Response of Posidonia oceanica (L.) Delile and Its Associated N2 Fixers to Different Combinations of Temperature and Light Levels

Abstract

The increasing occurrence of heat waves and water turbidity are threats to the persistence of seagrass meadows. Their effects on the productivity of seagrasses and the functioning of their associated microorganisms have not been studied extensively. The purpose of this study is to assess the effects of different light levels and temperatures on Posidonia oceanica; the endemic seagrass species in the Mediterranean Sea, and their N2-fixing community, which contributes importantly to the nitrogen requirements and high productivity of the plants. Aquarium experiments were conducted in winter, when the plants are more vulnerable to changes in temperature, subjecting them to short-term exposures to an ambient (15.5°C) and elevated temperatures (ambient+5.5°C), and at limited (13 μmol photons m−2 s−1) and saturating light conditions (124 μmol photons m−2 s−1). Primary production, chlorophyll content, reactive oxygen species production, polyphenols content, the nifH gene expression, N2 fixation, and alkaline phosphatase activities were measured in different plant tissues. Plants incubated at ambient temperature and high light exhibited an enhanced total chlorophyll production. Significantly higher gross and net primary production rates were also recorded under ambient temperature and high light conditions, which were approximately two-fold compared to the rest of the treatments. The oxidative stress analyses revealed an increased production of reactive oxygen species in young leaves incubated at ambient temperature and saturating light, while the polyphenols content in top leaves was considerably higher under elevated temperatures. In contrast, the N2 fixation and alkaline phosphatase rates were significantly higher under elevated temperature and low light levels. The presence of the N2-fixing phylotypes UCYN-A, -B, and -C was detected through genetic analyses, with UCYN-B demonstrating the highest nifH gene transcription levels at elevated temperatures. These findings emphasize the significant role of irradiance on the productivity of P. oceanica and the temperature dependence of the N2 fixation process in winter.

The Response of N2-Fixing Bacteria To PH Changes, Temperature, And Elevated Levels of CO2 Can Depend On Their Nutrient Status

“…The endemic Mediterranean seagrass Posidonia oceanica is a key component of the Mediterranean ecosystems and harbors a high diversity of diazotrophs associated, which can provide the entire demand of nitrogen [31] . From our early investigation, we described how global climate change (i.e., CO2 and temperature) can change the N2-fixation rates and the diazotrophic community structure on P. oceanica [32,33]. Considering the sensitivity of their cyanobacteria and heterotrophic bacteria associated to different abiotic factors (e.g., nutrients or anthropogenic pollution) [34][35][36][37], it is of utmost importance to understand and predict how diazotrophs associated with P. oceanica respond individually to global climate change.…”