Daily variation of the associated microbial community and the Hsp60 expression in the Maldivian seagrass Thalassia hemprichii

The microbial communities associated with plant's compartments and the expression of the chloroplast chaperonin Hsp60 have been simultaneously analyzed during the diel cycle in the marine seagrass Thalassia hemprichii from the lagoon of Magoodhoo island (Maldives), characterized by remarkable daily shifts in temperature and light. Plants showed a significant up-regulation of Hsp60 expression from 8.00 a.m. to 2.00 p.m., in correspondence with the increase in temperature and light, confirming their role as defense mechanism against photoinhibition and oxidative damage. However, a further significant increase of the Hsp60 level was also observed when irradiance and temperature dropped, suggesting that the cellular stress was still in progress. The plant-associated microbial communities showed differences by plant compartment and sampling time, with the aboveground compartment (leaves) being much more dynamic than the belowground one (roots/rhizomes). In the phyllosphere, a progressive shift during the day from the absolute dominance of the Gammaproteobacteria class, mainly constituted of Enterobacteraceae family, to the increase of the biodiversity due to the rise of Alphaproteobacteria was observed. Belowground, the microbial diversity was much lower than aboveground, being Gammaproteobacteria the most represented class throughout all sampling times. Vibrionaceae family was the most abundant at 8:00 a.m. and 6.00 p.m. decreasing slightly at 2.00 p.m., partially replaced by Halomonadaceae. The combined biochemical and microbial markers allows to assess the plant stress response and to deepen the knowledge on the seagrasses adaptation to harsh and changing environmental conditions, resulting useful to detect early signs of change in an organism's physiological state. Furthermore, the variation of Hsp60 expression and associated bacterial communities in response to light and temperature fluctuations support the ‘holobiont’ theory, which considers the plant/microorganisms association as a functional unit, as suggested for corals.