Depth-induced adjustment of fatty acid and pigment composition suggests high biochemical plasticity in the tropical seagrass Halophila stipulacea.
Halophila stipulacea is the dominant subtidal meadow-forming seagrass in the tropical Gulf of Aqaba (GoA; northern Red Sea), an area characterised by warm and oligotrophic waters. This species occurs across a wide range of environmental conditions, and is considered one of the most deep-growing seagrasses worldwide. This investigation assessed, for the first time, the morphological and biochemical responses of H. stipulacea populations adapted to different depths (6-21 m), focussing on total fatty acid (TFA) content and composition, and photosynthetic pigments. H. stipulacea leaves (collected July 2016) from greater depths contained 25% more TFA and 22% more photosynthetic pigments than plants from shallower depths. Increases in TFAs were mainly related to higher levels of polyunsaturated fatty acids (PUFAs) and a lower production of saturated fatty acids (SFAs). As PUFAs promote fluidity in chloroplast membranes as well as facilitate electron transport in the photosystems, their observed increase with depth may favour optimal photosynthetic activity under less favourable (e.g. low-irradiance) conditions. Cluster analysis of data on fatty acid composition derived from the literature for other seagrass species across a range of geographic locations highlights the fact that PUFA levels in H. stipulacea leaves are more similar to those found in seagrass species inhabiting higher latitudes, and thus colder regions, than in tropical or subtropical species. With H. stipulacea successfully spreading into non-native areas, such as the eastern Mediterranean and Caribbean seas, it is critical to understand the eco-physiological mechanisms that allow this species to adapt to a wide range of environmental conditions.