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 onPosidonia 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−2s−1) and saturating light conditions (124 μmol photons m−2s−1). Primary production, chlorophyll content, reactive oxygen species production, polyphenols content, thenifHgene expression, N2fixation, 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 N2fixation 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 highestnifHgene transcription levels at elevated temperatures. These findings emphasize the significant role of irradiance on the productivity ofP. oceanicaand the temperature dependence of the N2fixation process in winter.
Highlights
Posidonia oceanica (L.) Delile is an endemic and dominant seagrass species in the Mediterranean Sea, where it forms extensive meadows from the surface down to a maximum depth of about 45 m (Procaccini et al, 2003; Boudouresque et al, 2006)
The limited (13 μmol photons m−2 s−1) and saturating (124 μmol photons m−2 s−1) light levels, based on the photosynthesisirradiance parameters documented in the literature for shallow P. oceanica meadows during winter (Alcoverro et al, 1998; Lee et al, 2007), were factorially combined with the ambient temperature corresponding to the average annual temperature in the Balearic Sea at the time of the collection (∼15.5◦C; SOCIB, 2020) and 5.5◦C warmer (21◦C)
The results obtained for primary productivity and chlorophyll content of Posidonia oceanica suggest an enhancement in these values under saturating light conditions for the plants during winter, which is in line with previous studies that emphasize light availability as the primary factor influencing the photosynthetic performance of this Mediterranean species (Pergent-Martini et al, 1994; Alcoverro et al, 1995)
Summary
Posidonia oceanica (L.) Delile is an endemic and dominant seagrass species in the Mediterranean Sea, where it forms extensive meadows from the surface down to a maximum depth of about 45 m (Procaccini et al, 2003; Boudouresque et al, 2006). Among the threats that these ecosystems are facing, eutrophication from the waste waters and aquaculture, shoreline constructions, anchoring and trawling, dredging, introduced species, and climate change (warming and rising of sea-level) are considered the major causes of the decline of P. oceanica meadows over the last decades (Boudouresque et al, 2009; Champenois and Borges, 2018) Most of these impacts potentially or reduce water transparency and, the quality and quantity of the irradiance reaching the seagrass canopy (Duarte et al, 2004; Orth et al, 2006). These changes are predicted to have serious repercussions in the Mediterranean Sea given its confined nature, which makes it more susceptible to temperature increases, with the warming occurring at significantly higher rates compared to open oceans (Diffenbaugh et al, 2007; Vargas-Yáñez et al, 2008; Calvo et al, 2011)
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