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Growth rates
of unicellular organisms
-
Definition
of growth rates:
-
Primary
production: fixed carbon / time [µg
C l-1 h-1]
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Assimilation
Index: production normalized to biomass; helpful
when comparing production in systems with different phytoplankton biomass
[mg C (mg Chl.a)-1 d-1]
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Growth
rate based on cell number: change of cell
number over time; for unicellular organisms – exponential function; symbol:
µ, unit: d-1
N0 + DN
= N0 x eµt can
be transformed to:
µ = [ln(Nt) - ln(N0)]
x
1/t which
equals
µ = ln(Nt/N0)
x
1/t
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Doubling
time: time for a population to double its
cell number; unit: time [d]:
d = ln(2) / µ = 0.69 / µ
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Generation
time: reciprocal of doubling time; number
of generations produced per day; unit 1/time [d-1]
G = µ / ln(2) = µ /
0.69
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Growth
phases of algal cultures:
(1)
= lag phase; cells use new nutrients to replenish their internal pools
of nitrogen and phosphorous constitutents (proteins, chl.a)
(2)
= exponential phase; cells grow according to the exponential growth function;
if cell numbers are plotted on log scale versus time, the increase of cell
number is linear
(4)
= stationary phase; nutrients are exhausted, cell division and population
growth stops
(5)
= death phase; cells "starve"
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DNA replication
and cell division: occur during night in most
species
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Cell
growth (increase in cell volume) occurs during
day in parallel to photosynthesis
-
Makes
sense: phytoplankton use day to acquire energy
by photosynthesis, use this energy at night for DNA replication and cell
division metabolism (synthesis of proteins and structural material), divide
in the late night/early morning, so that daughter cells are „ready“ for
the next day of photosynthesis
-
Exceptions:
some algae (such as Synechococcus spp.) divide only at daylight
Nutrient Uptake
Kintetics
-
Nutrient
uptake can be described similar to photosynthesis
dependence on light
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Michaelis-Menten
Model: hyperbolic function of nutrient uptake
rate v with nutrient concentration S with vmax
= maximum uptake rate; km = half saturation constant
Ammonium uptake kinetics in two macroalgae
v = vmaxx
S / (km + S)
-
Monod
Model: describes the dependence of growth
rate µ on nutrient concentration
Growth rates of two diatom species in dependence of nitrate
and silicate concentrations
µ = µmaxx
S / (ks + S)
Resource limitation
of growth
-
Limitation:
The total yield or biomass of any organism will be determined by the nutrient
present in the lowest (minimum) concentration in relation to the requirements
of that organism (Liebig‘s law of the minimum,
1840); for zooplankton, food is the limiting
resource; light can also be a limiting resource for algae!
Justus von Liebig
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Competition:
under resource competition, those species with the lowest resource requirement
or with the highest ability to utilize low resources will succeed in competition
-
External
factors such as temperature may influence
growth rates regardless of limitation and competition
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Redfield
Ratio: elemental ratio of elements
C
: N : P = 106 : 16 : 1
(molar
ratio!) under non-limiting conditions
-
Size
is an important factor in the competition for limiting nutrients: smaller
cells are generally better adapted to low nutrient concentrations, because
their higher surface/volume ratio provides
more surface area per volume for nutrient uptake at the cell surface
With increasing size or body length, the surface/volume
ratio decreases
With increasing surface/volume ratio (which means
decreasing size!), the growth rate increases in one species of diatoms
The half-saturation constant for nitrate uptake
increases with mean spherical cell diameter, which means that bigger cells
have a higher kS, thus a lower nitrate uptake ability.
-
Regional
distribution: Because small phytoplankton is better adapted to deal with
limiting rescources (nutrients), pico- and nanoplankton are more dominant
in oligotrophic, nutrient-poor systems and large microplankton is dominant
in nutrient-rich upwelling regions
The contribution of picoplankton to total chl.a
decreases towards systems with higher chl.a concentrations; high chl.a
concentrations point towards higher nutrient concentrations as well to
sustain the higher plankton biomass
Resource limitation
of phytoplankton growth: Tilman's theory of equilibrium resource competition
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R*:
resource concentration at which growth = death, minimum requirement
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Competition:
if initial resource concentration R0 > R*, all species can grow; R will
be lowered; if R reaches R*1 of species 1, this species can no longer grow
if R is further lowered; in competition with another species 2 with lower
R*2, species 2 will outgrow species 1 (R*2 < R*1)
Ability
to compete: R* is an integrated measure of
the competitiveness for a limiting resource; species differ in their R*
for different nutrients / light
Two examples of plotting resource requirements
on two parameter plots. Lines that mark the minimum recource requirement
(starting from R* and parallel to the other axis) are called the ZNGI's
(zero net growth isolines). The line from the origin of the plot through
the intersection of the two ZNGI's represents the "optimum resource ratio".
Diagram of the two above species of diatoms,
showing nutrient ratios under which only one of the two species can grow,
where one will outcompete the other, and where the two species will co-occur
Theoretical construction of multi-species ZNGI's
plots, which indicate nutrient/resource ratios with allow for co-occurrence
of species or outcompeting by one species
Because of better adaptation to the growth situation,
Fragillaria will coutcompete Tabellaria even when introduced
late in the culture growth (upper graph); if introduced into a going culture
of Fragillaria,
Tabellaria is not able to compete (lower
panel).
As resources are depleted by phytoplankton growth,
different species will dominate along time, according to their R*
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