Effects of light intensity on the morphology, growth rate and phenolic content of Zygnema

Abstract

Zygnema is a freshwater alga that forms thick mats that float at the water surface. Cells located at the surface of mats are exposed to very high visible light and UVB compared to cells deep within the mats. Previous studies have shown that Zygnema grown at high visible light and UVA levels shows increased tolerance to UVB exposure that coincided with an increase in UVB absorbance suggesting the photo-induced production of UVB protective compounds. We investigated whether visible light or UVA exposure induces changes in UVB absorbance and whether increased production of phenolic compounds is responsible for the absorbance changes. We exposed two Zygnema isolates to different light treatments: high light with UVA, high light without UVA, low light with UVA, and low light without UVA. After 7 days of exposure, cells were photographed and cell lengths measured. Weighed samples were extracted in methanol. The extracts were scanned with a UV spectrophotometer to determine UVB absorbance and quantitatively tested for total phenolic compounds. Zygnema isolates exposed to high light exhibited a 296% increase in UVB absorbance which coincided with a 255% increase in total phenolic content compared to samples exposed to low light. Removing UVA at high visible light exposure resulted in decreased phenolic content for only one isolate. Cell length decreased 51% in high light treatments. Removing UVA resulted in significantly larger cells only at low visible light.