. The bacteria probably also differ with regard to the uptake of exudates; however, under favourable conditions they can adapt within a short time to the nutrients available. The incorporation of exu­dates by bacteria shows a time lag with respect to the phytoplankton development in spring but is subject to fluctuations similar to those of the primary production during the rest of the year. Experiments executed by Bolter (1981) show the uptake of exudates to conform to that of dissolved carbohydrates. With the entry of the algae into the stationary phase of growth, the exudation increases noticeably, and finally lysis of the cells occurs. A t this time the bacterial “ aufwuchs” on the algae strongly increases, and a heavier grazing by ciliates and rotifers sets in.

Meyer-Reil (1977) calculated an average bacterial biomass production in the water of 57 g C m-2 yr-1 in Kiel Fjord and 9 g C m-2 yr-1 in the central Kiel Bight in the period March 1974 — February 1975. This repre­sents approximately 30 and 15 % of the primary pro­duction in the two areas; for the whole area of the west­ern Baltic, bacterial production in the water might therefore be tentatively estimated at about 20 % of the primary production.

Of great importance for bacterial activity is also the presence of particulate organic matter, which occurs in large amounts after the death of planktonic organisms, particularly after the breakdown of phytoplankton blooms. Immediately after lysis of the cells a rapid growth of bacteria takes place. Because of its bacterial "aufwuchs” , this phytoplankton detritus represents a valuable substrate for grazers, and many animals prefer it to living phytoplankters (e.g. blue-green algae) as food. Observations made by Horstmann (personal communication) showed that ciliates and rotifers feed on this substrate.

Bacterial breakdown continues after sedimentation of the organic matter. As a result the bacterial content in the uppermost benthic zone is very high. The total bac­terial numbers vary between several hundred millions and many billions per cm3 wet sediment. During monthly investigations at a location with a sandy sedi­ment in the central Kiel Bight at approximately 12 m depth, between June and September 1974, 682-2301 x 106 bacteria were counted in 1 cm3 wet sediment. Among these 36—51 % were found free-living in the interstitial water and 4 9 - 6 4 % attached to the sand grains (Weise and Rheinheimer, 1979). Still greater are the bacterial counts of mud sediments, which can be higher by 1—3 orders of magnitude. Thus, in the up­

permost part of the sea floor in the shelf area a large reservoir of organic material may be present in the form of bacteria, which can be utilized by protozoa and other members of the micro- and macrofauna and strongly influences their quantity and composition.

In shallow waters with good illumination on the sea floor the greatest portion of the organic matter can be produced by the macro- and microphytobenthos. In some parts of the western Baltic there are fields of eel- grass and macro-algae; in others, microphytobenthos dominates.

Investigations by Meyer-Reil et al. (1980) of sandy sediments from a shore area of Kiel Bight without macrophytobenthos during the summer of 1977 gave a primary production by the microphytobenthos of 3 ■ 7 mg C m_2 h_l. From this 1 ■ 8 mg C m~2 h_1 or 50 % was transformed into bacterial biomass. The bacterial biomass of 43 mg C thus produced daily per m 2, is of about the same order of magnitude as the meiofauna present here, which was determined at 35 mg C m~2. This shows that the bacterial biomass production in these sediments is sufficiently high to supply carbon compounds for a daily turnover of the meiofauna. From the macrophytobenthos carbon a smaller proportion is transformed to bacterial biomass.

Meyer-Reil and Faubel (1980) found an inverse re ­lationship between meiofauna and bacterial carbon. Tracer experiments with tritiated glucose revealed a different incorporation of organic matter by various meiofaunal groups. Detritovore oligochaetes showed the highest incorporation rate, followed by turbellaria and nematodes. Bacteria and their extracellular pro­ducts seem to be the major part of the organic material taken up by these animals.

From the results cited above a rough estimate of the carbon flux in the food web of Kiel Bight can be made. About 20 % of the phytoplankton carbon produced yearly is transferred to bacterial carbon in the water. Up to 60 % of the primary production sinks to the bottom of Kiel Bight (mean depth 20 m). In the sediment about 15 % more of the total carbon produced by phyto­plankton is transformed to bacterial biomass. The pri­mary production of micro- and macrophytobenthos in Kiel Bight of about 150 g C m-2 yr-1 (M. Meyer, per­sonal communication) is in the same range as that of phytoplankton. From 30 to 40 % of the benthos pri­mary production is transformed to bacterial carbon. Among the grazers in the water and on the sediment surface, protozoa (mainly ciliates) play an important role. Especially in the case of summer phytoplankton blooms, large populations of ciliates and flagellates can grow, feeding to a great extent on bacteria.

