Preliminary observations on the Halophila stipulacea resilience to thermal stress at the northern tip of the Red Sea, Jordan

The present study aimed to evaluate the implementation of the biomarkers: Lipid Peroxidation (LPO) and Lactate Dehydrogenase (LDH) assays as an early warning tool for monitoring seagrass health status in relation to global and local environmental change in the Gulf of Aqaba, Jordan. To reach our objectives, we have developed a novel Seagrass Rack Semi-Automated Stress System (SR-SASS) to achieve comparable, short-term heat stress assays. The system consisted of twelve 15-L flow-through tanks that allowed the running of six independent temperature experiments simultaneously. The selected thermal stress assays: LPO and LDH, were adopted to collect an entire observation regarding the biological response of seagrass to thermal stress. The seagrass samples (Halophila stipulacea) were collected from quadrats measuring 0.25 m² placed 50 m apart along three transects at the Marine Science Station (MSS, a protected area with no anthropogenic impact) from a depth of approximately 15 m. The samples were immediately transferred to the aquaculture unit at the MSS and incubated in a monitored aquarium filled with artificial seawater with a salinity of 40.0 ± 0.5 psu and pH value of 8.0 ± 0.1. Furthermore, seagrass samples were randomly distributed over the SR-SASS, including two replicate tanks per temperature treatment. Consequently, all samples were kept at 25 °C overnight; and, the control tanks remained at 25 °C, and the treatment tanks were subjected to heat stress as (T1) Mild: (temp. 27 °C), (T2) Moderate (temp. 33 °C), and (T3) Extreme (temp. 35 °C) for one hour. Temperature profiles were successfully controlled across experiments. Our results showed that the control group exhibited normal LPO levels (0.124 μM/g). There was a significant increase in the LPO level in seagrass leaf homogenate from the treated groups by 4-fold in the case of T2 and 6-fold in the case of T3 compared with those of the control group (P<0.05). However, no significant increase was observed in T1 (P>0.05). Similar findings were noted regarding the LDH activity levels. A further increase was shown as 73.12 and 83.34 U/mL in the case of T2 and T3, respectively, compared to control (19.84 U/ml) as reflected in the elevation of malondialdehyde values which appear temperature-dependent. Moreover, our results demonstrate that the seagrass (H. stipulacea) shows above-average physiological thermal tolerance in different experimental exposures. H. stipulacea exhibits resilience to 7 °C above the maximum summer means temperatures. Our findings provide novel information on the effects of elevated temperature on the resilience of seagrass exposed to short-term heat stress challenges. 

Keywords: Halophila stipulacea; Global warming; Gulf of Aqaba