Acclimation to low light modifies nitrogen uptake in Halophila ovalis (R.Brown) J.D. Hooker

Abstract

Seagrass meadows can improve water quality through their nutrient removal capacity. However, this key ecosystem function might be affected by reduced light availability in the water column through impacts on seagrass photosynthesis. Photosynthesis provides energy and carbon skeletons for nitrogen uptake and assimilation in plants – a reduction in photosynthetic capacity should alter nitrogen metabolism. In our study, we explored how nitrogen metabolism in a common tropical seagrass, Halophila ovalis, responded as the plant photo-acclimated to reduced light levels. H. ovalis was exposed to various shading levels (0% control, 25% and 50%) in the field at Chek Jawa, Singapore for 28 days. Photo-acclimation was tracked by measuring light use characteristics through PAM fluorometry (rapid light curves) and protein expression pertaining to photosynthetic processes (RbcL, PsbA and PsbS). Nitrate and ammonium uptake rates were assessed in leaves and root-rhizome complexes, and plant tissue nutrients (carbon and nitrogen) were quantified at the end of the experiment. Shaded plants displayed significant photoacclimation responses, such as lower maximum electron transport rates (ETRmax) and saturating irradiance (Ek), as well as a down-regulation of PsbA and PsbS. Several parameters (ETRmax, Ek and PsbS protein levels) were identified to be promising candidates for monitoring impacts of light reduction on seagrass health and warrant further investigations. As the seagrass acclimated to reduced light levels, a shift in nitrogen use was observed through a reduction in relative nitrate uptake rates in the leaves and root-rhizome complexes, and an increase in relative ammonium uptake rates in the leaves. This signaled potential modifications in nutrient removal capacity, via nitrogen uptake, for seagrasses in low light conditions. Carbon-to‑nitrogen ratio in plant tissue and the expression of RbcL did not exhibit significant change with light reduction. Our findings highlight that inter-related processes of photosynthesis and nitrogen assimilation should be considered when evaluating the potential impacts of light reduction on seagrass meadows and the ecosystem functions they provide.