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NATURA MONTENEGRINA, PODGORICA, 3, 2004, 327-332 UDK 579.68:628.473.2(285.2)(497.7)(045)=111 COMPARATIVE INVESTIGATIONS OF ORGANOTROPHIC BACTERIA AND BIODEGRADABLE DISSOLVED ORGANIC MATER IN THE LAKE OHRID PELAGIC WATERS Vera NOVEVSKA 1 Synopsis Investigations of number of organotrophic bacteria and concentrations of dissolved biodegradable organic matter (DOM) were performed in the open waters of Lake Ohrid during one year period. Material was collected from several depths at a site of 200m depth. Organotrophic bacteria were cultivated on 20% MPA and incubated at 22 o C during 7 days. Counting of colonies was done with a binocular. Dissolved biodegradable matter was determined as KMnO 4 consumption. Key words: Lake Ohrid, organotrophic bacteria, DOM, mutual relations Sinopsis UPOREDNA ISTRAZIVANJA ORGANOTROFNIH BAKTERIJA I BIORAZGRADLJIVIH ORGANSKIH MATERIJA U VODI PELAGIJALA OHRIDSKOG JEZERA U toku jednogodišnjeg perioda, u vodi pelagijala Ohridskog jezera, istraživan je broj organotrofnih bakterija (fakultativno oligotrofnih) i količina rastvorenih biorazgradljivih organskih materija (DOM). Materijal je kolekcioniran sa stacionara na maksimalnoj dubini od 200 m sa nekoliko dubina. Organotrofne bakterije zasejavane su na 10% MPA-u, i inkubirane na 22 0 C u toku 7 dana. Brojanje izraslih kolonija vršeno je pomoću binokulara. Rastvorene biorazgradljive organske materije određivane su kao utrošak kalijum permanganata. Ključne reči: Ohridsko jezero, organotrofne bakterije, DOM, uzajamni odnosi. 1 Hydrobiological Institute, 6000 Ohrid, R. Macedonia; e-mail: [email protected] 328 Natura Montenegrina, 3/2004. INTRODUCTION Transformation of organic substrates of utilizable dissolved organic matter stored in the bacterial biomass is a crucial step in the processing of the organic matter by the organotrophic bacteria and its direction toward higher trophic levels in the aquatic ecosystems. Because of that interactions between bacteria and organic matter are of great interest in the lake functioning. These interactions are also important because of the structure of the organic matter which, together with other physicochemical factors directly controls the activity and structure of the microbial communities (W i t z e l , 1990). Aim of this work is to try and analyze mutual relations of number of heterotrophic bacteria and the content of dissolved organic matter in the pelagic water column of Lake Ohrid. STUDY AREA Lake Ohrid is a transboundary lake located in the southwestern part of the Republic of Macedonia with surface area of 358.18 km 2 , from which 118.9 km 2 belongs to the R. of Albania. Lake Ohrid is the largest and the deepest lake in the Dassaret lake group of Aegean lake zone with maximal depth of 286 m (Fig.1). Longterm investigations of Lake Ohrid have shown that this aquatic ecosystem has an oligotrophic character with unique ecological complex that makes possible the existance of special endemic and relic living world. MATERIAL AND METHODS Samples were collected (within one year period) with Ruttner water sampler at: 5m, 15m, 20m, 25m, 30m, 50m, 100m and 200m from the water column in the pelagic region of Lake Ohrid. From microbiological point of view a number of facultative oligotrophic bacteria developed on 10% MPA (P e t r o v i c 1998) were analyzed. Cultivated material was incubated at 22 o C during seven days. Content of the dissoved biodegradable organic matter was determined as KMnO 4 consumption. RESULTS AND DISCUSSION Organotrophic bacteria are main consumers of the organic matter in the natural waters and that is why good knowledge of mutual relation between bacterial metabolism and the trophic state is necessary. Numerous works indicate that the bacterial growth and the productivity positively corelate with concentrations of dissolved biodegradable organic matter in lakes. Relatively recent studies present that bacteria depend much more of the quality of the dissolved organic matter than of their ambiental concentration (C h r o s t , 1986; C h r o s t & R a i , 1993). 329 NOVEVSKA, V: Comparative Investigations Between Organotrophic Bacteria Fig. 1. Map of Lake Ohrid Crn Drim Sat eska reka Koselska reka Velgo{ka reka Reka ^erava Annual results for the spatial distribution of the average number of organotrophic bacteria in the pelagic region of Lake Ohrid are presented in Fig. 2. There can be seen an uneven distribution with evident changes in the trophogenic layer (5m-50m) in which, according S t a n k o v i c (1959) I situated main bacterial biomass. Their minimal number (222 bac·ml -1 water) is evidented at 5m depth, but there are two picks at 15m depth (608 bac·ml -1 water) and at 200 m depth (943 bac·ml -1 water). 