Feasting on terrestrial organic matter: Dining in a dark lake changes microbial decomposition

Area of Science:

• * Aquatic microbial ecology
• * Biogeochemical cycling
• * Boreal ecosystem science

Background:

• * Boreal lakes are crucial for the global carbon cycle due to microbial decomposition of organic matter.
• * Sediment bacteria break down both within-lake and terrestrially derived organic matter (t-OM).
• * The interplay between environmental factors and bacterial community composition in t-OM decomposition is not well understood.

Purpose of the Study:

• * To investigate how bacterial activity responds to varying quality and quantity of t-OM inputs in boreal lake sediments.
• * To identify the mechanisms driving bacterial responses to t-OM.
• * To compare these responses between a dark and a clear boreal lake.

Main Methods:

• * Experimental manipulation of t-OM inputs into littoral sediments of two boreal lakes (one dark, one clear).
• * Measurement of bacterial production (BP).
• * Analysis of bacterial operational taxonomic units (OTUs) and functional genes.

Main Results:

• * Bacterial production (BP) declined with increasing aromatic dissolved organic matter (DOM) in sediment pore water.
• * In the dark lake, bacteria increased extracellular enzyme production to degrade aromatic DOM.
• * In the clear lake, bacteria mineralized photo-degraded OM to CO2, potentially due to nutrient limitations or lack of genetic capacity for aromatic DOM degradation.
• * Dark lake sediments had higher abundance of t-OM degradation functional genes.
• * Dissolved organic carbon (DOC) and pH differentiated bacterial communities between the lakes.

Conclusions:

• * Bacterial decomposition of t-OM in boreal lakes is influenced by both environmental conditions and microbial community structure.
• * Future changes in t-OM inputs will have varied impacts on carbon cycling, depending on light exposure (photo-degradation potential) and resident bacterial communities.
• * Understanding these factors is critical for predicting the role of boreal lakes in future carbon budgets.