Microbial Food Webs and Nutrient Cycles

Question 1

What are the three major groups of microbes?

Answer 1

•Heterotrophic bacteria (small, break down carbohydrates)

• Require organic substrate (particulate or dissolved) for energy source
• During decomposition process re-mineralise essential plant nutrients

•Photosynthetic bacteria (blue/green cyanobacteria) - fix nitrogen

• Small primary producers like phytoplankton
• Often dominate in central ocean (oligotrophic) basins

•Archaea

• Biochemistry very different from bacteria (more like higher organisms)
• Some are extremophiles (e.g. vents, very salty environments)
• Can use organic substrate for energy source
• Extract energy from reduced compounds (CH4, NH4, H2S - Methane, Ammonium, Hydrogen Sulphite)

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Question 2

What organisms dominate the planet?

Answer 2

Microbes do!

(critical to the biology of the open ocean)

• Bacteria size typically 0.2-0.6 mm
• Abundance = 1029 (stars in the universe = 1021)
• Biomass > metazoan (zooplankton, fish, mammals)
• Very high metabolic rates (high surface area to volume ratios)

Question 3

What is the difference between micro-algae and bacteria?

Answer 3

Bacteria are heterotrophic (have external energy sources - food)

Question 4

What energy sources do microbes have?

Answer 4

Dissolved organic carbon (DOC)

• very large pool
• labile includes excretory products, cell lysis, sloppy feeding by zooplankton

Particulate organic carbon (POC)

• fecal pellets
• marine snow and detritus

•Heterotrophic bacteria & archaea have a wide range of enzymes capable of breaking down complex and refractory organic materials (lignin, cellulose, chitin & petroleum) into small simpler molecules that can be used by other microbes

•Resting stages (reduce metabolism, size) long periods of quiescence but capable of rapid growth when conditions allow

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Question 5

How is carbon in the microbial food web repacked and recycled?

Answer 5

50% of primary production consumed by bacteria

Cyanobacteria dominate in the oligotrophic ocean gyres (produce 50% of the worlds O2)

Question 6

Not a lot of organisms can feed on bacteria. how do they do it?

Answer 6

• Salps pump water through fine mucus net which is then ingested
• Capture very small particles, efficient grazers on components of the microbial loop
• Direct transfer to larger organisms but not a good food source for much else

Question 7

How efficient is the microbial web?

Answer 7

Bacteria and Nanoflaggelates have matching production rates (ratio)

• Grazer production lags bacterial production by only hours (cf. zooplankton-phytoplankton in classical food chain)
• Ecological transfer efficiency ~ 50 % (cf. typical 10-15 %)
• Bacteria do not ‘bloom’

Question 8

Briefly describe viruses

Answer 8

Viruses are the smallest unit of biological life but the most common

• infect most marine organisms
• cause 10-50% bacteria mortality
• contribute to cycling of organic matter and nutrients
• help maintain material non-sinkable form in the euphotic zone

Question 9

How do bacteria influence the nitrogen cycle in their critical role?

Answer 9

Nitrates go through a protein synthesis creating protoplasms which are consumed by plants and then animals. Bacteria help in this consumption process together with fungi causing decay.

Amino acids and organic residues together with aminifying bacteria create ammonia which is turned into nitrites by nitrite bacteria

• Denitrification is an anaerobic process occurs in OMZ and sediments reducing nitrate to nitrogen gas*
• Denitrification represents a loss of N from the system*

Question 10

What is anammox?

Answer 10

anaerobic oxidation of ammonium by nitrite to N2 gas

N2 fixation may be much more important than first assumed

Question 11

What mostly drives primary production in the tropics (stable water column, no seasonal overturn)?

Answer 11

regeneration/recycling of nutrients

the microbial loop ensures efficient cycling of nutrients, very tight coupling between bacteria and grazers keeps nutrients on euphotic zone

Question 12

What fuels 60-80% of primary production?

Answer 12

external systems (new production)

Question 13

What is new and regenerated production in the microbial loop?

Answer 13

• f-ratio = new production/(new production + regenerated production) - how much of total prodction is dependent on old vs generated sources
• Low f-ratio = dependence on regenerated production
• High f-ratio = upwelling and coastal areas

if N is removed (e.g. fish biomass) and not replaced the microbial loop can collapse or gets weakened