Algae and the origins of photosynthesis Flashcards
Why bother colonising land? the many benefits to living in the sea
High density medium (water) surrounds them
-therefore there is less need for investment in structural tissues
(e.g. lignin that we see in trees provides support in air which is alow density medium)
Relatively high ionic strength in sea water
so less investment in osmoregulation
Bathed in a dilute solution of nutrients
so no need for a root system
High specific heat capacity
therefore low risk of freezing and lower risk of photooxidation
What are algae?
A disparate group of organisms, spanning prokaryotic and eukaryotic kingdoms / domains
Predominately photosynthetic (but not always), simple reproduction, lack cuticles and are predominately aquatic.
many are microscopic
Share similar habitats
Algae are what algologists (now known as phycologists) study
About 5000-6000 species recognised from Britain and Ireland – a far higher number than all advanced land plants combined
Half of all global primary productivity
A very successful group of organisms
History of algae
Algae noticed and described by Theophrastus and Aristotle (4th century BCE).
Microscope invented in 17th century
by Anton van Leuwenhoek (Holland) and Robert Hooker (England)
Linnaeus described 14 genera of algae in Species Plantarum (1753)– Early studies often regarded microalgae as animals, as many are motile and/or not green
19th and early 20th century view was that algae were animals and not plants
(see notes for more detail)
starting point: cyanobacteria
The Cyanobacteria evolved first, early in the Precambrian (2.3 billion years ago) and all other algal groups are the result of a cyanobacteria-like cell being engulfed by another bacteria to become the chloroplast within a primitive eukaryotic cell. They are responsible for much of the oxygen generation in our atmosphere. Cyanobacteria are capable of nitrogen fixation
The first “endosymbiosis” then gave rise to the red and green algae lines.
Red algae
Rhodophyta / “red algae”
Domain: Eukaryotes
Kingdom: Plantae– Sub-kingdom: Biliphyta
Pigments: Chlorophyll a (green) + accessory pigments: phycoerythrin (reddish) & phycocyanin (blue) in the red algae the phycoerythrin is the dominant pigment but note that some red algaes do not appear red e.g. lemanea.
In green-blue algae the phycocyanin is dominant
this group includes chlorophyta and charophyta
Evolutionary history– probably evolved towards end of Pre-Cambrian (~600 million years ago)
Mostly marine
Complicated life cycles:
Sexual reproduction is rarely seen in the field
Often initiated by onset of adverse conditions.
Asexual reproduction and vegetative growth propagates the genotype
Sexual organs often necessary for identification of species
Asexual reproduction is the usual means of reproduction in algae but sexual reproduction does occur rarely
Asexual reproduction and vegetative propogation is how algae usually propogate their genotype . Sexual reproduction is stimulated by low temp and short days e.g. February weather, it very rarely occurs in the field except in times of extreme stress. However it is the sexual organs that are used to identify them.
Characeae: the most advanced of all algaes
A single main stem from which whorls of branches arise at intervals
May be calcified
Closest living relatives of higher plants
Not all green algaes are green!
Photoprotective pigments can give green alga a reddish tinge. These are carotenoids and xanthophylls.
Euglenophyceae
The Euglenophyta evolved through a second endosymbiosis in which a protozoan engulfed a primitive green alga.
Domain: Eukaryotes
Kingdom: Protist
Pigments: Chlorophyll a, Chlorophyll b
although some colourless representatives rely on heterotrophic feeding
Evolutionary history – unknown – small and without hard tissues so no fossil record
Mostly freshwater (some marine)
Plant or animal? Possess an eye spot and a flagellum engage in heterotrophic feeding/autotrophic photosynthesis (mixotrophic) and lack a fixed cell wall however they possess chloroplasts
Chromista
Arose from a further endosymbiosis involving an ancient protozoan engulfing a primitive red alga. All Chromists arose through this endosymbiosis, although many (including the organisms responsible for potato blight and malaria) subsequently lost their ability to photosynthesise. All of these endosymbioses also probably took place in the Precambrian (fossil evidence is weak).
Domain: Eukaryotes
Kingdom: Chromista– Pigments: Chlorophyll a, Chlorophyll c, carotonoids and xanthophylls
Evolutionary history– Result of second endoymbiotic event
Very diverse!
Marine and freshwater
A guide to the terminology of chromista
Chromista - Algae with chlorophyll c but not b, evolved from asecondary endoysmbiosis with a eukaryotic (red) alga,along with protists descended from these which have lostphotosynthetic capability and plastids
Heterokonta - Organisms possessing two flagellae of different lengths forat least part of the life cycle (hetero = different; kontos =punting pole)
Stramenopiles - A term that refers to the colour of the Heterokonta (Latin: stramen = made of straw, referring to their yellow-brown colour)
Ochrophyta - Similar meaning to Stramenopile, except from Greek ratherthan Latin roots (okhra = yellow)
Chromista example: Bacillariophyceae aka Diatoms
Marine and freshwater, benthic and planktonic
~ 2800 species in UK and Ireland freshwaters
~ 5 – 500 μm (mostly < 30 μm)
Mostly identified by silica cell wall (“frustules”)
Types of Diatom:
- Centrales (centric diatoms)
- Pennales (pennate diatoms):
-Araphidinae (pennates without raphes) - Raphidineae (pennates with raphes)
Further chromista examples
Chromista example: Phaeophyceae (brown algae)
Mostly marine littoral species
Kelps and wracks
Chromista example: Haptophyta aka dinoflagellates
The tertiary endosymbiosis event of a protozoan engulfing a chromista lead to the Haptophyta
Yet another Protozoan
Haptophyta have 2 flagellae (one in transverse furrow)
Chlorophyll a & chlorophyll c + accessory pigments (peridinin rather than fucoxanthin)
- Mostly marine, some freshwater
- Observed to move up and down within the water column
- Often mixotrophic
- Some can bioluminesce
Ceratium hirundinella – flagallae allow movement in the water column
Pigments
Traditional classification was based on colour/ pigments and flagella arrangement; modern classifications are also informed by molecular genetics (see notes)
The ‘Spring Bloom’: Asterionella formosa case study
Asterionella formosa blooms were first described by John Lundin in Windermere (1940s)
Numbers peak in April/May
Silica frustule in their cell walls make the diatoms heavier and less buoyant
Star-like colonies formed using mucous pads to stick together - function to slow sinking rate
These function to slow sinking rate
Grows at low temperatures
Carotenoids aid photosynthesis at low light levels
