Abstract
In some cyanobacteria, the color or prevalent wavelengths of ambient light can impact the protein or pigment composition of the light-harvesting complexes. In some cases, light color or quality impacts cellular morphology. The significance of changes in pigmentation is associated strongly with optimizing light absorption for photosynthesis, whereas the significance of changes in light quality-dependent cellular morphology is less well understood. In natural aquatic environments, light quality and intensity change simultaneously at varying depths of the water column. Thus, we hypothesize that changes in morphology that also have been attributed to differences in the prevalent wavelengths of available light may largely be associated with changes in light intensity. Fremyella diplosiphon shows highly reproducible light-dependent changes in pigmentation and morphology. Under red light (RL), F. diplosiphon cells are blue-green in color, due to the accumulation of high levels of phycocyanin, a RL-absorbing pigment in the light-harvesting complexes or phycobilisomes (PBSs), and the shape of cells are short and rounded. Conversely, under green light (GL), F. diplosiphon cells are red in color due to accumulation of GL-absorbing phycoerythrin in PBSs, and are longer and brick-shaped. GL is enriched at lower depths in the water column, where overall levels of light also are reduced, i.e., to 10% or less of the intensity found at the water surface. We hypothesize that longer cells under low light intensities at increasing depths in the water column, which are generally also enriched in green wavelengths, are associated with greater levels of total photosynthetic pigments in the thylakoid membranes. To test this hypothesis, we grew F. diplosiphon under increasing intensities of GL and observed whether the length of cells diminished due to reduced pressure to maintain larger cells and the associated increased photosynthetic membrane capacity under high light intensity, independent of whether it is light of green wavelengths.
Highlights
Due to fluctuations in parameters of the external environment, many biological organisms possess mechanisms for sensing changes in their habitat
Fremyella diplosiphon UTEX 481 cells were grown under increasing fluences of green light (GL) and reiteratively diluted over defined intervals to allow sequential generations of cells to be exposed to increasing intensities of GL (Figure 1)
The changes that are observed in cellular morphology in natural environments are, likely to result from a combination of light quality and quantity – low intensities of green-enriched light are associated with longer cells, whereas high intensities of red-enriched light are associated with spherical cells
Summary
Due to fluctuations in parameters of the external environment, many biological organisms possess mechanisms for sensing changes in their habitat. An ability to monitor environmental changes is vitally important for organisms that have limited mobility and must optimize their growth and development to maximize survival in the locations in which they find themselves. Photomorphogenesis is the process by which such organisms alter their growth and development in response to fluctuations in light. This process results in changes in growth, cellular development, and metabolism in ways that optimize organismal survival and/or limit damage. During CCA, cyanobacteria alter their pigment or protein composition to maximize light absorption for photosynthesis (Tandeau de Marsac, 1977), as well as sometimes exhibit changes in filament length and cellular morphology (Bennett and Bogorad, 1973). Changes in pigmentation have been associated definitively with increasing photosynthetic potential in this filamentous cyanobacterium (Campbell, 1996)
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