330 Natura Montenegrina, 3/2004. Organotrophic bacteria/ml water Depth (m) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 200 400 600 800 1000 Fig.2. Spatial distribution of the average number of organotrophic bacteria in the pelagic region of Lake Ohrid . Depth (m) In close corelation with the bacterial number is dissolved biodegradable organic matter which, in the pelagic zone of Lake Ohrid, mainly, is of autohtonous origin. Spatial distribution of average quantities during this investigated period are presented in Fig. 3. For this parameter maximal changes are evident in the trophogenic layer as well. KM nO4 mg/l water Minimal average value (3.50 mg·l -1 ) 3,20 3,40 3,60 3,80 4,00 4,20 4,40 waterwas evidented at 100m depth, and 5 10 15 maximal (4.30 mg·l -1 water) at 30m depth. 20 25 30 It is reasonable of their decrease in the 35 40 deeper layers. Depth of this aquatic 45 50 55 ecosystem, make available organic matter, 60 65 to be mineralized during sinking. From the 70 75 80 figures it can be seen that, mainly, bigger 85 90 number of the facultative organotrophic 95 100 105 bacteria respond to the lower quantity of the 110 115 120 dissolved biodegradable organic matter. 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 Fig. 3. Average spatial distribution of dissolved biodegradable organic matter values in the pelagic region of Lake Ohrid 331 NOVEVSKA, V: Comparative Investigations Between Organotrophic Bacteria 1000 6,00 800 5,00 4,00 600 3,00 400 2,00 200 1,00 0 KMnO4 mg/lit number of organotrophic bacteria/ml water Opposite situation of these parameters is shown at depth 5m and 30m, which indicate that bigger quantity of DOM is not always a precondition for more numerous bacterial population. We suppose that this situation can be the result of some inhibitory impact to the bacterial enzymatic system that disables utilization of substrate and transformation into bacterial biomass (V a s i l e s k a 2002). According to H e r n d a l e t a l (1992) solar radiation (UV-B, 280nm-320nm) can be the a suppressor of the bacterial activity at surface layer (up to 10m). Also, it should not be neglected that influence of the other plankton constituent component, the zooplankton community, as a predator, can influence bacterial number (K o s t o s k i 1998), G u r u n g et al (2000). Temporal review of the mutual relation of facultative oligotrophic bacteria and dissolved biodegradable organic matter are presented in Fig.4. 0,00 J/97 F M A M J J A organotrophic bacteria S O N D J/98 org.mater. Fig. 4. Temporal distribution of mean values of facultative oligotrophic bacteria and dissolved biodegradable organic matter in the pelagic region of Lake Ohrid It is remarkable that during longer period of these investigations there is a situation of greater quantity of organic matter and lower bacterial number, what indicates the that bacteria are not able to transform the organic matter in their own biomass. According to C h r o s t (1986), this situation could be explained as low bacterial utilization of the organic matter because of its form, and second, because of the absence of other auxiliary processes that support their utilization. During the period of May-June there is a situation where there is an increased bacterial development as its better utilization and transformation in the bacterial biomass. One of the components of the metabolic activities in the aquatic ecosystems is the constructive exchange in which, as the result of biochemical processes, there are utilized organic matter from the surrounding water and transformed in the bacterial biomass (D j u k i c , 2000). It is very important energy to be utilized for constructive exchange and the organic matter to be directed in the metabolic processes to the highest fesible extent 332 Natura Montenegrina, 3/2004. CONCLUSIONS According to the analyzes and results obtained during one year period of investigation of facultative oligotrophic bacteria and quantity of dissolved biodegradable organic matter in the pelagic water column of Lake Ohrid, it can be concluded that it is necessary to have an integral approach to analysis of these parameters. It is important to pay more attention to qualitative composition of the dissolved biodegradable organic matter in terms of their utilization within constructive exchange. Also, analysis of DOM can not be successful if the mutual relation of other constitutive elements of planktonic community, phytoplankton and zooplankton are not taken into account. REFERENCES C h r o s t R . J . , (1986): Algal-bacterial Metabolic Coupling in the Carbon and Phosphorus Cycle in Lakes. In Megusar F, Gantar (eds) Perspactives in Microbial Ecology. Slovene Society for Microbiology, Ljubljana, pp: 360-366 C h r o s t , R . 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