Metabolic responses of Halodule wrightii to hyposalinity

While seagrass growth is inhibited under acute high or low salinities, Halodule wrightii (shoal grass) is an exception for its wide salinity tolerances. Salinity reduction experiments were performed to determine how H. wrightii acclimates to hyposalinity. In laboratory experiments over 12 days, two salinity reduction scenarios (moderate-S15 (32→26→20→15) and severe-S5 (32→23→15→5)) were applied to H. wrightii. Periodic measurements of leaf growth, photosynthesis, quantum efficiency (Fv/Fm), and leaf respiration were made to assess the influence of moderate (S15) and severe (S5) hyposalinity treatments compared to a control (32). Regardless of salinity treatment, leaf respiration rates increased significantly with decreasing salinity. Significant depression of photosynthetic rates and Fv/Fm were found in the S5 treatment with greatest changes in the S5 plants compared to S32 controls. These results suggest that hyposalinity impairs photosynthesis and that H. wrightii survives severe short-term hyposalinity by increasing respiration rates to supplement its energetic requirements. Simultaneous low photosynthetic and high respiration rates could reduce the resiliency of H. wrightii under hyposalinity.