How the green tides turn purple

The visible effects of a "purple tide" -

• By proliferating, many species are able to change the color of seawater (usually red), according to a study published by our partner The Conversation.

• These stains due to sulfurous purple bacteria are harmless to humans or wildlife.

• The analysis of this phenomenon was carried out by Cédric Hubas, Lecturer in microbial ecology at the National Museum of Natural History (MNHN).

Every summer, there is a phenomenon that coast walkers all over the world are familiar with: the green tides.

These opportunistic algae grow rapidly and take advantage of the high nutrient contents of coastal areas as well as the strong summer light.

The algae then proliferate massively and invade our coasts and our foreshore (the parts of the coast periodically covered by the tide).

But sometimes, it happens that the green tide turns pink, even purple, which does not fail to disconcert users of the coast.

Is this a sign of pollution or simply an advanced state of degradation of the algae?

Habitat modification and bacterial mutual aid

With each green tide, a large quantity of algae wash up on the coast.

Algae generally grow continuously from spring to mid-summer.

This biomass needs carbon to develop and then absorbs carbon dioxide (CO2) on a massive scale.

This could therefore be good news for the fight against global warming, if all these algae were not intended to be degraded.

This is where things get complicated.

Many bacteria are eager to tear this fresh organic matter to pieces.

In doing so, they consume the oxygen in the surrounding sediment and make it completely anoxic (with no oxygen available).

In the middle of summer, the high temperatures (sometimes scorching) boost the bacterial metabolism so that the oxygen reserves in the sediments, under the deposits of green algae, are quickly depleted.

Oxygen disappears and aerobic bacteria too for lack of "fuel".

But as there is still a lot of organic matter to be degraded, many sulfate-reducing microorganisms take over.

These new bacteria and archaea (other microorganisms) do not breathe oxygen, but sulphate and can therefore proliferate, without problem, in this new anoxic environment.

Sulfate-reducing bacteria come from deep in the sediment and reduce sulfate to hydrogen sulfide, a poisonous gas characterized by its rotten egg smell.

This is when the foreshore turns pink.

Because, in the depths of the sediment, sulphurous purple bacteria coexist with sulfate-reducing microorganisms.

These bacteria are photosynthetic, but, unlike plants or algae, do not produce oxygen.

Instead, they oxidize hydrogen sulfide (H2S) to elemental sulfur which they can store in suitable vesicles.

Detail images of clusters of purple bacteria.

Note the presence of numerous vesicles (white arrows) which are used to store granules of elemental sulfur © C. Hubas

Usually, these purple bacteria live in the sediment at a depth that presents two important criteria: first, the presence of light which will have succeeded in crossing the superficial sediment layers by bouncing off the grains of sand, and the proximity of micro -sulfato-reducing organisms in order to recover the hydrogen sulfide (H2S) produced by them.

Since the sulphate-reducing microorganisms rise to the surface under the effect of the accumulation of green algae, the sulphurous purple bacteria soon follow them.

They then find themselves in a sort of paradise for purple bacteria: very abundant in light and in hydrogen sulphide (H2S);

which is perfect for their photosynthesis.

Carpet with purple bacteria visible to the naked eye © C. Hubas

They then develop massively on the surface and form purple spots visible to the naked eye.

These plaques do not fail to challenge walkers who might wonder about the quality of the water and the potential link with a source of pollution.

Practical workshop: the Winogradsky column

In normal times, it is quite possible to observe these purple bacteria without waiting for a possible green tide.

Our dossier "Green algae"

Sergei Nikolaevich Winogradsky (1856-1953), a Russian microbiologist specializing in biogeochemical processes and pioneer in microbial ecology, developed a simple process to study the bacteria that inhabit our soils and sediments.

This is the Winogradsky column.

To perform this experiment, you will need to collect the equivalent of a small bucket of very fluid mud.

• Reserve a little vase and mix it with a source of carbon (very fine pieces of newspaper) and a source of sulfur (egg yolk…).

• Fill the bottom of a transparent tube (vertical vase, water bottle) about 1/3 of the way up.

• Tamp the mud well to remove the air and continue adding mud to reach 2/3 (or even ¾) of the container.

• Complete the last 1/3 (or the last ¼) with water taken from the site.

• Leave an empty space at the top of the device and seal everything tightly.

• Place the column near a window and wait (this may take several weeks, be patient).

You can experience it at home from mud collected from the foreshore or even from a pond if you have one near you.

Microbial mats: the HLM of bacteria

For those who would like to observe the different bacterial communities of the sediments, but would not have the adequate material to set up a Winogradsky column;

there is a solution.

Some sediments are already naturally ultra-structured (much like a natural Winogradsky column).

They are found in hypersaline areas such as salt marshes for example.

Photo of a microbial mat (Marennes d'Oléron).

We notice that the bacteria organize themselves in "slices" © C. Mazière & C. Hubas

In these zones, the microorganisms organize themselves in mille-feuilles according to the depth according to the gradients of light, oxygen and hydrogen sulfide.

By delicately taking a slice of sediment (generally these are fairly compact vases) it is possible to see a pink band made up of purple sulfur bacteria.

This band is generally found sandwiched between a layer of non-sulphurous bacteria (which take on a rather orange color) and a layer of very dark sediment (or even completely black) which indicates a sediment devoid of oxygen and inhabited by numerous bacteria whose sulfate-reducing bacteria.

These natural structures are a very good example of the structuring and interaction between microorganisms, as each stage depends on the activity of the top and / or bottom stage for its survival.

A great example of a self-sufficient ecosystem!

Microbial blooms: an ancient story

Some algae release toxins into the water which can be extremely problematic for local fauna, in particular filter-feeding organisms (oysters, mussels, etc.).

Nothing to worry about, however, from our dear purple bacteria.

Unlike some marine planktonic blooms, purple sulphurous bacteria do not release toxins into the environment and are therefore harmless to humans or wildlife.

Some algal blooms, however, are more destructive.

The oldest (and best-known) archive on the subject potentially dates from the 7th century BC.

AD in the book of the exodus.

A proliferation of toxic algae could be the cause of the first of the ten plagues of Egypt (the waters of the Nile turned into blood).

There are, in fact, a very large number of species capable of massively proliferating and changing the color of sea water and estuaries (salt or brackish water).

These efflorescences generally result in a strong coloring of the water in red.

The growth and persistence of these algal blooms depend on the direction and strength of the wind, temperature, nutrients and salinity.

In France, Ifremer and its partners have launched since 2013 across Brittany, Phenomer, a participatory science project which invites citizens to report colored water phenomena due to the proliferation of microalgae.

Most of the time, these are dinoflagellates or diatoms which are photosynthetic microalgae.

Unfortunately, the Ifremer site does not allow the reporting of mats with sulphurous purple bacteria.

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This analysis was written by Cédric Hubas, Lecturer in microbial ecology at the National Museum of Natural History (MNHN).

The original article was published on The Conversation website.

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