Response of the seagrass Posidonia oceanica and its associated N₂ fixers to high business‐as‐usual climate change scenario in winter

Abstract

We investigated the effects of increasing seawater temperature and CO2 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 N2‐fixing activities of microbes associated with different plant parts during winter when the plants may be thermally more vulnerable. Our results suggest that elevated CO2 and temperature benefit the overall photosynthetic performance of P. oceanica. Despite the benefits, the magnitude of respiration increased with elevated CO2 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 CO2, 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 N2 fixers. Of the several groups of N2 fixers tested for nifH expression (a proxy for activity of nitrogenase, the enzyme required for N2 fixation), only the unicellular N2‐fixing cyanobacterial phylotypes, UCYNB and UCYNC, actively transcribed with a positive nifH transcription response of UCYNC to elevated CO2 and temperature. Our results suggest that in future climate scenarios, the structure and diversity of N2 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.

The Diversity and Functional Capacity of Microbes Associated with Coastal Macrophytes

“…Future experiments should explore the spatial extent of these metabolisms and test hypothesized interactions between the host, its microbiome, and carbon and nitrogen cycling. The importance of seaweeds and seagrasses to coastal productivity, and the demonstrated sensitivity of both the host and microbes to increasing temperatures and pH ( 14 , 38 , 52 ), pathogens ( 51 ), and other anthropogenic stressors, underline the importance of further studies of host-microbiome interactions in these foundational species.…”

Response of Posidonia oceanica (L.) Delile and its associated N2 fixers to different combinations of temperature and light levels

“…However, significantly higher leaf net photosynthetic rates were still exhibited by plants subjected to 15.5 °C and high light treatment. On the other hand, in Agawin et al (2021), positive responses to higher temperature were observed in the photosynthetic activity of P. oceanica , while leaf respiration rates did not increase, suggesting that ocean warming scenarios may not necessarily have adverse effects on the carbon balance of the plants in winter. This agrees with the higher NPP rates estimated from the phyllosphere of P. oceanica at elevated temperature and high light conditions during the experiment.…”

“…In contrast, the APA values were on average the lowest under equal temperature and saturating light, regardless of the more favorable pH values for the phosphatases in this treatment, as they were higher compared to the low light treatments. This result could be attributed to the fact that at the final stage of the experiment the mean phosphate concentration for this treatment was several orders of magnitude higher than the rest (Table 1), given that APA in seagrasses decreases under elevated phosphate content and, vice versa, increases with phosphorus limitation (Invers et al, 1995;Martínez-Crego et al, 2006;Agawin et al, 2021). As for the significantly lower APA rates demonstrated by the rhizomes, this may suggest that the metabolic activity occurring in this part of the plants is relatively This is a provisional file, not the final typeset article low compared to the others, consequently, its inorganic phosphorus demand is low as well.…”

“…Several studies have been carried out in order to determine the effects of light reduction on P. oceanica , comparing its response along bathymetric/spatial gradients (Alcoverro et al, 2001; Ruiz and Romero, 2003; Dattolo et al, 2014) and experimentally though the modification of the light environment with shading screens (Ruiz and Romero, 2001; Mazzuca et al, 2009; Serrano et al, 2011; Gacia et al, 2012). Investigations on the impacts of sea warming, considering climate change scenarios, have focused on its isolated effects (Marbà and Duarte, 2010; Guerrero-Meseguer et al, 2017; Hernán et al, 2017; Ruiz et al, 2018; Traboni et al, 2018) and the interaction with other stressors (Hendriks et al, 2017; Ontoria et al, 2019; Agawin et al, 2021). However, research on the combined effects of ocean warming and water turbidity in P. oceanica meadows remains relatively scarce and understanding them is crucial for the future management of these coastal ecosystems.…”

“…The purpose of this study was to assess the response P. oceanica and its N 2 fixing community to different combinations of temperature and light levels, in terms of primary production and respiration rates, chlorophyll content, alkaline phosphatase activity, oxidative stress indicators and N 2 fixation activities of the diazotrophs associated with different plant tissues. The experiment was performed during winter, when the plants are thermally more vulnerable to temperature increases (Agawin et al, 2021).…”