Correlation of changes in pigment content with photosynthetic capacity of seaweeds as a function of water depth

Abstract

We conducted a study of the relationship between changes in photosynthetic pigment content and photosynthetic capacity as a function of water depth in Great Harbor near Woods Hole, Massachusetts, USA, on the green algae Ulva lactuca and Codium fragile and the red algae Porphyra umbilicalis and Chondrus crispus. Seaweeds were attached to vertically buoyed lines at 0.5 and 10 m and were allowed to adapt to the ambient light field. All species showed greater pigment content with depth, and the ratio of accessory pigments to chlorophyll a increased with depth. Seaweed samples from 0.5 and 10 m were placed in tandem pairs of stoppered bottles and hung at prescribed depths. The rates of O₂ evolution were calculated from changes in dissolved O₂ content, both as a function of biomass and chlorophyll a concentration. Our results indicate that intensity and/or chromatic adaptation enhance the photosynthetic capacity of a seaweed in limiting light conditions. The strategy of seaweeds in manipulating their photon-gathering antennae is not to maximize photosynthetic rate, but rather to optimize the photosynthetic rate. They can change pigment rations, or simply increase the total amount of pigment, or both. Further, if a seaweed is optically thick, as are Codium fragile and Chondrus crispus, it does not matter what color it is. We conclude that the red algae are phylogenetically no better adapted to utilize the ambient light at great depth than their green counterparts. The ambient light conditions alone do not determine the limit for the vertical distribution of the red algae relative to the green algae.