© 2000, Annual Reports of the Zoological Institute RAS.

Microflora of bottom sediments of the White Sea Basin

Valentina N. Galkina

Zoological Institute, Russian Academy of Sciences, Universitetskaya nab., 1, St. Petersburg, 199034, Russia
 

The estimation of the White Sea biological resources is impossible without understanding of the principles governing the production processes. An important role in these processes belongs to the microflora of seawater and bottom deposits, since only photosynthesis surpasses bacterial oxidation and bacterial biosynthesis in biological importance.

Material and methods

Quantitative characterization of microflora in deepwater bottom sediments of the central part of the White Sea Basin was given. Samples of the deposits were collected at 9 stations: in spring (May-June), summer (July), autumn (early October) and winter (late November). The samples were preserved in 40% formaldehyde. The number of bacteria in each morphological group (cocci and rod-like) and their linear size were determined in the laboratory. The method of direct counting on nuclearpore filters (pore size 0.2 mm) treated with Sudan black B was used. Bacterial cells sedimented on the filters were stained with acridine orange and observed under fluorescent microscope (×1250). The biomass was calculated on the basis of the average cell volume and total number of the cells. The obtained value was expressed in mg of wet biomass per g of wet sediments. The bacterial production was estimated by the time, required for the doubling of the number of bacteria (Razumov, 1932; Poglazova & Mitzkevitch, 1984; Kuznetsov & Dubinina, 1989).

Results and discussion

The microbiological investigation of the deepwater part of the White Sea Basin demonstrated that the microflora density in bottom sediments amounts to 0.01-1.66 billion cells per g of wet mud depending on the season and depth of localization in the sea floor. The maximum density 1.27-1.66 billion cells per g of wet mud was discovered in the surface layer of hydrated mud everywhere in the Basin of the White Sea (Table 1).

Samples obtained from different layers of bottom sediments revealed a constant decrease in bacterial density with depth (Table 2). Bottom sediments at a depth of 0-1 cm contain 1.16 billion cells per g of wet mud in average over the stations, i.e. 21% less than surface hydrated mud. The density of bacteria decreases further with depth and does not exceed 0.01-0.07 billion cells per g at a depth of 25 cm, which makes only 3% of the value registered at the zero level. However, deep layers of the deposits are significantly populated with bacteria and probably they can be encountered in deeper layers of the sediments. However, the role of those bacteria is apparently confined to geochemical processes. In biological processes only microflora of upper layers of sediments (5-10 cm) is likely to be of any importance.

Table 1. Total density (N) and wet biomass (B) of bacterial population in the surface layer of the ground in he bottom sediments of the White Sea Basin

The value of bacterial density in the sediments of the deepwater part of the White Sea Basin can be assumed equal to 0.82-1.39 billion cells per g of wet sediment. Close values of bacteriobenthos density were registered in shallow water bottom deposits in the Baidaratskaya Inlet of the Kara Sea (Baitaz & Baitaz, 1993). It should be emphasized that the temperature of deep waters of the White Sea and in the investigated region of the Kara Sea is more or less the same and similar methods were used in both cases. The microflora density of about 1 billion cells per g of wet sediment must be typical for muddy bottom sediments at a temperature close to negative values and minimal anthropogenic load.

It is most likely that the bacteria community of bottom sediments of the White Sea Basin is mainly represented by psychrophilic forms with the enzymatic system adapted to the low temperature.

Table 2. Total density (N) of bacterial population at different depth from the surface (cm) in the bottom sediments of the White Sea Basin

Microflora of bottom sediments of the White Sea Basin contains 90% of cocci cells of an average volume of 0.14 mm3 in all the analyzed samples.

Bacterial biomass in the upper surface layer of mud at different stations varied from 0.18 to 0.23 mg per g of wet mud, yielding an average value of 0.21 mg per g, which makes the total of 21 g/m² for a 10 cm-thick layer. At the depth of 0-1 cm the biomass of bacteria was slightly lower and depending on their number at different stations varied from 16-19 g/m² for a 10 cm-thick layer. Assuming the content of organic matter in the deposits of the White Sea equal to 3.6% of dry weight (Klenova & Gorshkova, 1937), we obtain for the bacterial biomass a value of 2.2-2.9% of the total content of organic matter.

