Within the department for plant- and environmental sciences we are a group studying the
ecology of microalgae. Especially we are now part of the platform for Marine Chemical
Ecology at the faculty where we cooperate with research groups from three other departments.
Within this area we are proposing two different examworks. They both can be adjusted to fit
to different lengths and levels.
Induced defences in Skeletonema marinoi
Background: Diatoms are probably the single most important primary producers in the
world, they are responsible for a third of the carbon fixed in the ocean and more than 10% of
the total carbon fixed globally. Size of diatoms determines the flow of energy and materials to
higher trophic levels and thereby the function of aquatic food webs (Smetacek 1999). Species
of Skeletonema dominate spring blooms in temperate waters and drive the biogeochemical
cycle of carbon and other nutrients in these parts of the ocean.
Size matters for a marine diatom
Changes in growth pattern and consumer selectivity can have large effects on ecosystems.
The diatom Skeletonema marinoi can exist as solitary cells or form chains of more than 20
cells. The reason why S. marinoi and some other species of diatoms form chain-like colonies
and what determines the length of the chains are not clear, but other organisms (bacteria and
Phaeocystis) can start to grow in filaments or colonies after sensing cues from grazers (Long
et al. 2007). It has been showed that copepod grazers choose to graze diatoms in chains of two
cells or more to a larger extent than single cells. We have seen that the chain length of one
strain of Skeletonema is modified when exposed for grazers. We do not, however, know if
this is a general phenomenon among other strains and what triggers the modification.
Hypothesis: Skeletonema and other species of chain forming diatoms can sense the presence
of grazers and change its growth pattern to fall outside the size span of the grazer (grow in
chains or grow as single cells).
Methods: Different strains of Skeletonema and other species of chain forming diatoms are
incubated with different grazers (copepods and dinoflagellates) and the change in chain length
is recorded.
Changes in PUA production according to growth pattern.
Studies have shown that diatoms produce compounds that induce egg mortality and
production of abnormal larvae in copepods (Pohnert 2005). The effect has been shown for a
wide range of organisms, including bacteria, echinoderms and molluscs. The compounds
responsible for these effects are reactive volatile polyunsaturated aldehydes (PUA), the
production of the PUAs is initiated when the cell is wounded, as would occur during grazing.
The ecological function of this production is, however, not clear and we have formulated a
hypothesis suitable to be tested within a master project.
Hypothesis: PUA production in Skeletonema differs when it grows as single cells versus
chains.
Methods: Skeletonema is grown on a plankton wheel in order to form chains and with air
bubbling to get single cells. Cells are collected on filter and PUA is measured. The
experiment will be followed up with experiments to investigate the effect of the different algal
strains on copepod egg hatching success
References
Long, J. D., G. W. Smalley, et al. (2007). "Chemical cues induce consumer-specific defenses
in a bloom-forming marine phytoplankton." Proceedings of the National Academy of
Sciences of the United States of America 104(25): 10512-10517.
Pohnert, G. (2005). "Diatom/copepod interactions in plankton: The indirect chemical defense
of unicellular algae." Chembiochem 6(6): 946-959.
Smetacek, V. (1999). "Diatoms and the ocean carbon cycle." Protist 150(1): 25-32.
Contact:
Sten-Åke Wängberg
E-mail: sten-ake.wangberg@dpes.gu.se
Johanna Bergkvist
E-mail: johanna.bergkvist@dpes.gu.se