Microscopic algae known as phytoplankton are photoautotrophic organisms. That is to say that like higher plants, they use minerals (C02, P, N …) and energy from the sun to grow and multiply. As the base of the marine food chain, they are also the base of organic matter production and the sequestration of atmospheric C02. Because of this, understanding the processes controlling their biomass and degradation is a major challenge for scientists.

The degradation of microscopic algae has long been regarded as essentially biological in nature (bacterial remineralization). However, it appears that in some areas these processes are negligible compared to non-biological phenomena such as light degradation (photodegradation). This is what interests Rémi Amiraux. He highlighted that although ubiquitous at all longitudes, photo-degradation processes are particularly intense in Polar Regions. Besides, they may also limit biological degradation. His objectives are twofold: to characterize the photo-degradation of algae based on perceived light intensities (in the Arctic), and to determine the extent of the impact of light degradation on bacterial degradation so that they can better understand how these processes work and will evolve in the future.

Sea ice extent in mid-September in the Arctic. Adapted from NOAA.

In effect, the Arctic is one of the zones most impacted by climate change. Increasing temperatures are inducing a reduction in the extent and thickness of the winter sea ice and are permitting more light to penetrate into the water column. His research aims to determine if an increase in light penetration induces an escalation or diminution of photodegradation processes and consequently, in bacterial degradation. Given that photodegradation is incomplete and does not extend to the mineralization of organic matter (but only a simplification of molecules), it can be assumed that an increase in photodegradation would, by short-circuiting bacterial mineralization, result in better export and sequestration of organic matter (C02metabolized by algae) to the deep ocean.

How photo-degradation works and how it impacts bacteria?

Photo-degradation in our daily lives

Photo-degradation is also responsible, at least in part, of the aging of our skin! Indeed, the pigment contained in our skin is able to capture the light energy and to produce some excited oxygen molecules. These oxygen species, called radicals have the potential to degrade a lot of our cellular molecules! Degradation of the lipids contained in our skin induces, in addition, a loss of our skin elasticity and thereby, the appearance of wrinkle. Our body naturally synthesizes anti-radical compounds, but they can’t stop all the attacks which explain the progressive aging of the skin. UVA and UVB are even capable to degrade efficiently our lipids without using the “pigment pathway”. Thus, do not forget to wear sunscreen in summer!

Photo-degradation in living or dead algae

When algae are alive, they use 99% of the light energy they captured with their pigments (chlorophyll) through photosynthesis. This energy allows their growth and multiplication which can lead to a bloom. The other 1% induces the production of excited oxygen molecules, these latter having the ability to degrade cellular and extracellular components and thus inducing photo-degradative damages. To counter this process, algae are equipped with photoprotective systems. However, when algae are dead, the capture of light energy is entirely directed towards the production of excited oxygen molecules and is therefore harmful. This oxygen quickly saturates the photoprotective systems inducing degradation of a large number of cellular and extracellular components that adjoin the algae, such as bacteria. Thus, the photodegradation of algae can induce a short-circuiting of the bacterial mineralization by killing these latter!