Artificial light helps people drive, walk and see safely at night, but manmade light can be hazardous for nocturnal creatures, affecting their ability to find food, reproduce, migrate and survive. Though certain hues can minimize these negative effects on coastal land animals, colored light pollution might threaten aquatic organisms such as phytoplankton, which rely on light to create food. Recent work by Christina Diamantopoulou and colleagues at the University of Ioannina, Greece, and the University of Glasgow, UK, suggests that red and green light alternatives can increase the growth of the tiny organisms and have a detrimental effect on the diversity and composition of phytoplankton communities.
To test how different colors of artificial light affect phytoplankton, Diamantopoulou and colleagues grew the tiny organisms under either white, red or green light inside laboratory flasks of water with nutrients. The team first looked at how colored light affects the growth of the green algae Tetraselmis suecica. The researchers counted the number of individual algal cells every 2 days as the colony grew and measured the amount of chlorophyll, which converts sunlight into energy, after 18 days. The team suspected that the algae grown under artificial white light would have (produced the most chlorophyll and grown produced the most cells) the most, because white light is more readily absorbed by the pigment.
However, the flasks of algae exposed to green light contained the most cells and chlorophyll and grew the fastest, while the algae grown under red light were less prolific, although they still produced more chlorophyll than the algae in flasks left under white light. This suggests that green glows spur both the growth and productivity of green algae, while red light just increases phytoplankton's energy production, possibly because the light receptors that guide the phytoplankton toward green tones affect their movement.
The team then nurtured flasks containing multiple species of algae to examine how different colored lights affect the growth and energy production of entire phytoplankton communities. They found that containers exposed to red, green and white light contained more phytoplankton cells after 12 days than those left in the dark. However, the number of cells in each flask was similar, suggesting that artificial light boosts growth in phytoplankton communities, regardless of the color. However, algae grown in red light had much more chlorophyll, suggesting that red hues increase the tiny organism's energy production, possibly because chlorophyll readily absorbs red light during photosynthesis.
To investigate whether artificial light color affects the structure of phytoplankton communities, Diamantopoulou and colleagues examined the liquid communities under a microscope to determine the number of different algal species present. The team also calculated how many cells there were of each species and found that although light color did not affect the number of different algal species, it did affect which species were most abundant. The most dominant species, which tend to vary in proportion from community to community in the dark, were present in almost equal numbers when grown under red or green light. In other words, red and green light affect the demographics of phytoplankton communities.
Though green and red artificial light pollution might be less damaging than white light for some land-based animals, excessive phytoplankton growth owing to colored glows could cause algal blooms along coastlines, stripping oxygen from the water and killing other aquatic organisms, such as fish. To redress the damaging effects of light pollution, researchers and policymakers must balance the benefits of alternative hues for land animals against the potential harm to aquatic organisms.