Relationship Between Lifestyle and Structure of Bacterial Communities and Their Functionality in Aquatic Systems

Community stability of free-living and particle-attached bacteria in a subtropical reservoir with salinity fluctuations over 3 years

Changes in salinity have a profound influence on ecological services and functions of inland freshwater ecosystems, as well as on the shaping of microbial communities. Bacterioplankton, generally classified into free-living (FL) and particle-attached (PA) forms, are main components of freshwater ecosystems and play key functional roles for biogeochemical cycling and ecological stability. However, there is limited knowledge about the responses of community stability of both FL and PA bacteria to salinity fluctuations. Here, we systematically explored changes in community stability of both forms of bacteria based on high-frequency sampling in a shallow urban reservoir (Xinglinwan Reservoir) in subtropical China for 3 years. Our results indicated that (1) salinity was the strongest environmental factor determining FL and PA bacterial community compositions - rising salinity increased the compositional stability of both bacterial communities but decreased their α-diversity. (2) The community stability of PA bacteria was significantly higher than that of FL at high salinity level with low salinity variance scenarios, while the opposite was found for FL bacteria, i.e., their stability was higher than PA bacteria at low salinity level with high variance scenarios. (3) Both bacterial traits (e.g., bacterial genome size and interaction strength of rare taxa) and precipitation-induced factors (e.g., changes in salinity and particle) likely contributed collectively to differences in community stability of FL and PA bacteria under different salinity scenarios. Our study provides additional scientific basis for ecological management, protection and restoration of urban reservoirs under changing climatic and environmental conditions.

Linking lifestyle and foraging strategies of marine bacteria: selfish behaviour of particle-attached bacteria in the northern Adriatic Sea

Microbe-mediated enzymatic hydrolysis of organic matter entails the production of hydrolysate, the recovery of which may be more or less efficient. The selfish uptake mechanism, recently discovered, allows microbes to hydrolyze polysaccharides and take up large oligomers, which are then degraded in the periplasmic space. By minimizing the hydrolysate loss, selfish behaviour may be profitable for free-living cells dwelling in a patchy substrate landscape. However, selfish uptake seems to be tailored to algal-derived polysaccharides, abundant in organic particles, suggesting that particle-attached microbes may use this strategy. We tracked selfish polysaccharides uptake in surface microbial communities of the northeastern Mediterranean Sea, linking the occurrence of this processing mode with microbial lifestyle. Additionally, we set up fluorescently labelled polysaccharides incubations supplying phytodetritus to investigate a 'pioneer' scenario for particle-attached microbes. Under both conditions, selfish behaviour was almost exclusively carried out by particle-attached microbes, suggesting that this mechanism may represent an advantage in the race for particle exploitation. Our findings shed light on the selfish potential of particle-attached microbes, suggesting multifaceted foraging strategies exerted by particle colonizers.

The role of organic nutrients in structuring freshwater phytoplankton communities in a rapidly changing world

Carbon, nitrogen, and phosphorus are critical macroelements in freshwater systems. Historically, researchers and managers have focused on inorganic forms, based on the premise that the organic pool was not available for direct uptake by phytoplankton. We now know that phytoplankton can tap the organic nutrient pool through a number of mechanisms including direct uptake, enzymatic hydrolysis, mixotrophy, and through symbiotic relationships with microbial communities. In this review, we explore these mechanisms considering current and projected future anthropogenically-driven changes to freshwater systems. In particular, we focus on how naturally- and anthropogenically- derived organic nutrients can influence phytoplankton community structure. We also synthesize knowledge gaps regarding phytoplankton physiology and the potential challenges of nutrient management in an organically dynamic and anthropogenically modified world. Our review provides a basis for exploring these topics and suggests several avenues for future work on the relation between organic nutrients and eutrophication and their ecological implications in freshwater systems.

A comparative whole-genome approach identifies bacterial traits for marine microbial interactions

Microbial interactions shape the structure and function of microbial communities with profound consequences for biogeochemical cycles and ecosystem health. Yet, most interaction mechanisms are studied only in model systems and their prevalence is unknown. To systematically explore the functional and interaction potential of sequenced marine bacteria, we developed a trait-based approach, and applied it to 473 complete genomes (248 genera), representing a substantial fraction of marine microbial communities. We identified genome functional clusters (GFCs) which group bacterial taxa with common ecology and life history. Most GFCs revealed unique combinations of interaction traits, including the production of siderophores (10% of genomes), phytohormones (3-8%) and different B vitamins (57-70%). Specific GFCs, comprising Alpha- and Gammaproteobacteria, displayed more interaction traits than expected by chance, and are thus predicted to preferentially interact synergistically and/or antagonistically with bacteria and phytoplankton. Linked trait clusters (LTCs) identify traits that may have evolved to act together (e.g., secretion systems, nitrogen metabolism regulation and B vitamin transporters), providing testable hypotheses for complex mechanisms of microbial interactions. Our approach translates multidimensional genomic information into an atlas of marine bacteria and their putative functions, relevant for understanding the fundamental rules that govern community assembly and dynamics.

Variation in Photosynthetic Performance Relative to Thallus Microhabitat Heterogeneity in Lithothamnion australe (Rhodophyta, Corallinales) Rhodoliths

Rhodoliths are free-living, coralline algae that create heterogeneous structure over sedimentary habitats. These fragile ecosystems are threatened by anthropogenic disturbances that reduce their size and three-dimensional structural complexity. We investigated how physical disturbance from boat moorings affects photosynthetic performance in the rhodolith Lithothamnion australe. Photosynthetic parameters were measured for intact rhodoliths and crushed rhodolith fragments of two sizes (ca. 1 and 2 cm diameter), while chlorophyll fluorescence was measured at the surface of rhodoliths of these two sizes, between the interior branches of the larger rhodoliths, and at the surface of 52 various sized (0.4-3.5 cm diameter) rhodoliths. Gross productivity and net productivity were 15% and 36% higher, respectively, in the smaller L. australe, while respiration was 10% higher in the larger individuals. Thallus crushing reduced gross productivity by 20% and 41%, and net productivity by 9% and 14% in the smaller and larger rhodoliths, respectively. It also reduced respiration by 33% and 60% in the smaller and larger rhodoliths, respectively. Fluorescence parameters were all greater at the surface of the larger L. australe than the smaller individuals, and greater at the surface than in the interior parts of the larger individuals. Across a range of rhodolith sizes, surface fluorescence parameters were at their maxima in 1.54 to 2.32 cm diameter individuals. These results show that L. australe's complex structure creates heterogeneity in photosynthesis and respiration between their surface and interior parts and among rhodolith sizes. This information can help predict how rhodoliths may respond to disturbance and environmental stressors.