Investigations by Gast (1983) showed that an in­crease in protozoa is strictly correlated with a decrease in bacteria.

In feeding experiments with Uronema marinum and Euplotes spec. 6 • 8 mg bacterial carbon were converted to 1 3 mg ciliate carbon. Thus, about 20 % of the bac-

149

terial biomass consumed is transferred to ciliate biomass. The rest is used for the energy demand of the ciliate cells and, to a lesser extent, excreted as indigesti­ble material.

Very important for the growing of bacteriovore ciliates is the bacterial concentration. The threshold values for Uronema marinum could be estimated at greater than 1 million bacteria cm"3. Thus, only in waters with more than 1 million bacteria cm-3 can a population of ciliates be expected. The greater the bac­terial concentration the greater is the ciliate production. The feeding rate at increasing densities is nearly linear until a saturation value is reached.

Owing to the short generation time (2—3 hours for Uronema marinum), ciliates can quickly react to favourable nutrient conditions and influence the bac­terial population in the ecosystem.

The transfer of energy from primary producers via bacteria to primary consumers is of particular impor­tance if phytoplankton is not directly eaten by zoo­

plankton as is the case with blue-green algal blooms in the Baltic.

Another factor to be considered is that bacteria are able to concentrate organic material from very low concentrations in the water. Thus, it can be brought into the food web as a valuable protein nutrient.

Parasitic microorganismsMany bacteria and fungi live as parasites on a great variety of marine plants and animals and may cause diseases which lead to the death of the affected organisms. They are particularly important where cer­tain plants and animals occur in large numbers in re­stricted space, such as, for example, plankton blooms, mussel banks, or fish shoals. In cases of such mass occurrence of one particular species, epidemics have been observed which have led to complete destruction of large crops of plants or stocks of animals. Organisms that live in greater isolation, however, are much less

E nvironm ental fa c to rsLight

Zooplankton

P hytoplanktonDeath ra te

Fishing m orta lity ' N death rate

Fish

Tempe­ra ture Salin ity

/^erobic bacteria

Anaerobicbacteria

POM

Phytobenthos

DOM

Physical and chemical / interactions \

DIN

Legend :

D Plant

production

<§)• S torages,

populations

O Consumers

O " Input

W ork gate w ith

'' energy sink

POM » Particulate organic matter

DOM - Dissolved organic matter

D IN = Dissolved inorganic nutrien ts

Figure 6. Abstraction of the ecosystem of Kiel Bight with special reference to small heterotrophs, using Odum symbols. The hatched boxes indicate the interference of small heterotrophs (after Bolter, 1977, modified).

150

prone to attacks by pathogenic microorganisms, be­cause of the reduced chances for infection.

Although parasitic microorganisms have been discov­ered in rather large numbers in marine plants and ani­mals, the course of infection has mainly been studied in detail in cases of economically important organisms such as seaweed, oysters, crayfish, and commercially important fish species.

Very little is known about bacterial diseases in plankton algae, and there are only few reports on bac­terial pathogens in lower animals.

However, there is some information on bacteria- induced diseases of oysters from European and Ameri­can coasts.

A series of microbial diseases are known to affect economically useful fish. Investigations have increased during recent years with the establishment of marine fish farms in coastal waters.

In brackish water of the Baltic and along the coasts of the North Sea Vibrio anguillarum has attacked eels, causing the red pest or saltwater eel disease. Particularly in warm summers the eel stock of the western Baltic can suffer from this disease. Furuncolosis caused by Aero- monas sp., fish tuberculosis due to infection with mycobacteria, and vibriosis are rather common. Other fish pathogens among the bacteria are Haemophilus, Corynebacterium, and Cytophaga. Viruses and fungi are also very important as fish pathogens. However, in most cases the animals affected are already weakened.

Conclusion

From microbiological studies carried out mainly during the past two decades in different regions of the shelves, the complex role of small heterotrophs in shelf ecosys­tems has become evident. We can now roughly sketch the functions of bacteria and protozoa in the ecosystem of Kiel Bight (Fig. 6).

The main gaps in our knowledge at present concern the degradation of high molecular organic compounds, the transfer of energy from bacteria to the primary consumers (protozoa) in the food web, and the influ­ence of microbial parasites on the ecosystem.

Thus, for a better understanding of the shelf ecosys­tem, intensive studies on these questions will be our task in the coming years.

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