Samenvatting

Light is a form of energy produced by a luminous object which occurs as electromagnetic radiation and is closely related to other forms like radio waves, microwaves, gamma rays, infrared, UV and X radiation. The different kinds of radiation can be classified by using wavelengths. Only a small section of the entire light spectrum is visible to the human eye, this is between 380 and 780 nm from blue to red.
Plants including algae use light for photosynthesis, which is the process of capturing sunlight and converting it into organic molecules and biomass. Not all sunlight can be used, before the light reaches a photosynthetic cell, light can be reflected (bent for example by the water surface) or diffused (dispersed into the surroundings). Light that reaches the plant will then either be reflected, the wavelengths reflected determine the color we observe, or absorbed. ltimately, the amount of light absorbed by a plant determines the amount of light that can be used for photosynthesis.

Chlorophyll a (Chl a) is used by algae as their major light harvesting pigment for photosynthesis. Other pigment compounds like Chl b and Chl c, carotenoids, phycoerythrin and phycocyanin are known as accessory pigments. These pigments extend the optical collection window, thereby improving the absorption efficiency and adaptation capabilities regarding the exposure to light (Trees, Clark, Bidigare, Ondrusek, & Mueller, 2000). Phycoerythrin is an accessory pigment also responsible for photosynthesis mainly found in cyanobacteria, red algae and cryptophytes (e.g. Rhodomonas). Phycocyanin functions cooperatively with chlorophyll in photosynthesis. Specifically, phycocyanin increases the efficiency of chlorophyll's oxygen production under low light conditions. This pigment is mainly found in cyanobacteria. The different pigments each have specific wavelengths at which most light is absorbed, some pigments show a second peak (e.g. chlorophyll-a has a peak around 430-440 nm and at 665-670 nm).

Algal cultures are grown using different types of lighting because of the light and color specific absorption peaks they have. The most common types are fluorescent (TL) and (light emitting diode) LED light. TL-light contains a broad light spectrum, with wavelengths emitted within the visible light 400-700 nm spectrum as well as small amounts of UVB (280-315 nm), UVA (315-400 nm) and ultraviolet (>700 nm) (Brown, 2021). Within this light spectrum not all wavelengths are (efficiently) used by algae. The companies Zeeschelp and Roem van Yerseke are aiming to replace their TL-light system to LED light. LED lights can be installed to have a specific wavelength when corresponding to the pigments in algae to ensure efficient absorption in the appropriate wavelengths only. In addition, due to providing light in the used wavelength, LED is more efficient and effective regarding growth in biomass, cell density and lipid production, less energy is needed to obtain comparable growth reached using TL-light.

Wavelength, as well as the light intensity will influence growth rate, density of a culture and biochemical content (ratio protein-lipids and fatty acid composition) of algae. Aim of this study is to provide the optimal wavelength(s) and light intensity for a selection of algal species that are currently being cultured at both companies. For 8 species (Chaetoceros calcitrans, Pavlova lutheri, Rhodomonas baltica, Isochrysis galbana, Chaetoceros neogracilis, Skeletonema costatum, Skeletonema marinoi, Thalassiosira weissflogii) we have performed spectrophotometric measurements to identify the absorption spectrum of each of these algal species. In order to give insight in the wavelength(s) that will be best absorbed. In addition, the results of a short literature review focused on the effects of light wavelengths and intensity on algal growth rate and production is also included in this report.