The influence of light quality on the growth of some phytoplankton species

Summary

The pigment composition of an alga determines which part of the solar spectrum can be used for photosynthesis. Chlorophyll absorbs red and blue light, phycocyanin (C-PC) absorbs orange light, while phycoerythrin (C-PE) absorbs green light. The phycobiliproteins (C-PC and C-PE) are possessed by red and brown algae and by cyanobacteria. The ability to adapt the pigment composition to the light quality is called chromatic adaptation;e.g. in green light C-PE is formed and in orange light C-PC is the main phycobiliprotein (TANDEAU DE MARSAC, 1977). In the deeper layers of a lake the light-climate will be green-because of the absorption in the epilimnion of red and blue light by the chlorophyll of the phytoplanktonso that C-PE-containing species are to be expected and are indeed found (FAAFENG and NILSSON, 1981; KONOPKA, 1980).

The influence of the light quality on the growth of the green algaScenedesmus protuberans and two cyanobacteria, the ‘green’ (only C-PC containing)Oscillatoria agardhii and the ‘red’ strainO.agardhii var. 7821 (which possess both C-PC and C-PE) was investigated in dense, light-limited continuous cultures. The energy balance of algae, when limited in the light supply, can be described according to GONS and MUR (1975):

$$\mu = c.q_E - \mu _e $$

where μ is the growth-rate (h⁻¹), μ_e is the specific maintenance rate energy constant (h⁻) and q_E is the specific light uptake rate (h⁻¹); c is a growth efficiency factor which determines the efficiency of the transformation from absorbed light to biomass.

It was found for the two cyanobacteria, when grown in green light, that there were no significant differences in the efficiency factor c. Furthermore it was found that c was influenced by the incident irradiance; c decreased when the incident irradiance increased. For the greenOscillatoria it appeared that the decrease in green light was greater than in white light. The maximum efficiency found was 0,22–0,25. This is close to the theoretical maximum of 0,25–0,28 (GONS, 1977). In red light, when there is a poor absorption of C-PC, the decrease in the c-value did hardly occur. The results suggest that the decrease of the efficiency factor c under green and white light was caused by a decrease of the efficiency of the transport of absorbed light from C-PC and C-PE to the chlorophyll-a of the reaction centres since no decrease in c occurred when chlorophyll was the main active pigment (i.e. in red light).

The efficiency factor c ofS.protuberans was also investigated. c remained constant when the incident irradiance increased. In green light c was lower (0,1) than in white light (0,15). This was probably due to the relative great absorbance of nonphotosynthetic active components of the cell in green light.

Growth kinetics, determined in dilute turbidostats, showed that in green lightO.agardhii var. 7821 grew faster than the green strain when the average irradiance was lower than 6 W.m⁻², while in white light the green strain always grew faster than the red strain.Scenedesmus had. a greater growth-rate in white light thanO.agardhii at high irradiances.

These results confirm the hypothesis that the redO.agardhii var. 7821 will inhabit the lower zones of the epilimnion or the metalimnion in stratified lakes (it possess a good buoyancy regulation) where exists a green lightclimate of low intensity, while the greenO.agardhii andS.protuberans are found in more shallow waters.