Microalgae or microscopic algae were amongst the first life forms on our planet. Among them, some unicellular organisms are particularly tiny and form the picophytoplankton group.
The term phytoplankton comes from the Greek words “phyton” meaning plant and “plagktos” meaning wandering, because these organisms are taken along marine currents. The prefix "pico" probably comes from a Spanish word and means "tiny". The picophytoplankton is constituted of both evolved organisms, called "eukaryotes", which have well-defined nuclei and organelles (such as mitochondria or plastids) and of more primitive organisms, called "prokaryotes", in which the cell components (DNA, ribosomes, membranes, etc.) are not separated into defined compartments.
The first eukaryotic picoplanktonic organisms were discovered in the 1950's in the Atlantic Ocean.
Picophytoplankton is a group of organisms which perform photosynthesis, a biological process during which the light energy is used to convert carbonic gas (CO2) into carbohydrates, one of the essential foods for living organisms. This process releases oxygen (O2) and its apparition some 2.5 Ma ago has been a key factor for the development of many kinds of living forms on Earth. As these organisms perform photosynthesis and are quite numerous, they are responsible for capturing an important part of the atmospheric carbon which is central in today’s preoccupations concerning global warming.
The Oceanic biological pump, R. Stewart, 2004.2
credit picture: oceanworld.tamu.edu/.../NMEA%20Talk%2020032.jpg
Eukaryotic phytoplankton, although numerically inferior to cyanobacteria, is a major basic element of the food chain (as shown on the diagram above). In fact “the community structure and ecological function of marine ecosystems is critically dependent on these organisms” (P. Falkowski et al., 2004)1. The fishes we eat are there thanks to these organisms and to their intense reproduction or blooms in the summer time.
There are three principal eukaryotic phytoplankton groups that came to dominate the modern seas: dinoflagellates, coccolithophores and diatoms:
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Akashiwo sanguinea (Dinoflagellate). size: 50 µm | Emiliania huxleyi (coccolithophore). size: about 5 microns | Skeletonema costatum (diatom) |
Both dinoflagellates and coccolithophores are excellent stratigraphic markers in marine sediments. In fact, the cyst (resting spore) of dinoflagellates is well preserved in the sediments and has allowed to study the age of the sediments and to make biostratigraphic inferences especially in the context of oil exploration.
As these organisms have very important functions, it is important to know them and learn about the biodiversity they represent. In the laboratories, this is being done through cloning and sequencing techniques.
In order to get an image of this biodiversity that is as large as possible, scientists are investigating these organisms in many different types of environments (e.g. low or rich in nutrient) by organizing oceanographic cruises throughout the world oceans.
During these cruises:
- Water samples are collected, at different stations and depths.
- Samples can be analysed on board in order to count the cells by using modern techniques such as flow cytometry. The cells (e.g. cyanobacteria or eukaryotes) can also be discriminated very quickly by the same technique
- Other samples are frozen and brought back to the land laboratory to be analysed there.
These scientific expeditions last one or two months generally and bring together many scientists and sailors.
One very fascinating cruise was BIOSOPE (Biogeochemistry and Optics South Pacific Experiment) that took place in November and December 2004, in the South East Pacific in the most nutrient-depleted region of the world oceans. (BIOSOPE web site: http://www.obs-vlfr.fr/proof/vt/op/ec/biosope/bio.htm)
World map showing the concentration of the chlorophyll (contained in the phytoplankton) in the ocean analysed by the colour captor SeaWiFS of the NASA. The purple zone in the South Pacific shows the region where the concentration of chlorophyll is the lowest in the world’s oceans. This type of environment is said to be oligotrophic. This zone was explored by the oceanic cruise BIOSOPE in 2004.
© SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE
http://www2.cnrs.fr/presse/communique/564.htm
In the Mediterranean, another cruise PROSOPE (“PROductivité des Systèmes Océaniques PElagiques” = Productivity of oceanic pelagic systems) was achieved in 1999 with the aim to study the relationship between nutrient levels and phytoplankton abundance in the Mediterranean Sea.
(PROSOPE web site: http://www.cnrs.fr/cw/dossiers/dosclim/biblio/pigb14/08_campagne.htm)
Credit picture:
http://www.cnrs.fr/cw/dossiers/dosclim/biblio/pigb14/00_grandes/08_01_trajet.htm
From the samples obtained during this cruise, scientists were able to isolate several thousand different genetic sequences belonging to many different eukaryotic groups such as the chlorophyte algae, the alveolates or the acantharea (see tree of life).
