Abstract
Current models and observations indicate that bacterial respiration should increase and growth efficiency (BGE) should decrease with increasing temperatures. However, these models and observations are mostly derived from data collected in temperate regions, and the tropics are under-represented. The aim of this work was to compare bacterial metabolism, namely bacterial production (BP) and respiration (BR), bacterial growth efficiency (BGE) and bacterial carbon demand (BCD) between tropical and temperate ecosystems via a literature review and using unpublished data. We hypothesized that (1) tropical ecosystems have higher metabolism than temperate ones and, (2) that BGE is lower in tropical relative to temperate ecosystems. We collected a total of 498 coupled BP and BR observations (Ntotal = 498; Ntemperate = 301; Ntropical = 197), calculated BGE (BP/(BP+BR)) and BCD (BP+BR) for each case and examined patterns using a model II regression analysis and compared each parameter between the two regions using non-parametric Mann–Whitney U test. We observed a significant positive linear regression between BR and BP for the whole dataset, and also for tropical and temperate data separately. We found that BP, BR and BCD were higher in the tropics, but BGE was lower compared to temperate regions. Also, BR rates per BP unit were at least two fold higher in the tropics than in temperate ecosystems. We argue that higher temperature, nutrient limitation, and light exposure all contribute to lower BGE in the tropics, mediated through effects on thermodynamics, substrate stoichiometry, nutrient availability and interactions with photochemically produced compounds. More efforts are needed in this study area in the tropics, but our work indicates that bottom-up (nutrient availability and resource stoichiometry) and top-down (grazer pressure) processes, coupled with thermodynamic constraints, might contribute to the lower BGE in the tropics relative to temperate regions.
Highlights
Freshwater ecosystems are critical bioreactors in the global carbon cycle as they process a large fraction of the organic matter exported from terrestrial ecosystems (Cole et al, 2007; Tranvik et al, 2009)
Two parameters derived from bacterial production (BP) and bacterial respiration (BR) are useful tools to understand the role of bacteria in ecosystem functioning: bacterial carbon demand (BCD), which represents the total amount of carbon processed by bacteria (BCD = BP + BR) and bacterial growth efficiency (BGE), which is the proportion of carbon taken up by bacteria that is converted into biomass (BGE = BP/[BP + BR])
Statistical tests on confidence intervals (“ma” function from “smart” package) showed no significant differences between slopes for temperate and tropical sub-sets, for both regressions
Summary
Freshwater ecosystems are critical bioreactors in the global carbon cycle as they process a large fraction of the organic matter exported from terrestrial ecosystems (Cole et al, 2007; Tranvik et al, 2009). Heterotrophic bacteria play an important role in processing this organic matter and in releasing CO2 and inorganic nutrients (Cole et al, 1994; Odum et al, 2004), and re-integrating dissolved organic matter (DOM) to the food web through the microbial loop (Azam et al, 1983). Bacterial production (BP), and bacterial respiration (BR) are key processes in the carbon cycle of all aquatic systems. Higher BGE leads to higher energy and organic matter availability to higher trophic levels, i.e., the microbial loop. Lower in BGE may result in higher carbon mineralization rates through CO2 production
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