L3 - Zooplankton grazers, microbes, and predators

Question 1

How do water molecules attack and ‘stick’ to each other?

Answer 1

Hydrogen bonds

Question 2

How do hydrogen bonds benefit some insects?

Answer 2

It creates tension on the water surface, this tension allows some insects to walk on water

Question 3

What is viscosity?

Answer 3

a fluid’s resistance to flow
* can also be through of as ‘internal friction’ measured in centipoise (CP)

Question 4

How does the size of an organism affect the properties of water?

Answer 4

• Larger organism: water seems more turbulent (inertia is more important)
• Smaller organism: water seems more viscous (inertia is less important)

Question 5

What is the equation which can roughly quantify the balance of inertial and viscous forces?
HINT: Reynold’s numbers (Re)

Answer 5

Inertial forces/viscous forces = Re - Ul/v

Question 6

what is inertia?

Answer 6

Inertia is an object’s resistance to changes in its motion or state of rest, determined by its mass.

Question 7

‘Small organisms or structures are always surrounded by a boundary layer that sticks to them’. If these boundary layers are at low Re, what do they impact?

Answer 7

→ How zooplankton grazers eat
→ How microbes move, sink, and absorb nutrients
→ How macroinvertebrates live in running water

Question 8

How do Cladocerans like Daphnia use these filters?

Answer 8

use their carapace as an enclosed, high pressure chamber that forces water through the filter

Question 9

How do Copepods use their “filters”?

Answer 9

they use their “filters” like paddles to create currents and steer algae to their mouth - they handle their food and can reject toxic algae

Question 10

How does Cladocera’s and copepods feeding strategies differ?

Answer 10

Copepods select their food items; Cladocerans do not, which leads to big differences in impacts of grazing on algal communities…

Copepods prefer big algae: select for smaller algae
Cladocerans eat everything: select for larger algae

Question 11

Explain the microbial food web loop for aquatic microbes

Answer 11

• When algae are N or P limited, they excrete Dissolved Organic Carbon (DOC), getting rid of excess carbon
• This DOC is then lost from the food web and waster - leaving zooplankton grazers with no way to eat
  BUT DOC is food for heterotrophic bacteria which feed on detritus from decaying matter
• Heterotrophic = eat organic matter as a source for carbon, (not eating/absorbing CO2 for this) - not producing it themselves
• Heterotrophic bacteria are good food for (phagotrophic) protozoa (unicellular zooplankton)
• Protozoa are great food for zooplankton grazers * which brings the microbial food chain back into the classical food chain - so carbon isn’t lost

Question 12

Why is the aquatic microbial food chain called the microbial loop?

Answer 12

because it comes off the ‘classic’ food chain as a separate loop

Question 14

What is the viral shunt (1999)?

Answer 14

Viruses attack bacteria and algae, releasing more DOC and nutrients (N & P)

Question 15

How is the microbial loop and the viral shunt critical for the nutrient cycle?

Answer 15

Of all the carbon fixed by algae (entering into the food web):
- 40% goes through the microbial loop
- 6-26% goes through the viral shunt

Question 16

What % of Heterotrophic bacteria represent living biomass in ocean photic zones

Answer 16

40%

Question 17

Explain how the algal production and microbial loop operate in oxygenated water

Answer 17

When oxygen is available:
- Algae photosynthesis - use sunlight energy to produce sugar and O2 from CO2 and water
- Heterotrophic bacteria use aerobic respiration to reverse everything and get chemical energy back out as ATP

Question 18

What happens in anoxic conditions?

Answer 18

When respiration is higher than photosynthesis in seep, stratified water, oxygen can’t diffuse from the surface, and so doesn’t mix with surface water > Oxygen is depleted

Question 19

What happens when oxygen is depleted in anoxic conditions?

Answer 19

Many bacteria can switch to anaerobic respiration (fermentation), but it’s much less efficient

Question 20

what is anoxic?

Answer 20

A type of habitat

Question 21

Anoxic freshwater is dominated by specialist bacteria… Explain…

Answer 21

The specialist bacteria come in pairs that use each others waste products:
Pair 1: Sulphur metabolisms
- Purple and green sulphur bacteria
- Sulphate reducing bacteria
Pair 2: Methane metabolisms
- Methanogenic archea
- Methanotrophic bacteria

Question 22

Explain how phototrophs use anoxygenic photosynthesis, in comparison to sulphate reducing bacteria

Answer 22

Phototrophs that use anoxygenic photosynthesis: they use sunlight to fix CO2 like algae, but…
- Instead of using H2O and producing O2, they use H2S and produce SO4

Sulphate reducing bacteria use anaerobic sulphate respiration, using SO4 to produce H2S (instead of O2 which produces H2O)

Question 23

How do Methanogenic archaea produce CH4?

Answer 23

use anaerobic methanogenic respiration using CO2 or small organic carbon molecules to produce CH4
* with 2 major reactions taking place within the anaerobic respiration: Hydrogen: 4H2 + CO2 → CH4 + 2H2O
Acetate: CH3COOH + H2O → CH4 + HCO3

Question 24

How do Methanogenic bacteria use CH4?

Answer 24

Use methane as a carbon source
* but still need oxygen as an electron acceptor for aerobic respiration
- 5CH4 + 8O2 → 2(CH2O) + 3CO3 + 8H2O

Question 25

“Most aquatic consumers are typically gape-limited” … What does this mean?

Answer 25

Can’t tear apart their food, so they can only eat things smaller than their mouths

Question 26

How does being gape-limited impact aquatic consumers?

Answer 26

→ What species can eat what, at what life stages
→ What adaptations can protect prey from predators
* Many species undergo ontogenetic diet shifts (at different life stages, they eat different things - as their mouths grow in size)

Question 27

Why do aquatic primary producers (alage) grow rapidly?

Answer 27

Due to binary fission

Question 28

Why do pelagic habitats often have inverted biomass pyramids? (food webs)

Answer 28

because there is more biomass in higher trophic levels as there is high turnover so algal standing biomass doesn’t represent the algal productivity rate (i.e. algal biomass is quickly consumed by grazers and moved up the pyramid)