Phosphorus Availability Promotes Bacterial DOC-Mineralization, but Not Cumulative CO2-Production

Abstract

The current trend of increasing input of terrestrially derived dissolved organic carbon (DOC) to boreal freshwater systems is causing increased levels of carbon dioxide (CO 2) supersaturation and degassing. Phosphorus (P) is often the most limiting nutrient for bacterial growth and would thus be expected to increase overall mineralization rates and CO 2 production. However, high carbon (C) to P ratios of terrestrially derived DOC could also cause elevated cell-specific respiration of the excess C in heterotrophic bacteria. Using data from a survey of 75 Scandinavian lakes along an ecosystem gradient of DOC, we estimated in situ CO 2 production rates. These rates showed a unimodal response with DOC-specific CO 2 production negatively related to DOC:total phosphorus (TP) ratio, and a turning point at 5 mg C L −1 , indicating higher DOC turnover rates in productive than in unproductive lakes. To further assess the dependency of bacterial respiration (BR) on DOC and P, we monitored CO 2 production in incubations of water with a gradient of DOC crossed with two levels of inorganic P. Finally, we crossed DOC and P with a temperature gradient to test the temperature dependency of respiration rates [as oxygen (O 2) consumption]. While total CO 2 production seemed to be unaffected by P additions, respiration rates, and growth yields, as estimated by ribosomal gene copy numbers, suggest increased bacterial growth and decreased cell-specific respiration under non-limited P conditions. Respiration rates showed a sigmoid response to increasing DOC availability reaching a plateau at about 20 mg C L −1 of initial DOC concentrations. In addition to these P and DOC level effects, respiration rates responded in a non-monotonic fashion to temperature with an increase in respiration rates by a factor of 2.6 (±0.2) from 15 to 25°C and a decrease above 30°C. The combined results from the survey and experiments highlight DOC as the major determinant of CO 2 production in boreal lakes, with P and temperature as significant modulators of respiration kinetics.

Factors Governing Biodegradability of Dissolved Natural Organic Matter in Lake Water

“…Moreover, few dystrophic lakes have high levels of inorganic ions [64]. This is consistent with the findings of Allesson et al [18], who also suggested that at a community level, bacterial production increases relative to the bacterial respiration in nutrient-rich lakes. We thus suggest that in meso-and eutrophic lakes, the bacterial community uses a large proportion of the DNOM to grow and respire, although only a small part is used to provide energy for this growth.…”

“…Abbott et al [54] found that DP in their permafrost leachate samples was a good positive indicator of the percentage of BDOC. Similarly, Allesson et al [18] reported a higher turnover of BDOM in lakes in which P was in surplus. In our incubation experiment, all treatments received N and P to avoid the effects of nutrient limitation, in order to specifically test the role of DNOM quality on respiration.…”

“…Optical sensors for dissolved oxygen have previously been used to measure the biodegradability of organic pollutants under incubation [16]. The estimation of the respiration rate of organic matter with optical sensors has been previously undertaken in our group [17][18][19][20]. The large scale of the current study allows a standardisation of the method, including the development of a script for the extraction of biodegradability parameters and the comparison of the respiration rates in a large variety of samples.…”

Experimental evaluation of the role of inorganic phosphorus for terrestrial carbon degradation in stream hyporheic zones

“…It is possible that the hyporheic microbial community in these nutrient polluted streams shifted its metabolism towards a higher assimilation of the provided DOM soil than respiration under P-enriched conditions. Similar effects have been found in Scandinavian lake waters where P additions increased bacterial growth and decreased cell-specific respiration at the same time (Allesson et al, 2020). However, more evidence is needed to confirm this mechanism here.…”

Regrowth of Escherichia coli in environmental waters after chlorine disinfection: shifts in viability and culturability

“…, phosphate) regulates carbon mineralization, and it is the most frequently reported limiting nutrient for bacterial production. 60–62 The sampled Nomi River water contains a higher phosphate concentration (2.73 mg L −1 ) than Tama River water (1.23 mg L −1 ). Considering that Nomi River water is more nutritious than Tama River water in terms of AOC and MAP, it was reasonable to expect more regrowth in Nomi River water under all conditions, consistent with our observation (Fig.…”