Most all of us are aware of the deleterious effects of too much sun exposure, specifically by ultraviolet radiation (UV). We know that UV-A and UV-B can damage our skin, resulting in a sunburn, but other organisms can also get too much sun exposure. For example, in microscopic plants known as phytoplankton, it can inhibit photosynthesis. Humans are not the only species that applies suntan lotion to prevent sunburns. Microbes all over the globe produce their own suntan lotion, called MAAs, to protect themselves from the sun’s harmful rays.
MAAs: The sunscreen for microscopic marine plants.
One strategy that phytoplankton and bacteria use to protect themselves is via the production of mycosporine-like amino acids, abbreviated MAAs, to act as a photo-protectant, much like the suntan lotion that we use to protect our skin. We use sunscreen to block UV-B (i.e. 290 – 320 nm wavelengths) and UV-A (i.e. 320-400 nm wavelengths) rays. Similarly, MAAs absorb electromagnetic radiation between these wavelengths. Although the chemical composition of MAAs is different than that of sunscreen, it offers similar protection. However, unlike man-made suntan lotions, scientists do not know the chemical composition of MAAs produced in the ocean and cannot predict the microbes’ response to varying UV radiation or environmental conditions.
The dinoflagellate Akashiwo sanguinea featured in this image is one of many phytoplankton species known to produce high concentrations of MAAs. Image taken by Françoise Morison using Flowcam® imaging.
The study site: Sub-Antarctic Zone and Polar Frontal Zone
Between January and February of 2007, aboard the RV ‘Aurora Australis’, scientists incubated phytoplankton at the sea surface for one to two days under different UV radiation treatments. Incubation experiments were conducted in marine waters to the southwest and southeast of Tasmania in the Sub-Antarctic Zone and in the Polar Frontal Zone, which are known to have high and low abundances of phytoplankton, respectively. Using phytoplankton from these sites was advantageous because it permitted scientists to see how different environmental conditions such as light, vertical mixing, and nutrient availability influenced MAA production.
Experimental set-up: Ultraviolet radiation treatments
Dr. Oubelkheir and his research crew wondered whether or not phytoplankton respond to changes in UV radiation by applying more sunscreen, meaning that they increase their MAA production. To test this question, Dr. Oubelkheir’s team phytoplankton to several treatments, consisting of natural surface irradiance, called photosynthetically active radiation (PAR), and ultraviolet radiation (UV), over an incubation period of two days. To create each treatment, they used three different materials, in which each was designed to vary the degree of UV exposure to phytoplankton. It was similar to testing UV effects on phytoplankton placed under three different kinds of sunglasses, ranging from protective to inefficient. The first material, UV-transparent Plexiglas, allowed the passage of UVA, UVB, and PAR. A more effective material to block the sun, Mylar-D, allowed the passage to UVB and PAR and an even more efficient screen, a UV opaque Plexiglas, only allowed the passage PAR. These experiments helped the scientists figure out how phytoplankton respond to UV radiation and in particular, to see if phytoplankton adapt to changing light conditions by altering MAA production. The graph below shows that the UV transparent Plexiglas (closed circles) allowed the passage of much more ultraviolet light compared to the UV opaque Plexiglas (open circles).
Fig. 2. Transmission of the UV transparent Plexiglas and UV opaque Plexiglas used for the incubation experiments between 250 and 450 nm.
In addition, scientists collected samples at different depths to measure the amount of UV radiation at each depth and the corresponding concentration of MAAs to determine its distribution in the sub-Antarctic waters and sub-tropical waters south of Tasmania.
The Findings
The researchers discovered that some species of phytoplankton naturally accumulate MAAs over time. In general, it appears that if phytoplankton are well-protected from ultraviolet radiation and only exposed to PAR – equivalent to a human using very efficient sunglasses to block out UV rays but let in natural light – MAA production is low. However, MAA production increases when the same phytoplankton are exposed to UVB radiation, and to a lesser extent by UVA radiation. Interestingly, Dr. Oubelkheir and his crew found that phytoplankton’s response to increased UV radiation can occur in as little as two days!
In addition, the phytoplankton response to ultraviolet radiation differed depending on location. Within the Sub-Antarctic Zone, incubated phytoplankton responded by producing similar amounts of MAAs across UV treatments. However, in the Polar Frontal Zone, the incubated phytoplankton responded very differently across treatments. The scientist attributed this difference to vertical mixing. In the Polar Frontal Zone, waters are deeply mixed to a depth of greater than sixty meters below the water’s surface. Here, phytoplankton are well acclimated to low UV radiation and are therefore not producing a lot of MAAs in the water column. It is only when they were brought to the surface for the two-day incubation, that they produced high concentrations of MAAs to protect themselves from UV radiation. In comparison, the stratified waters of the Sub-Antarctic Zone only have vertical mixing to a depth of about thirty meters. As such, the phytoplankton are already well acclimated to high UV radiations due to their high MAA production rates. Therefore, when the Sub-Antarctic phytoplankton were brought to the surface for the incubation experiment, MAAs production could not increase much more. The observations provide evidence that phytoplankton can adapt to increased UV by producing UV-absorbing compounds such as MAAs.
Changes in the types of phytoplankton present can also explain variability in MAA concentration. Previous research discovered that some species, like dinoflagellates, typically produce lots of MAAs, while others produce less MAAs. The present study found that a specific kind of MAA, called porphyra-334, dominated the Polar Frontal Zone, whereas palythenic acid, a different MAA, dominated north of the Sub-Antarctic Zone. The shift in the type of MAA was coincident with the presence of dinoflagellates and cyanobacteria. The figure below shows that as dinoflagellate and cyanobacteria concentrations increased, MAA concentration increased.
Fig. 10 Relationship between pMAAs (MAAs in the particulate fraction) and the chl a concentration associated with dinoflagellates-A (peridinin-containing dinoflagellates) and cyanobacteria in the SAZ-N.
Significance
Each phytoplankton species respond differently to UV radiation. Just as some people are more sensitive to the sun than others, phytoplankton species differ in their sensitivity to UV radiation and in response seek out more or less sun protection by varying production of MAAs. The length of exposure to UV radiation, the amount of mixing, and the type of phytoplankton species present all contribute to how much MAAs get produced. The ability of phytoplankton use photo-adaptation processes has global implications regarding the primary productivity of our oceans.
