Light is a fundamental resource for phytoplankton. To utilize the available light, most phytoplankton species possess pigments in taxon-specific combinations and quantities, which in turn result in a specific use of certain wavelengths. This optimizes the light use efficiency, allows for a complementary use of light, and may be an additional driver for community structure. While the effects of light intensity on phytoplankton biomass production and community composition have been intensively studied, here we focused on the effects of specific light spectrum quality (thus light color) on a natural phytoplankton community. In a controlled mesocosm experiment we reduced the supplied wavelength range to its blue, green, or red part of the light spectrum and compared the responses of each treatment to a full spectrum control over 28 d. Highest community growth rates were observed under blue, lowest under red light. Light absorption by the communities showed adaptation toward the supplied wavelength range. Community composition was significantly affected by light quality treatments, driven by Bacillariophyta and Chlorophyta, whereas pigment composition was not. Furthermore, lower species richness but higher evenness occurred when communities were exposed to red light compared to the full spectrum. We expected the response of phytoplankton communities to changes in the light spectrum to be driven by a combination of species sorting and pigment acclimation; however, the effect of species sorting turned out to be stronger. Our study showed that, even if species might acclimate, changes in the available light spectrum affect primary production and phytoplankton community composition.
The natural environment of phytoplankton is variable in manifold ways. Light, as essential resource for photosynthetic phytoplankton, fluctuates in its intensity (quantity) as well as spectrum (quality) over great temporal scales in aquatic ecosystems. To elucidate the significance of temporal heterogeneity in available light spectrum for phytoplankton, we analyzed the growth of four marine North Sea species (chlorophyte Tetraselmis sp., cryptophyte Rhodomonas salina, cyanobacteria Pseudanabaena sp., raphidophyte Fibrocapsa japonica), in monoculture as well as the dynamics of these species in pairwise competition experiments under blue and green light. These species were chosen as they differ in their absorption of light, the colors were chosen to contrast the absorption by chlorophylls (blue), carotenoids (partially green) and phycobiliproteins (green). Light colors were either supplied constantly or along a gradient of fluctuation frequencies (hourly to weekly alternation) between blue and green but always with the same photon flux density. When constantly supplied (no change in color), the color of light led to significant differences in growth rates and carrying capacities of the species, with Pseudanabaena sp. being the only one profiting from green light. Under alternating light color, the maximum growth rate of R. salina was higher with faster light color fluctuations, but lower for Pseudanabaena sp. and did not show significant trends for F. japonica and Tetraselmis sp. Accordingly, competition was significantly affected by the light color treatments, under constant as well as fluctuating supply conditions. However, we did not detect considerable changes in competitive outcomes between fluctuating light colors vs. constant light color supply. As the underwater light in natural ecosystems is rather variable than constant, our results of fluctuations within the light spectrum highlight their frequency-dependent effects on growth and competition. While fluctuating light colors affect the growth and capacity of species, our tested fluctuations did not have major effects on species competition.
Underwater light is spatially as well as temporally variable and directly affects phytoplankton growth and competition. Here we systematically (following the guidelines of PRISMA‐EcoEvo) searched and screened the published literature resulting in 640 individual articles. We mapped the conducted research for the objectives of (1) phytoplankton fundamental responses to light, (2) effects of light on the competition between phytoplankton species, and (3) effects of climate‐change‐induced changes in the light availability in aquatic ecosystems. Among the fundamental responses of phytoplankton to light, the effects of light intensity (quantity, as measure of total photon or energy flux) were investigated in most identified studies. The effects of the light spectrum (quality) that via species‐specific light absorbance result in direct consequences on species competition emerged more recently. Complexity in competition arises due to variability and fluctuations in light which effects are sparsely investigated on community level. Predictions regarding future climate change scenarios included changes in in stratification and mixing, lake and coastal ocean darkening, UV radiation, ice melting as well as light pollution which affect the underwater light‐climate. Generalization of consequences is difficult due to a high variability, interactions of consequences as well as a lack in sustained timeseries and holistic approaches. Nevertheless, our systematic literature map, and the identified articles within, provide a comprehensive overview and shall guide prospective research.
In 1883, Theodor Wilhelm Engelmann, a German scientist, wrote his essay "color and assimilation" (Ger.: "Farbe und Assimilation") describing the state of the art in photosynthesis research, his recent findings, and further assumptions based upon his presented results. Nearly 140 years later, many of his assumptions were proven correct. By his still well-known bacteria experiments using aerotactic, heterotrophic bacteria, he identified the chloroplasts as the location in which photosynthesis and oxygen production takes place. Furthermore, by evaluating the effects of different light spectra, he constructed the first action spectra that demonstrated the implication of the "green gap" of chlorophylls. He further posited that accessory photosynthetic pigments existed to extend the absorption range of chlorophyll. Although infrequently cited, his work was foundational for current ecological research of the vertical appearance of algae species within the underwater gradient in light spectrum due to specific harvesting of different light spectra, hence complementary chromatic adaptation of communities. This short retrospective highlights this piece of literature that represents an early step toward our current understanding of ecological competition for light spectra.
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