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[ACG53-12] Bacterial production dynamics in the presence of benthic animals at the bottom of a lake
Keywords:Sediment, DNA, Arthropod, Oligochaetes, Nucleotide metabolism
1. Introduction
A decrease in benthic animals such as amphipods due to hypoxia has been reported on the bottom of Lake Biwa (Ishikawa et al. 2023). Because the generation time of benthic animals is long, ranging from several months to a year (Ishikawa and Urabe 2002), even if the oxygen level returns to normal, the population will not recover for some time. So even if hypoxia occurs only briefly, frequent occurrences will cause significant damage to the population. Many benthic animals live on the lake bed and feed on organic matter, contributing to the nutrient cycle on the lake bed through the consumption and excretion of organic matter (Devine and Vanni 2002). In addition, there are reports that the disturbance of lake bed sediments by benthic animals not only promotes the circulation of materials in the water and on the lake bed, but also activates the production of bacteria in the surrounding area (van de Bund et al. 1994). However, the functions of these benthic animals are not well recognised in Japanese lakes, and knowledge of their functions is limited. In this study, we aimed to establish a method for measuring the production of benthic bacteria in the deep lake bed of Lake Biwa, and to evaluate the impact of benthic arthropods such as amphipod Jesogammarus annandalei, freshwater shrimp Palaemon pucidens and asellus Asellus hilgendorfii on the production of benthic bacteria.
2. Materials and methods
Since the structure of the surface layer of sediment can change significantly when it collapses, including the structure of the microbial community, we used an undisturbed column core sampler that can collect sediment while maintaining the stratum structure to collect sediment cores from depths of 90 m and 60 m in Lake Biwa. The water above the bottom sediment in the core was replaced with 0.2 µm filtered lake water. The sediment cores with benthic animals were incubated with 15N-dA (1 µM) for 20 hours under 8 ℃ condition. The experimental condition was supplemented with each of the following benthic animals: amphipod J. annandalei (5 inds/core), freshwater shrimp P. pucidens (1 ind/core), and asellus A. hilgendorfii (5 inds/core). The control condition was not supplemented with benthic animals. After incubation, surface layer of the sediment was collected from three locations in the core and the microbial reaction was stopped using an equal volume of ethanol. The 15N-dA incorporation rate (bacterial production) was quantified using LC-MS/MS according to Tsuchiya et al. (2015).
3. Results and Discussion
In the experimental conditions for the amphipod, freshwater shrimp and asellus, there was a tendency for the bacterial production to increase compared to the control. These arthropods disturbed the surface of the sediment during the incubation period, and the surface layer of the sediment became more like suspended sediment than before incubation. Therefore, it is thought that the increase in the contact area and contact opportunities between bacteria and organic matter led to the increase in the bacterial production. In addition, the animals used in the experiment release carbon, nitrogen and phosphorus into the surrounding water through their excrement, etc., so it is also possible that these nutrients also had an effect on the increase in bacterial production. From the above results, it is suggested that the presence of benthic arthropods increases the bacterial production in the sediment and increases the rate of organic matter decomposition.
From the above, it is suggested that benthic arthropods are an important group of organisms that promote material cycling, and that the conservation of benthic animals by preserving the benthic environment is an important issue in maintaining the sound material cycle of the entire lake.
Acknowledgement: This research was performed by the Environment Research and Technology Development Fund (JPMEERF20232M02) of the ERCA provided by MoE of Japan.
A decrease in benthic animals such as amphipods due to hypoxia has been reported on the bottom of Lake Biwa (Ishikawa et al. 2023). Because the generation time of benthic animals is long, ranging from several months to a year (Ishikawa and Urabe 2002), even if the oxygen level returns to normal, the population will not recover for some time. So even if hypoxia occurs only briefly, frequent occurrences will cause significant damage to the population. Many benthic animals live on the lake bed and feed on organic matter, contributing to the nutrient cycle on the lake bed through the consumption and excretion of organic matter (Devine and Vanni 2002). In addition, there are reports that the disturbance of lake bed sediments by benthic animals not only promotes the circulation of materials in the water and on the lake bed, but also activates the production of bacteria in the surrounding area (van de Bund et al. 1994). However, the functions of these benthic animals are not well recognised in Japanese lakes, and knowledge of their functions is limited. In this study, we aimed to establish a method for measuring the production of benthic bacteria in the deep lake bed of Lake Biwa, and to evaluate the impact of benthic arthropods such as amphipod Jesogammarus annandalei, freshwater shrimp Palaemon pucidens and asellus Asellus hilgendorfii on the production of benthic bacteria.
2. Materials and methods
Since the structure of the surface layer of sediment can change significantly when it collapses, including the structure of the microbial community, we used an undisturbed column core sampler that can collect sediment while maintaining the stratum structure to collect sediment cores from depths of 90 m and 60 m in Lake Biwa. The water above the bottom sediment in the core was replaced with 0.2 µm filtered lake water. The sediment cores with benthic animals were incubated with 15N-dA (1 µM) for 20 hours under 8 ℃ condition. The experimental condition was supplemented with each of the following benthic animals: amphipod J. annandalei (5 inds/core), freshwater shrimp P. pucidens (1 ind/core), and asellus A. hilgendorfii (5 inds/core). The control condition was not supplemented with benthic animals. After incubation, surface layer of the sediment was collected from three locations in the core and the microbial reaction was stopped using an equal volume of ethanol. The 15N-dA incorporation rate (bacterial production) was quantified using LC-MS/MS according to Tsuchiya et al. (2015).
3. Results and Discussion
In the experimental conditions for the amphipod, freshwater shrimp and asellus, there was a tendency for the bacterial production to increase compared to the control. These arthropods disturbed the surface of the sediment during the incubation period, and the surface layer of the sediment became more like suspended sediment than before incubation. Therefore, it is thought that the increase in the contact area and contact opportunities between bacteria and organic matter led to the increase in the bacterial production. In addition, the animals used in the experiment release carbon, nitrogen and phosphorus into the surrounding water through their excrement, etc., so it is also possible that these nutrients also had an effect on the increase in bacterial production. From the above results, it is suggested that the presence of benthic arthropods increases the bacterial production in the sediment and increases the rate of organic matter decomposition.
From the above, it is suggested that benthic arthropods are an important group of organisms that promote material cycling, and that the conservation of benthic animals by preserving the benthic environment is an important issue in maintaining the sound material cycle of the entire lake.
Acknowledgement: This research was performed by the Environment Research and Technology Development Fund (JPMEERF20232M02) of the ERCA provided by MoE of Japan.