Tree of life presenting eukaryotic lineages issued from molecular and structural data.
The red arrows indicate the chlorophyte algae, the alveolates and the acantharae (part of the radiolarians).
Document extracted from Baldauf 20033
Researchers are now trying to determine the phylogenetic relations between these sequences (and the organisms from which they originate) and learn about their evolution. They are also studying the biogeography of these organisms to understand at which specific place and depth of the ocean they proliferate, which should reflect their adaptation to these different environments.
In order to know which organisms are present where, scientists start by analysing a specific part of the organism genome called a genetic marker. In the Biological station of Roscoff (partner #2), scientists look more specifically at the gene coding for the 18S RNA subunit of the ribosome. By using the Polymerase Chain Reaction technique (PCR), they can amplify and this specific region which can then be sequenced. Then they establish phylogenetic relationships between the different sequences and learn about the evolution of the corresponding organisms.
Other techniques such as FISH (Fluorescent In Situ Hybridization) or quantitative PCR are also used in parallel to determine the abundance of certain groups of organisms (for example a certain species).
Understanding which organism is found in which type of environment can give scientists insight into their adaptative capacities. For example, different groups of Ostreococcus (belonging to the class of Prasinophyceae, themselves belonging to the Chlorophyte algae division, see tree of life) were identified by cloning and sequencing technique. Some originate preferentially from surface waters and others from deeper parts of the ocean. This information led scientists to analyze in details the photosynthetic pigments of these organisms. Indeed, they discovered that the species present deeper have an additional pigment compared to those living near the surface, which could explain their adaptation to low light levels.
The oceanographic cruises are the result of collaboration of many different laboratories interested in marine biology, chemistry or physics. About 70 researchers from all around the world were implicated in the BIOSOPE cruise. The PROSOPE cruise was financed by the CNRS, the INSU, the IFREMER, diverse European contacts and the NASA.
Microalgae are of critical importance to us as they play a major role in marine ecosystems by participating in biogeochemical cycles such as the carbon cycle and by being one of the main basic elements of the marine food webs.
The Earth's various sources and sinks for carbon.
The role of phytoplankton is put forward on this diagram as it can absorb carbon from the atmosphere through the surface of the ocean by the processes of photosynthesis.
(Graphics by Inez Fung/UC Berkeley)
www.berkeley.edu/.../08/images/carbon_cycle.jpg
But microalgae could also be a solution concerning the development of future renewable energy scenarios:
Biomass (constituted by organic matter) is considered a good potential renewable energy source as it could be turned into bio-fuels. In fact, these bio-fuels are cleaner than fossil fuels (coal and petroleum for example) because of their low nitrogen and sulphur contents. In order to turn biomass into fuel, organic matter (microalgae for example) has to be pyrolysed (Pyrolysis= degradation of macromolecular materials with heat alone in the absence of oxygen).
According to W. Peng et al., 20014, microalgae are considered to be an optimal candidate for pyrolytic fuel production because of the advantages of larger biomass, faster growth, and higher content components preferable for pyrolysis. This makes them, indeed, an interesting alternative solution for the development of renewable energy scenarios.
Microalgae can be cultivated under difficult agro-climatic conditions and are also able to produce a wide range of commercially interesting by-products such as fats, oils, sugars and functional bioactive compounds.
For more information, go to: http://www.fao.org/ag/ags/Agsi/MICROALG.htm
However, these applications (concerning renewable energy and commercially interesting by-products) are no yet in use and remain hypothetical.
References:
1: Paul FALKOWSKY, Miriam E. KATZ, Andrew H. KNOLL, Antonietta QUIGG, John A. RAVEN, Oscar SCHOFIELD, F. J. R. TAYLOR. The Evolution of Modern Eukaryotic Phytoplankton. 2004. Science. Vol 305. Pages 354 to 360.
2: Robert STEWART. What Every Student Ought to Know About the Oceans. The Oceanographer’s Perspective. Texas A&M University. Given at the National Marine Educators Association Annual Meeting, Saint Petersburg, Florida. 19–22 July 2004. web site: Ocean World: http://oceanworld.tamu.edu
3: S. L. BALDAUF. The deep roots of Eukaryotes. 2003. Science. Vol 300. Pages 1703 to 1706
4: WEIMING Peng, QINGYU Wu, PINGGUANG Tu, NANMING Zhao. Pyrolytic characteristics of microalgae as renewable energy source determined by thermogravimetric analysis. 2001. Bioresource Technology. Vol 80. Pages 1 to 7.
Contributed by Stephanie Ries
