Assessing the ecological status of seagrasses using morphology, biochemical descriptors and microbial community analyses. A study in Halophila stipulacea (Forsk.) Aschers meadows in the northern Red Sea

Seagrasses are one of the most valuable marine ecosystems on earth, yet they are declining worldwideat alarming rates. With most of seagrass monitoring based on long term responses to environmentalpressures, there is growing interest in developing alternative diagnostic tools that more effectively iden-tify changes in seagrass ecological status at an early stage. Besides morphological indicators, functionaland biochemical descriptors may provide a good understanding of plant’s responses to environmentalchanges. Moreover, the epiphytic microbial communities of seagrasses may also shift in response tochanges in environmental conditions, although these have been seldom used as a descriptor of environ-mental change. In this study three Halophila stipulacea (Forsk.) Aschers meadows, found in the Gulf ofAqaba (northern Red Sea), were characterized using an integrated approach to highlight possible differ-ences in the meadows ecological status. Plant descriptors, including leaves morphometrics (leaf size, leafnumber/plant, leaves with lost apex), photosynthetic pigments (Chlorophylls, Carotenoids) and total phe-nols contents, were investigated and coupled with the plants’ epiphytic microbial community structureand composition, studied using pyrosequencing. The entire suite of descriptors highlighted differencesamong the meadows ecological status based on changes in plants’ morphology and biochemistry, andtheir associated microbial communities, in response to the different environmental conditions (watercolumn turbidity, seawater and sediment nutrients) and the geomorphological features (bottom slope,granulometry) of the stations. Leaf morphology and photosynthetic pigment content were modulated inH. stipulacea in response to light availability and hydrodynamics in the Gulf of Aqaba. The highest leafsurface area and photosynthetic pigment contents were observed at the lowest irradiance and hydro-dynamics/granulometry among stations. Total phenol content showed differences among stations withincreasing concentrations from north to south. The microbial communities showed differences amongstations and plant compartments, with high incidence of Gammaproteobacteria and Bacteroidetes in lightlimiting conditions, while Cyanobacteria and Rhodobacteraceae thrived in conditions of high light avail-ability and hydrodynamics. The mutual response of the seagrass plants and the microbial communitiesprovided evidence of their functional relationship, which undoubtedly needs further investigation. Tothe best of our knowledge, this is the first time that such descriptors have been used in an integratedapproach. We provide evidence of their effectiveness in discriminating seagrass ecological status, even atsmall spatial scales. This work constitutes a new approach to the assessment of seagrasses and a steppingstone in the application of microbial communities as a putative marker in a changing environment.