AQUATIC PLANT PHYSIOLOGICAL ECOLOGY:

ABIOTIC FACTORS

Up to this point we have discussed the general environmental preferences for particular taxa. Now we will cover the physiological bases for such preferneces in a more theoretical sense.

The concept of limiting factors suggests that only one factor that constrains algal biomass or growth/productivity rate. However, therein lies the problem: limiting to what? Growth rate of individuals/of populations? Biomass? Reprod. success per individual/of the population?

It is also a steady state concept, and in the real world conditions are dynamic.

More than one factor probably limits an individual organism and certainly a community, esp. for dissimilar factors, e.g. chemical (salinity, nutrients) and physical (light, temperature).

The level of one factor determines the response to other factors, even in steady state and esp. in rapidly changing conditions.

To make sense out of such complex factor interactions, we feeble-minded scientists often study them in isolation or very simple combinations. Keep that in mind as we go through the various factors.

LIGHT

Light is the ultimate resource for plants as it provides all of the energy for primary production, and so drives the entire ecosystem.

Light attenuates exponentially through water:

I_z = I_o·e^-kz where k = extinction coefficient, which depends on both dissolved substances and suspended particulates (sediment, detritus, plankton)

Operationally, this means that if 10% of surface PFD (I_z/I_o = 0.1) reaches depth X, then 1% of surface PFD (the nominal limit of the euphotic zone, where Ps can occur) will occur at 2X, assuming no vertical variation in absorption properties.

Plants respond to light quantity (PFD or PFR) in the form of a saturation-type, quasi-hyperbolic P-I curve

The above P-I response is an "instantaneous" (given the minutes to hours time frame of actual measurements) characterization of a plant in a given physiological state.

Many plants, especially algae, have the ability to reversibly photoacclimate to prevailing light conditions by altering the amounts of photosynthetic pigments and enzymes to make the most efficient use of light

Note increase in P_m and R_d and decrease in slope w/ acclimation to high PFD (incident light basis)

R_d is typically about 5-15% of P_m, but remember that R_d probably occurs all night, so can consume a large % of daily productivity

Because of nocturnal respiration, growth rate is less and I_c for growth higher than predicted from diurnal Ps

Temperature has a large effect on P_m and R_d, usually much less on alpha.

Nutrient availability also may change P-I parameters.

When PFD fluctuates between saturating and limiting at high frequencies (seconds or less) the net Ps rate may be higher than predicted (the "flicker effect") from the steady state P-I model

In addition to effects on Ps, light also controls photoperiodism (triggering of reproduction, etc.) and photomorphogenesis (plant development modified by PFD or spectral quality)

It is possible that in many supposed instances of photoperiodism, the photoperiod may simply act to entrain an endogenous circannual rhythm; further research is needed

NUTRIENTS

1. Physiological role of elements (from Graham & Wilcox 2000, Table 2-1)

N - proteins, nucleic acids, chlorophyll, phycobilins

P - nucleic acids, ATP, phospholipids, ion transport regulation, bioenergetics

Si - diatom frustules, silicoflagellate skeletons, synurphyte scales & stomatocyst walls

S - some amino acids (Met, Cys), nitrogenase, thylakoid lipids, Coenzyme A, carrageenan, agar, biotin

Mg - chlorophyll

Ca - alginates, calcium carbonate, calmodulin

Na - nitrate reductase

Cl - photosynthetic O₂-evolution

Fe - ferredoxin, cytochromes, nitrogenase, nitrate & nitrite reductases, catalase

Mn - photosynthetic O₂-evolving complex

Zn - carbonic anhydrase, some superoxide dismutases, alcohol dehydrogenase, glutamic dehydrogenase

Cu - plastocyanin, some superoxide dismutases, cytochrome oxidase

Co - cyanocobalamine (B₁₂)

Mo - nitrate reductase, nitrogenase

V - bromoperoxidase, some nitrogenases

vitamins - some algae require (cannot synthesize) certain water-soluble vitamins: thiamine (B₁), cyanocobalamine (B₁₂), biotin (H)

1. Nutrient uptake kinetics

a) Michaelis-Menten equation: V = V_m (S/(K_m + S)), where Km = S at 0.5V_m (half-saturation) - decribes a hyperbolic saturation function

b) Types of uptake experiments: data from these are used to construct M-M curves

parallel independent incubations

cumulative disappearance (problem: cells become satiated w/ time so uptake rate underestimated at low conc.)

Nutrient uptake rate depends on light, esp. in nuts-sufficient conditions.

There is a distinction between uptake from the medium and assimilation into organic compounds, esp. N:

NO₃^- --*--> NO₂^- --*--> NH₄⁺ ------> amino acids, etc.; depends on ability to store inorganic ions, rate of enzymatic steps, and cell needs (enzymes: * nitrate reductase; * nitrite reductase)

Cells/plants can acclimate to chronically low nutrient levels by increased surge uptake capacity (V_m), carbos & lipids; decreased cell quota (Q), Chl & protein per cell

2. Nutrient-limited growth (u = specific growth rate)

a) Monod model: based on external concentration, which may be below detection limits (but still biologically relevant) in oligotrophic waters:

u = u_m (S/(K_m + S)), where K_m = [S] at 0.5u_m (half-saturation)

b) Droop model: based on internal conc., which is often more important and easier to measure (because higher) than instantaneous external conc.:

u = u_m (1- (Q_o /Q)), where Q_o = minimum cell quota (u = 0)

for that nutrient

Note that "external" nutrient concentration is a scale problem: phytoplankton cells may "perceive" micropatches of nutrients in ul volumes whereas our measurements integrate many ml (10³x higher).

3. Nutrient-spiking (enrichment) experiments

Designed to determine which nutrient is limiting to the population or community

Add nutrients singly or in combination, observe which induces increased cell #, Chl, Ps or u

Note time scale problem: immediately after NO₃^- addition the Ps rate may decrease due to competition for reductant (NADPH) and ATP, and only later will Ps increase

TEMPERATURE

All chemical processes are temperature sensitive

Enzymatic reactions are (+) related to temperature up to some threshold, beyond which the enzyme denatures; The typical temperature coefficient, Q₁₀, is about 2:

Q₁₀ = rate of process at (T+10°K) / rate of process at T

= (k₂/k₁)^^(10/T₂^-T₁⁾

where k₁, k₂ = rates at temperatures T₁, T₂

The Q₁₀ for Ps < Q₁₀ for R_d, thus P/R ratio decreases w/ increasing temperature

Similarly, because many processes control growth rate (u), u_m vs. T is not equal to P_m vs. T

FACTOR INTERACTIONS