Importance of Bacterial Maintenance Respiration in a Subarctic Estuary: a Proof of Concept from the Field

Effects of Tropical Cyclone Passage on Plankton Community Respiration in a Phosphate-Limited Freshwater Ecosystem

Plankton community respiration (CR) in aquatic ecosystems varies with environmental factors, which could be altered during tropical cyclones (TCs). A potential increase in CR resulting from the effects of TCs is generally understudied. Here, we examined the relationship between plankton CR and environmental factors, including during TC-affected periods, in a phosphate-limited freshwater ecosystem. We conducted an intensive in situ sampling in Fei-Tsui Reservoir (FTR) from January 2010 to December 2015 during TC periods and non-TC periods. Our results showed a consistent temporal pattern that plankton CR increased between March to October and declined between November to February. These changes in plankton CR, primarily supported by bacterial biomass, were positively influenced by euphotic depth-averaged temperature. The CR also significantly increased with euphotic depth-averaged NO2-concentrations and decreased with euphotic depth-averaged NO3-concentrations. These results indicated that these factors typically influenced CR dynamics in the FTR. During TC periods, plankton CR was increased further due to a higher and ideal euphotic depth-averaged temperature (23-27°C) and increased supply of limiting nutrient resources via stream runoff. Overall, this study showed that a TC positively influences plankton CR by creating favorable water conditions. Notably, with a higher frequency of intense TCs projected for the Western North Pacific in most climate change scenarios, the impact of TCs on CR may increase in the near future.

Prokaryotic maintenance respiration and growth efficiency field patterns reproduced by temperature and nutrient control at mesocosm scale

The distribution of prokaryotic metabolism between maintenance and growth activities has a profound impact on the transformation of carbon substrates to either biomass or CO₂ . Knowledge of key factors influencing prokaryotic maintenance respiration is, however, highly limited. This mesocosm study validated the significance of prokaryotic maintenance respiration by mimicking temperature and nutrients within levels representative of winter and summer conditions. A global range of growth efficiencies (0.05-0.57) and specific growth rates (0.06-2.7 d^-1 ) were obtained. The field pattern of cell-specific respiration versus specific growth rate and the global relationship between growth efficiency and growth rate were reproduced. Maintenance respiration accounted for 75% and 15% of prokaryotic respiration corresponding to winter and summer conditions, respectively. Temperature and nutrients showed independent positive effects for all prokaryotic variables except abundance and cell-specific respiration. All treatments resulted in different taxonomic diversity, with specific populations of amplicon sequence variants associated with either maintenance or growth conditions. These results validate a significant relationship between specific growth and respiration rate under productive conditions and show that elevated prokaryotic maintenance respiration can occur under cold and oligotrophic conditions. The experimental design provides a tool for further study of prokaryotic energy metabolism under realistic conditions at the mesocosm scale.

Response of Coastal Shewanella and Duganella Bacteria to Planktonic and Terrestrial Food Substrates

Global warming scenarios indicate that in subarctic regions, the precipitation will increase in the future. Coastal bacteria will thus receive increasing organic carbon sources from land runoff. How such changes will affect the function and taxonomic composition of coastal bacteria is poorly known. We performed a 10-day experiment with two isolated bacteria: Shewanella baltica from a seaside location and Duganella sp. from a river mouth, and provided them with a plankton and a river extract as food substrate. The bacterial growth and carbon consumption were monitored over the experimental period. Shewanella and Duganella consumed 40% and 30% of the plankton extract, respectively, while the consumption of the river extract was low for both bacteria, ∼1%. Shewanella showed the highest bacterial growth efficiency (BGE) (12%) when grown on plankton extract, while when grown on river extract, the BGE was only 1%. Duganella showed low BGE when grown on plankton extract (< 1%) and slightly higher BGE when grown on river extract (2%). The cell growth yield of Duganella was higher than that of Shewanella when grown on river extract. These results indicate that Duganella is more adapted to terrestrial organic substrates with low nutritional availability, while Shewanella is adapted to eutrophied conditions. The different growth performance of the bacteria could be traced to genomic variations. A closely related genome of Shewanella was shown to harbor genes for the sequestration of autochthonously produced carbon substrates, while Duganella contained genes for the degradation of relatively refractive terrestrial organic matter. The results may reflect the influence of environmental drivers on bacterial community composition in natural aquatic environments. Elevated inflows of terrestrial organic matter to coastal areas in subarctic regions would lead to increased occurrence of bacteria adapted to the degradation of complex terrestrial compounds with a low bioavailability.