The Tropical Invasive Seagrass, Halophila stipulacea, Has a Superior Ability to Tolerate Dynamic Changes in Salinity Levels Compared to Its Freshwater Relative, Vallisneria americana

Abstract

The tropical seagrass species, Halophila stipulacea, originated from the Indian Ocean and the Red Sea, subsequently invading the Mediterranean and has recently established itself in the Caribbean Sea. Due to its invasive nature, there is growing interest in understanding this species’ capacity to adapt to new conditions. One approach to understanding the natural tolerance of a plant is to compare the tolerant species with a closely related non-tolerant species. We compared the physiological responses of H. stipulacea exposed to different salinities, with that of its nearest freshwater relative, Vallisneria americana. To achieve this goal, H. stipulacea and V. americana plants were grown in dedicated microcosms, and exposed to the following salt regimes: (i) H. stipulacea: control (40 PSU, practical salinity units), hyposalinity (25 PSU) and hypersalinity (60 PSU) for 3 weeks followed by a 4-week recovery phase (back to 40 PSU); (ii) V. americana: control (1 PSU), and hypersalinity (12 PSU) for 3 weeks, followed by a 4-week recovery phase (back to 1 PSU). In H. stipulacea, leaf number and chlorophyll content showed no significant differences between control plants and plants under hypo and hypersalinities, but a significant decrease in leaf area under hypersalinity was observed. In addition, compared with control plants, H. stipulacea plants exposed to hypo and hypersalinity were found to have reduced below-ground biomass and C/N ratios, suggesting changes in the allocation of resources in response to both stresses. There was no significant effect of hypo/hypersalinity on dark-adapted quantum yield of photosystem II (Fv/Fm) suggesting that H. stipulacea photochemistry is resilient to hypo/hypersalinity stress. In contrast to the seagrass, V. americana exposed to hypersalinity displayed significant decreases in above-ground biomass, shoot number, leaf number, blade length and Fv/Fm, followed by significant recoveries of all these parameters upon return of the plants to non-saline control conditions. These data suggest that H. stipulacea shows remarkable tolerance to both hypo and hypersalinity. Resilience to a relatively wide range of salinities may be one of the traits explaining the invasive nature of this species in the Mediterranean and Caribbean Seas.

The Seagrass Holobiont: What We Know and What We Still Need to Disclose for Its Possible Use as an Ecological Indicator

“…In the Gulf of Aqaba, under different anthropogenic pressure (i.e., coastal urbanization, fish farming activities, commercial maritime facilities, etc. ), depths, topography and hydrodynamics (adaptation at small spatial scale) H. stipulacea was found to modulate: (i) leaf morphology and photosynthetic pigment content according to light availability, depth and hydrodynamics; (ii) total phenol content according to the anthropogenic pressure (and to irradiance) and (iii) the microbial communities according to local environmental conditions and plant The invasiveness of H. stipulacea is probably due to its capabilities to thrive under different ecological conditions including different salinity, light and water temperature [148][149][150][151], being able to easily adapt its morphophysiological parameters to changing environmental conditions [24,25,105,135,150,[152][153][154][155][156][157][158][159][160]. H. stipulacea is also able to modify its morphology: it shows increasing leaf size along depth gradients [25,[152][153][154]156,157,159,160] and small leaf size (width and length) under high light levels and/or high temperature and hydrodynamics [25,152,155].…”

Responses of Invasive and Native Populations of the Seagrass Halophila stipulacea to Simulated Climate Change

“…Collected plants were transferred to our seagrass dedicated microcosm facility in zip lock bags filled with seawater (Figure 2A; recently described in Oscar et al, 2018). Here plants were planted in 15 aquaria (40 cm width, 33 cm height, 45 L of seawater in each aquarium) layered with 20 L (∼7 cm high) of natural sediment (sediment collected from the shores of the GoA, Eilat; Figure 2A) and filled with artificial seawater (Red Sea Salt, Israel).…”

Differences in flowering sex ratios between native and invasive populations of the seagrass Halophila stipulacea

“…The seagrass Halophila stipulacea was suggested to be highly adapted to a wide range of physiological conditions, such as salinities, water temperatures, light intensities, and nutrient levels (Por 1971, Gambi et al 2009, Sharon et al 2009, Oscar et al, 2018. The ongoing tropicalization of the Mediterranean Sea (Bianchi and Morri 2003), accompanied by the recent expansion of the Suez Canal (July 2015), could contribute to the spreading of H. stipulacea in the Mediterranean, potentially threatening the local Mediterranean seagrass species (Marbá et al 2010).…”