Response of the seagrass Posidonia oceanica and its associated N 2 fixers to high business‐as‐usual climate change scenario in winter

Abstract

We investigated the effects of increasing seawater temperature and CO 2 concentration based on a high business‐as‐usual climate change scenario by year 2100 on the photosynthetic performance and productivity of Mediterranean seagrass Posidonia oceanica and alkaline phosphatase and N 2 ‐fixing activities of microbes associated with different plant parts during winter when the plants may be thermally more vulnerable. Our results suggest that elevated CO 2 and temperature benefit the overall photosynthetic performance of P. oceanica . Despite the benefits, the magnitude of respiration increased with elevated CO 2 resulting in a negative carbon balance for P. oceanica in winter. This trend is contradictory to the general notion of decreased respiration in plants with increasing CO 2 , and warrants future investigation on the mechanisms behind the opposite trend. Changes of alkaline phosphatase activities found here may not be a direct consequence of the different treatments, but indirectly, through changes in the demand for dissolved inorganic phosphorus for N 2 fixers. Of the several groups of N 2 fixers tested for nifH expression (a proxy for activity of nitrogenase, the enzyme required for N 2 fixation), only the unicellular N 2 ‐fixing cyanobacterial phylotypes, UCYNB and UCYNC, actively transcribed with a positive nifH transcription response of UCYNC to elevated CO 2 and temperature. Our results suggest that in future climate scenarios, the structure and diversity of N 2 microbial communities associated with seagrasses may change and high‐light the importance of investigating the responses of different groups individually in their natural habitat substrates.

Bibliographic Information