2026 Ecosystem Transformation 9 (4), 121-136

Current state of periphyton and benthos bacterial communities in natural-anthropogenic biotopes of the Northern Caspian Sea

In the summer and autumn periods of 2023, bacterioperiphyton and bacteriobenthos were studied at the bottom survey stations installed in the northern part of the Caspian Sea in the area of the Yuri Korchagin offshore gas-condensate and oil field. At these stations saprotrophic bacteria were found, their abundance averaged millions of cells: cells/cm2 in periphyton and cells/g of dry weight in benthos. The number of hydrocarbon-oxidizing bacteria in studied samples reached several thousand cells, while the abundance of sulfate-reducing and thionic bacteria did not exceed hundred cells in both periphyton and grounds. The total bacterial count averaged several million cells per cm2 in periphyton and several hundred million cells per gram of dry weight in benthos. The total bacterial biomass ranged from tenths of mg C/m2 to several g C/m3. The most represented morphological group of bacteria in periphyton at the bottom survey stations were rods (72%), followed by coccoid forms (20%), while curved forms were found in smaller amounts (8%). In benthos, on the contrary, cocci were dominant (69%), rods were subdominant (23%), while vibrios (8%) were also a minor group. The analysis of the summer–autumn dynamics of the studied bacteriocenosis revealed a decrease in the studied parameters in the autumn period by an average factor of 2.0–2.5. The proportion of saprotrophs in the total bacterial count was substantial (it ranged from 4.4 to 5.2% for periphyton and from 3.8 to 5.8% for benthos, on the average, across the seasons of survey) and varied within a wide range (from 0.5 to 20.0%). The proportion of hydrocarbon-oxidizing group in the total bacterial count amounted to hundredths, thousandths, and ten-thousandths of a percent, which is typical for bacteriocenoses of the Northern Caspian Sea. At the same time, the proportion of sulfate-reducing and thionic bacteria amounted to thousandths and ten-thousandths of a percent. Overall, the bacteriocenosis of the studied bottom stations was represented by a stable bacterial community inhabiting the natural-anthropogenic biotopes of the Northern Caspian Sea.