Systematic microbial production of optically active dissolved organic matter in subarctic lake water

Abstract

The ecology and biogeochemistry of lakes in the subarctic region are particularly sensitive to changes in the abundance and optical properties of dissolved organic matter (DOM). External input of colored DOM to these lakes is an extensively researched topic, but little is known about potential reciprocal feedbacks between the optical properties of DOM and internal microbial processes in the water. We performed 28-day dark laboratory incubation trials on water from 101 subarctic tundra lakes in northern Sweden, measuring the microbial decay of DOM and the resulting dynamics in colored (CDOM) and fluorescent (FDOM) DOM components. While losses in dissolved oxygen during the incubations corresponded to a 20% decrease in mean DOM, conversely the mean CDOM and total FDOM increased by 22% and 30%, respectively. However, the patterns in microbial transformation of the DOM were not the same in all lakes. Notably, along the gradient of increasing ambient CDOM (water brownness), the lakes showed decreased microbial production of protein-like fluorescence, lowered DOM turnover rates and decreasing bacterial growth per unit of DOM. These trends indicate that browning of subarctic lakes systematically change the way that bacteria interact with the ambient DOM pool. Our study underscores that there is no unidirectional causal link between microbial processes and DOM optical properties, but rather reciprocal dependence between the two.

Weak mineralization despite strong processing of dissolved organic matter in Eastern Arctic tundra ponds

“…This study shows that four nearby tundra ponds can have different patterns in DOM photochemical and microbial transformation, which are likely driven by the quality of the DOM pool at the start of the experiment. This is in agreement with results from studies in subarctic lakes showing changes in composition and biological reactivity along DOM gradients (e.g., Berggren et al 2019). Notably, the photodegradability of DOM has proven to be related to its color (Lapierre and del Giorgio 2014).…”

“…These general trends may be inadequate for thermokarst lakes deeply affected by permafrost soil erosion, receiving organic matter with different reactivities to microbial and photochemical degradation than allochthonous matter from nonpermafrost regions or from regions not affected by thermokarstic erosion. Moreover, as pointed out by Berggren et al (2019), the common assumption that CDOM has a terrestrial origin may need to be revised.…”

“…The consumption of chromophoric/aromatic molecules concurrent with a production of fluorescing molecules was also found in lacustrine systems of the temperate zone, and largely depended on DOM source (Guillemette and del Giorgio 2012). The production of CDOM and FDOM has also been observed in laboratory incubations of subarctic lake DOM (Berggren et al 2019). Bacteria tend to be presented in the literature as low-molecular-weight nonaromatic DOM consumers, with photodegradation often causing DOM properties to shift in an opposite direction than biodegradation (Hansen et al 2016).…”

The dominant role of sunlight in degrading winter dissolved organic matter from a thermokarst lake in a subarctic peatland

“…broadly observed in boreal and arctic lakes (e.g. Berggren et al, 2020;Laurion et al, 2021). The study of Berggren et al (2020) on 101 boreal lakes suggests the reciprocal dependence between microbial processes and DOM optical properties.…”

“…Berggren et al, 2020;Laurion et al, 2021). The study of Berggren et al (2020) on 101 boreal lakes suggests the reciprocal dependence between microbial processes and DOM optical properties. Considering the concurrent increase in BP and FDOM, we assume that bacteria were responsible for this synthesis of fluorescent molecules.…”