Quantification of Carbon and Phosphorus Co-Limitation in Bacterioplankton: New Insights on an Old Topic

Abstract

Because the nature of the main resource that limits bacterioplankton (e.g. organic carbon [C] or phosphorus [P]) has biogeochemical implications concerning organic C accumulation in freshwater ecosystems, empirical knowledge is needed concerning how bacteria respond to these two resources, available alone or together. We performed field experiments of resource manipulation (2×2 factorial design, with the addition of C, P, or both combined) in two Mediterranean freshwater ecosystems with contrasting trophic states (oligotrophy vs. eutrophy) and trophic natures (autotrophy vs. heterotrophy, measured as gross primary production:respiration ratio). Overall, the two resources synergistically co-limited bacterioplankton, i.e. the magnitude of the response of bacterial production and abundance to the two resources combined was higher than the additive response in both ecosystems. However, bacteria also responded positively to single P and C additions in the eutrophic ecosystem, but not to single C in the oligotrophic one, consistent with the value of the ratio between bacterial C demand and algal C supply. Accordingly, the trophic nature rather than the trophic state of the ecosystems proves to be a key feature determining the expected types of resource co-limitation of bacteria, as summarized in a proposed theoretical framework. The actual types of co-limitation shifted over time and partially deviated (a lesser degree of synergism) from the theoretical expectations, particularly in the eutrophic ecosystem. These deviations may be explained by extrinsic ecological forces to physiological limitations of bacteria, such as predation, whose role in our experiments is supported by the relationship between the dynamics of bacteria and bacterivores tested by SEMs (structural equation models). Our study, in line with the increasingly recognized role of freshwater ecosystems in the global C cycle, suggests that further attention should be focussed on the biotic interactions that modulate resource co-limitation of bacteria.

Bridging factorial and gradient concepts of resource co‐limitation: towards a general framework applied to consumers

“…1, Box 1). Our categorisation complements previously described and established types (Harpole et al 2011;Dorado-Garc ıa et al 2014) with additional types mentioned in other studies (see Table S1 for detailed summary of nomenclature). Simultaneous co-limitation can be considered as a case of 'true' co-limitation (Harpole et al 2011), and the type of independent co-limitation can be further specified according to the extent of the supply effect of both resources in super-additive, additive, sub-additive and antagonistic ( Fig.…”

“…; Dorado‐García et al . ) with additional types mentioned in other studies (see Table S1 for detailed summary of nomenclature).…”

Evaluating the stoichiometric trait distributions of cultured bacterial populations and uncultured microbial communities

“…The 100:1 C:P R growth medium also represents a C limited resource environment 39 , which may impose limits on bacterial growth not seen in the 1000:1 C:P R growth medium. C-limitation has been shown to more abruptly halt bacterial replication compared to P depletion due to the role of C in catabolic metabolism whereas P is more closely linked to anabolic metabolism and required in smaller amounts 59, 60 .…”

Evaluating the stoichiometric trait distributions of cultured bacterial populations and uncultured microbial communities

“…The 100:1 C:P R growth medium also represents a C limited resource environment (Godwin and Cotner, 2015a), which may impose limits on bacterial growth not seen in the 1000:1 C:P R growth medium. C limitation has been shown to more abruptly halt bacterial replication compared to P depletion due to the role of C in catabolic metabolism whereas P is more closely linked to anabolic metabolism and required in smaller amounts (Monod, 1949;Dorado-García et al, 2014). P limitation has been shown to result in increased stoichiometric plasticity of microbial biomass across a range of isolates (Godwin and Cotner, 2015a;.…”

“…Among growth phases we expected that all traits would be more constrained during exponential growth and have increased richness during lag and stationary phases as has been previously observed for macromolecular composition (Hall et al ., ). We also expected trait richness would increase under resource limitation as had been suggested from previous studies of phytoplankton poulations (Dorado‐García et al ., ). Differences in cellular stoichiometry among growth phases appears to be due to constraints on macromolecular composition during exponential growth that are weaker during stationary phase (Hall et al ., ).…”