Albert Luiz Suhett

Tropical freshwater ecosystems have lower bacterial growth efficiency than temperate ones.

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(BGE) and bacterial carbon demand (BCD) between tropical and temperate ecosystems via a literature review and using unpublished data.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.

Exposure to humic material modulates life history traits of the cladocerans Moina macrocopa and Moina micrura

Humic substances (HS) are ubiquitous biogeochemicals and major constituents of all surface waters. HS are internalised by exposed organisms which, in turn, respond with anti-stress reactions. However, the outcome of these reactions is not necessarily negative; rather, it can lead to increases in individual lifespan and lifetime reproductive output. If clones of two closely related species respond differently, HS have the potential to shape the community structure. We obtained clones of Moina macrocopa and Moina micrura from Brazilian habitats and this potential to affect the community structure was tested with HuminFeed® (HF), a commercial HS preparation exotic to the animals, and with regional HS from a Brazilian coastal lagoon. Upon exposure to HF, the M. macrocopa clone responded with only a slight increase in lifespan and reproductive output. However, when exposed to the Brazilian HS, a strong lifespan extension and a reduction in lifetime reproductive output occurred in this clone. Upon exposure to HF, the M. micrura clone responded with a reduced lifespan and reproductive output. Upon exposure to HS, none of the Moina clones showed a delay in the onset of reproduction. Our results show that HF impacts the clones of two sister Moina species differently, and hence has the potential to shape zooplankton communities.

An overview of the contribution of studies with cladocerans to environmental stress research

Cladocerans are microcrustaceans component of the zooplankton in a wide array of aquatic ecosystems. These organisms, in particular the genus Daphnia, have been widely used model organisms in studies ranging from biomedical sciences to ecology. Here, we present an overview of the contribution of studies with cladocerans to understanding the consequences at different levels of biological organization of stress induced by environmental factors. We discuss how some characteristics of cladocerans (e.g., small body size, short life cycles, cyclic parthenogenesis) make them convenient models for such studies, with a particular comparison with other major zooplanktonic taxa. Then we illustrate the contribution of cladocerans to stress research with examples encompassing stress responses spanning from the molecular to the populational level. Most worth of note are recent studies that presented evidence of beneficial consequences of mild stress caused by natural stressors (cross-tolerance), which may be passed along across generations, favoring individual survival and species persistence in fluctuating environments. This would be particularly relevant for environments prone to frequent natural environmental fluctuations, such as coastal lagoons and other shallow aquatic ecosystems. Based on reviewed studies, a conceptual model is presented summarizing the potential effects of a first stressor on the organisms resistance to a second one. We finish by highlighting some gaps on environmental stress research that could benefit from further studies using cladocerans as model organisms.