Flowering plants in the sea, part 1 - photosynthesis

Just as whales are mammals that have gone back to the sea, so seagrasses are flowering plants that have gone back to the sea. Interestingly, seagrasses appear to have gone back to sea several times, but have evolved many strikingly similar features for survival in the marine environment.

Seagrass in the genus Zostera

The feature that gives seagrasses their name, is the strap-like leaves which look similar to the leaves of the true grasses. Although the leaf shape is similar among the different groups of seagrass there are enough differences to indicate that it's a trait that has evolved separately in each of the seagrass lineages. Something that is supported by genetic analyses.

Seagrass growing at depth appears blue because water absorbs more light at the red end of the spectrum

One of the important selection pressures on seagrasses is access to light for photosynthesis. Water absorbs light and absorbs some wavelengths of light better than others, which is what makes it appear blue. Suspended particles in the water column and organisms growing on the leaf surface (epiphytes) attenuate light available to seagrass for photosythesis further. The light environment of seagrasses is, therefore, very different from that experienced by terrestrial plants.

Seagrass from the genus Halodule with epiphytic organisms growing on its leaves

Seagrasses have several adaptations that allow them to survive in low-light conditions. Their chloroplasts occur in shallower tissues than terrestrial plants. The leaves also contain very little structural material, so a larger proportion of the plant is capable of photosynthesis than most terrestrial plants.

Another property of the marine environment that differs from terrestrial plant habitats is wave action. Wave action moves the seagrass backwards and forwards exposing different sides of the leaf to alternating high and low light levels. As a adaptation to these conditions both sides of the seagrass leaf are equally capable of photosynthesis. This is unlike many terrestrial plants where photosynthesis occurs mostly in the upper surface of the leaf.

Light levels can vary strongly in the marine environment on short and long time scales. For instance, light levels can vary on short time scales with changing tide heights and increased turbidity due to wind and wave action or can vary on long time scales with changes in day length and the number of organisms growing on the seagrass. To cope with periods of low light, seagrasses make use of rhizomes. Rhizomes are modified stems that grow horizontally and are used to store the products of photosynthesis when times are good for times when little or no photosynthesis can be achieved.

The seagrass Zostera marina showing the rhizome in the bottom of the image

It's the energy-rich rhizomes that make seagrass an important source of food for many marine animals and water birds, such as dugongs, manatees, turtles and swans.