Abstract
The light-to-nutrient hypothesis proposes that under high light-to-nutrient conditions, bacteria tend to be limited by phosphorus (P), while under relatively low light-to-nutrient conditions, bacteria are likely driven towards carbon (C) limitation. Exploring whether this light-to-nutrient hypothesis is fitting for alpine lakes has profound implications for predicting the impacts of climatic and environmental changes on the structures and processes of aquatic ecosystems in climate-sensitive regions. We investigated the environmental conditions and bacterioplankton community compositions of 15 high-elevation lakes (7 above and 8 below treeline). High light-to-nutrient conditions (denoted by the reciprocal value of the attenuation coefficient (1/K) to total phosphorus (TP)), high chlorophyll a (Chl a) concentrations, low TP concentrations and low ratios of the dissolved organic carbon concentration to the dissolved total nitrogen concentration (DOC:DTN) were detected in above-treeline lakes. Significant positive correlations between the bacterioplankton community compositions with 1/K:TP ratios and Chl a concentrations indicated that not only high light energy but also nutrient competition between phytoplankton and bacteria could induce P limitation for bacteria. In contrast, low light-to-nutrient conditions and high allochthonous DOC input in below-treeline lakes lessen P limitation and C limitation. The most abundant genus, Polynucleobacter, was significantly enriched, and more diverse oligotypes of Polynucleobacter operational taxonomic units were identified in the below-treeline lakes, indicating the divergence of niche adaptations among Polynucleobacter oligotypes. The discrepancies in the light-to-P ratio and the components of organic matter between the above-treeline and below-treeline lakes have important implications for the nutrient limitation of bacterioplankton and their community compositions.
Highlights
The amount of solar energy and the amount of nutrients are two major factors that determine the structures and processes of aquatic ecosystems
The light-to-nutrient hypothesis indicates that the balance between light and nutrients manipulates the ‘nutrient use efficiency’ in lacustrine food webs [2] and that under high light-to-nutrient conditions, bacteria tend to be limited by phosphorus (P) because high light energy and low nutrient availability decrease phytoplankton biomass production and promote the exudation of labile carbon (C), which can be utilized by bacteria [3]
All the oligotypes were divided into two groups: twelve were driven by the chlorophyll a (Chl a) concentrations in the lakes above treeline, and the remaining oligotypes were enriched in the lakes below treeline; these oligotypes were further roughly divided into two clusters, one correlating with the terrestrial humic-like C1 component and total phosphorus (TP) and the other correlating with the microbial humic-like C4 component and the nitrogen compounds DTN, NH4-N and NO2-N (Fig. 5b)
Summary
The amount of solar energy and the amount of nutrients (such as nitrogen and phosphorus) are two major factors that determine the structures and processes of aquatic ecosystems. Solar radiation is the main energy source for aquatic ecosystems, and photosynthetically active radiation (PAR) from the 400700-nm wavelengths of visible light plays a decisive role in the growth of various organisms and the primary production of lakes. The light-to-nutrient hypothesis indicates that the balance between light and nutrients manipulates the ‘nutrient use efficiency’ in lacustrine food webs [2] and that under high light-to-nutrient conditions, bacteria tend to be limited by phosphorus (P) because high light energy and low nutrient availability decrease phytoplankton biomass production and promote the exudation of labile carbon (C), which can be utilized by bacteria [3]. Under relatively low light-to-nutrient conditions, relatively abundant P increases phytoplankton biomass production and reduces the exudation of labile C. In some ecosystems where DOC cycling is governed by allochthonous inputs, the light-to-nutrient hypothesis might be modified. Studies on whether such aquatic ecosystems with abundant allochthonous C fit the above interpretation and the mechanism behind it are lacking
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