Synergistic effects of UVR and simulated stratification on commensalistic phytoplankton–bacteria relationship in two optically contrasting oligotrophic Mediterranean lakes

[摘要] An indirect effect of global warming is a reduction in the depth of the uppermixed layer (UML) causing organisms to be exposed to higher levels ofultraviolet (UVR, 280–400 nm) and photosynthetically active radiation (PAR,400–700 nm). This can affect primary and bacterial production as well asthe commensalistic phytoplankton–bacteria relationship. The combined effectsof UVR and reduction in the depth of the UML were assessed on variablesrelated to the metabolism of phytoplankton and bacteria, during in situexperiments performed with natural pico- and nanoplankton communities fromtwo oligotrophic lakes with contrasting UVR transparency (high-UVR versuslow-UVR waters) of southern Spain. The negative UVR effects on epilimneticprimary production (PP) and on heterotrophic bacterial production (HBP),intensified under increased stratification, were higher in the low-UVR thanin the high-UVR lake, and stronger on the phytoplanktonic than on theheterotrophic bacterial communities. Under UVR and increased stratification,the commensalistic phytoplankton–bacteria relationship was strengthened inthe high-UVR lake where excretion of organic carbon (EOC) rates exceeded thebacterial carbon demand (BCD; i.e., BCD : EOC(%) ratio < 100). This didnot occur in the low-UVR lake (i.e., BCD : EOC(%) ratio > 100). Thegreater UVR damage to phytoplankton and bacteria and the weakening of theircommensalistic interaction found in the low-UVR lake indicates that theseecosystems would be especially vulnerable to UVR and increasedstratification as stressors related to global climate change. Thus, ourfindings may have important implications for the carbon cycle in oligotrophiclakes of the Mediterranean region.

[学科分类] 地球化学与岩石