The Microbial Loop

Question 1

What are the 4 fates of carbon?

Answer 1

1. Grazing food chain (classic food web)
2. Physically sink in the ocean (fecal pellets/dead zooplankton)
3. Microbial loop (DOM—>bacteria
   —>protozoa—>zooplankton)
4. Turn into CO2

Question 2

What are the two pathways in which primary production transferred to other trophic levels?

Answer 2

1. Food chain
2. Microbial loop

Question 3

What is the microbial loop?

Answer 3

phytoplankton —> DOM
- Phytoplankton release DOM (this is the main source of organic carbon into the loop)
DOM —> bacteria
- Bacteria consume DOM converting it into bacterial biomass
bacteria —> protozoans
- Protozoans graze on bacteria transferring energy up the microbial food web
protozoans —> zooplankton
- Zooplankton consume protozoans incorporating the microbial loop derived energy into the traditional food web
zooplankton —-> carbon sinking
- Some carbon gets exported to deep waters via fecal pellets or dead zooplankton (biological pump)

Question 4

Why are protozoan grazers a crucial part of the microbial loop?

Answer 4

• 10:1 rule!
• They are the link that allows for the microbial loop energy to be transferred to the traditional food web (bacteria = too small for zooplankton to eat directly)

Question 5

How can the microbial loop be a link?

Answer 5

C from bacteria feeds upper trophic levels

Question 6

How can the microbial loop be a sink?

Answer 6

C from bacteria turns into CO2 (fecal pellets/dead zooplankton) and does not make it to upper trophic levels

Question 7

What determines which pathway will be dominant in a food web?

Answer 7

• The size of primary producers
• Small = microbial loop more important
• Increased level of recycling carbon

Question 8

__% of what goes through ___ via DOC?

Answer 8

50% of primary production goes through bacteria via DOC

Question 9

Where would the microbial loop be most important?

Answer 9

• The open ocean (oligotrophic)
• Microbial loop energy is likely to be incorporated back into the food web
  Small plankton = dominant

Question 10

Where would the microbial loop be least important?

Answer 10

• Coastal upwelling zones (eutrophic)
• Microbial loop will serves as a sink
  Large plankton = dominant

Question 11

What minimizes the return of bacterial energy to upper trophic levels?

Answer 11

The 10% rule!
Only 10% of energy is passed to each trophic level

Question 12

What are archaea?

Answer 12

• Diverse prokaryotes
• 1/3 of ocean life
• Poorly studied

Question 13

What 2 biochemical pathways are important for archaea?

Answer 13

• Parasitism/symbiosis
• Saprotrophy

Question 14

Describe the mycoloop

Answer 14

• Chytrid fungi infect large phytoplankton (via sapotrophy) that is otherwise hard to consume by zooplankton
• Chytrids produce zoospores
• Zooplankton feed on zoospores
• Energy from primary produces is linked to higher trophic levels