2 Ecological and biological background
due to its serial arrangement of units with maximum frequency fmax (or speed per
length) Vogel, 2013. The output power is thus ultimately limited by the maximum
force times maximum speed of the muscle, which then scales with the volume or mass
of muscle as Pmax = max fmax L3, i.e., linear with motor mass.
This maximum power could relate to a constant upper bound in measured massspeci
c respiration rates, that has been found for a wide range of size scales and
not only for muscle motors Makarieva et al., 2008; Kirboe and Hirst, 2014. The
maximum frequency, on the other hand, can be related to maximum speeds in living
organisms Meyer-Vernet and Rospars, 2015. To obtain the maximum power limit we
here assume that maximum frequency and maximum tension can be realised independently.
However, this is generally not the case. The force output of muscles is highest
at the lowest, even negative speed, while the power output (force times speed) reaches
its maximum at intermediate speeds Wilkie, 1950.
2.6 Study organisms
During this Ph.D. study we investigated the hydrodynamics of dierent marine plankton.
Here I will introduce our main study organisms, haptophytes, choanoagellates,
and pelagic tunicates. Since copepods are one of the most important marine grazers,
which feed on agellates, and since bacteria form a main prey component of all
studied organisms, I will further introduce those life forms. I will avoid specialised
biological and taxonomical terminology and introduce the relevant organisms in view
of their overall trophic role, morphology, and functionality in terms of their physical
interaction with their biotic and abiotic environment.
A classical model organism for many hydrodynamic studies of biagellates is Chlamy-
domonas, which is a phototrophic algal cell with an almost spherical cell body of a
few micrometres in diameter, which swims in a breast-stroke type fashion with two
left-symmetrically arranged agella. This model organism is especially important for
the study of the origin of multicellular life Goldstein, 2015. However, mixotrophy,
i.e., the ability to collect prey in addition to photosynthetic activity, is very common
in unicellular marine species, while pure phototrophy as in Chlamydomonas or pure
heterotrophy as in choanoagellates (section 2.6.2) is rather rare in agellates.
Our model organisms in the study on biagellates are mixotrophic and belong to the
group of haptophytes Nygaard and Tobiesen, 1993; Dolger et al., 2017b. Haptophytes
usually have two, sometimes four agella, which like the rest of the cell surface may be
covered by scales (coccoliths) Hausmann et al., 2003. A third slender structure, the
haptonema, is typical for haptophytes and extends from the cell similar in structure
as the agellum, but it is not involved in motility. Its length varies between species
and individuals and its function can vary, from xation and gliding on surfaces to food
capture Hausmann et al., 2003; Kawachi et al., 1991; Kawachi and Inouye, 1995.