The organic matter suspended in the waters of the White Sea Basin is known to be uniformly distributed in the water mass with a concentration of 0.2-3.46 mg/l (Agatova et al., 1994). Consequently, the total content of organic matter reaches the value of 60-1038 g/m² in a 300 m - thick layer. If a value of 15 cm per hour (Kiselev, 1981) is assumed for the average sedimentation velocity of suspended particles, the above indicated amount of organic matter will sink to the bottom within 83 days. During a year the amount of organic matter may reach 258-4463 g/m², which is 4.3 times higher. About 85% of organic substances, contained in the sinking suspension, is assumed to be oxidized (Novitski, 1990). Hence, a total amount of organic matter of 38.7-669.0 g/m² reaches the sea floor within a year. If we assume that autochtonous substance is mainly formed during the vegetation period and do not take account of dissolved organic matter, even then the amount of autochtonous substance delivered within a year makes 19-334 g of organic matter per m2 with an average value of 178 g of organic matter per m². That organic matter contains 50% of proteins, 20-30% of carbohydrates and no more than 10% of lipids (Agatova et al., 1994). This testifies to a high energy and trophic value of that source for microorganisms. Therefore this substrate may be considered to be of crucial importance for the existence of microflora of the sea floor. Apparently, the indicated energy and trophic value of the suspension delivered to the bottom sediments makes possible such an active development of microflora in the deepwater part of the White Sea Basin, where, notwithstanding the low temperature, the density of bacteria reaches high values.

An approximate estimate of the bacterial biomass production can be given on the basis of the generation time, number and average volume of bacterial cells. In summer 1999 the average generation time was equal to 352 hours, i.e. 14.6 days. The number of bacteria being relatively constant during the whole period of time, it can be presumed that the quantity of newly formed bacteria is equal to that of the eliminated ones. Hence, the bacterial biomass is reproduced 25 times within one year (365 days), which yields 525 g of the organic matter of bacterial cells per m² a year for a biomass of 21 g/m² in the 10 cm-thick layer of deposits. Thus, daily production amounts to 0.014 mg of the organic matter per g of wet deposits.

The value of P/B coefficient, characterizing the intensity of the biomass recycling can be calculated on the basis of the obtained data. For the bottom sediments of the White Sea Basin the P/B value is 0.7 per day. Comparison of the data on different water basins yielded the following values of generation time, production and P/B coefficient. In the bottom sediments of the Rybinskoe Reservoir the generation time of microflora was equal to 5-44 hours in summer and up to 600 hours in winter (1972), in the Barents Sea to 34-379 hours (Teplinskaya, 1985), in the coastal region of the White Sea to 22-132 hours in autumn (Teplinskaya & Moskvina, 1987). The daily production in the deepwater sediments in the Pacific Ocean amounted to 0.04-0.28 mg of wet bacterial biomass per g of wet sediments, P/B = 0.01-0.05 (Sorokin, 1970). In coral reefs the value of production was up to 0.99 mg of wet bacterial biomass per g of wet sediments per day, P/B = 0.2-0.4 (Sorokin, 1990). The value of P/B coefficient for bacterioplankton of northern seas (The Laptev Sea) varies in the ranges from 0.06 to 0.33 (Sorokin et al., 1993).

According to the classification, proposed by Sorokin (1977), waters manifesting the following production characteristics: B = 0.05-0.4 mg of wet bacterial biomass per g of wet sediments; P = 0.005-0.06 mg per g of wet weight per day; daily P/B = 0.1-0.15 belong to the mesotrophic type. From comparison of the above-mentioned data with the results of our investigations it can be concluded that judging by the level of bacteriobenthos biomass the White Sea Basin can be ascribed to the mesotrophic type of water basins (Table 1). However, the value of production, equal to 0.014 mg of wet bacterial biomass per 1 g of wet deposits per day, and the rate of bacterial biomass recycling (daily P/B = 0.07) are significantly lower than the criteria assumed for the mesotrophic type and correspond rather to the oligotrophic type. Therefore, on the basis of size-structure and production parameters of the development of bacteria in the bottom sediments the White Sea Basin, apparently, should be considered as intermediate between oligo- and mesotrophic water types.

Therefore, the results, obtained in our investigations of the bottom sediments of the White Sea Basin, reveal active processes of both destruction of organic substances, delivered to the sea floor, and formation of bacterial biomass. Both processes are of crucial importance for the functioning of sea floor ecosystems, since oxidation of a large amount of organic matter prevents from the formation of azoic zones. Microflora density in the White Sea bottom deposits is sufficient for use by suspension feeders and deposit feeders (Sorokin, 1974). Hence the newly formed bacterial biomass should be recognized as an important nutrition source for sea floor organisms.

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