The world dyed by algae
By EcoEvo@TCD • Perspectives • 19 Jun 2013
Seamount, Cortes Bank
The term “algae” does not refer to a single taxonomic group but instead comprises a diverse array of
species, from prokaryotic cyanobacteria to many types of unicellular and multicellular eukaryotes. It’s well
known that algae make great contributions to sustaining the diversity and productivity of our natural world.
It has been estimated that the number of algae species could range from 30,000 to more than 1 million. So
it’s not surprising to find that algae, so high in diversity and abundance, have significantly altered the
appearance of our planet.
Just as plants add colour to our terrestrial world, algae are responsible for the diverse colours of many
aquatic systems. Without the existence of macroalgae, most marine zones would be barren places and we
would miss the beauty of park-like intertidal zones and magnificent marine forests comprised of gigantic
algae such as the Giant kelp (Macrocystis pyrifera). In the shallow zone of rivers and lakes, attached
filamentous macroalgae can flourish when the pressure of competition from aquatic plants is low, making
the river and lake much greener than it would be otherwise. Microalgae also play their own role in dyeing
the environment. In marine areas with high concentrations of phytoplankton, the existence of large
amounts of chlorophyll absorbs the red and blue portions of the light spectrum and reflects green light.
Therefore, sea water in these regions is not blue but appears as different shades; from blue-green to
green. Attached microalgae, such as some benthic diatom species, can easily occupy diverse substrates,
such as stones and aquatic plants, and decorate that substrate with their own colour. Despite their tiny
volumes, only several μm in length or diameter, the adherent strength of these algae is quite strong and it
is not easy to remove them from the substrate (as I found out when helping Lindsay with her field
experiments!).
Some people might say they prefer multicoloured coral reefs to algae. However, even the beautiful colours
of coral reefs depend on Symbiodinium, one group of dinoflagellate algae which live in a symbiotic
relationship with coral polyps. However, this relationship is not absolutely stable and severe environmental
stress can sometimes weaken the symbiotic bond. Increased sea water temperatures and ocean
acidification can cause coral polyps to expel their algae. As a consequence, the corals lose both their color
and any phototrophic capabilities and eventually become boringly white; a process termed coral bleaching.
Bleached coral reef
Healthy coral
Algae are also responsible for the beautiful colours of some sea slug species. In contrast to the
harmonious symbioses between algae and corals, some sea slugs directly consume algae and use the
algae’s pigment to produce remarkable body colors which can either be apatetic or aposematic in their
functions.
Sometimes, however, algal colouration is not always welcome. Blooms of intensively growing algae can
produce very ugly and unpleasant colours. Cyanobacterial blooms are one of the most common
environmental problems in freshwater systems including eutrophic lakes, reservoirs and the backwater
zone of rivers. During bloom periods, lake surfaces can be partly or fully occupied by cyanobacterial cells,
creating an unappealing dark blue-green colour.
A cyanobacterial bloom in Chao Hu, the fifth largest freshwater lake in China
Other algal groups, such as some dinoflagellates, are commonly found in marine blooms. The colours of
algal blooms in marine systems are more diverse depending on the type and density of bloom species.
However, one kind of bloom, Noctiluca scintillans, is especially unattractive. Proliferation of this species
creates a bloody red colour remarkably similar to the one described in Exodus, which may be one of the
earliest recorded instances of a red tide (“… and all the waters that were in the river turned to blood. And
the fish that were in the river died, and the water stank …”).
A Noctiluca scintillans bloom off the coast of eastern Australia
Author
Qiang Yang: qiang.00.yang[at]gmail.com
Photo credits
Brian Skerry, National Geographic
Wikimedia commons
finance.ifeng.com
coolage.in