Microbial Processes and Material Flow in Ocean Ecosystems

Outline

• Ecological terms to be used in lecture

• Concept of size-structured food webs

• Brief history of the development of our

current understanding of microbial processes

in marine systems

• Role of microbes in nitrogen and carbon

cycling

DefinitionsAutotroph: Grows on non-organic forms of carbon and energy. For example, phytoplankton are autotrophs - they use CO2 for their carbon and use sun light for their energy

Heterotroph: Uses carbon and energy contained in pre-formed organic carbon for growth. For example, herbivorous zooplankton consume phytoplankton for their carbon and energy needs.

Oligotrophic: Refers to low nutrient and low productivity environments. For example the subtropical gyres are oligotrophic regions

Eutrophic: Refers to high nutrient and high productivity environments. For example the coastal upwelling areas are eutrophic regions

Optimal Prey Size of Pelagic Animals

Our conceptualization of marine food webs is built on the general rule that preferred prey size is

approximately 1/10 consumer size

Marine Food Webs are Size-Structured

Traditional Food ChainConcept (early1970’s)

Traditional Bacterial Concentrations Estimated from Transmission Light Microscopy and Culture-Plate Colony Counts

Epifluorescent Microscopy, in combination with fluorescent DNA stains, dramatically increased our estimates of bacterial concentrations in the ocean and also enabled us to distinguish large autotrophic cells (chlorophyll containing) from heterotrophic cells (lacking chlorophyll)(circa 1975-1985)

Comparison of Bacterial Concentrations Before and After the Introduction of Epifluorescent Microscopy

New view of marine food webs that recognizes the importance of high bacterial biomass and a large fraction of nanoflagellates (2- 20 um diam.) that are heterotrophic

Cycling of Organic Carbon from Phytoplankton via Exudates & Cell Senescence to Heterotrophic Bacteria

The Term Microbial Loop is Coined by Azam et al. (1983) to Describe the Role Microbes play in Marine Ecosystems

Sink Versus Link Controversy ends with the Recognition that Most Carbon Entering the Heterotrophic Bacteria is Eventually Respired Back to Carbon Dioxide

Summary: Early 1970’s versus Early 1980’s

Discovery of an Important New Bacteria-Sized Autotroph

In 1988 Sally Chisholm and Others Published a Paper Describing the Presence of a New Type of Very Small Autotroph that is Present in High Abundance - Especially in Oligotrophic Regions

The Discovery was Made using a New Technique called Analytical Flow Cytometry

This Important New Autotroph Came to be Known as Prochlorococcus

Simple Diagram of Flow Cytometeric Method

Relative Abundance of Prochlorococcus and Heterotrophic Bacteria

New View (1990’s) of Marine Food Webs that Recognizes the Importance of Prochlorococcus

Relative Importance of Prochlorococcus in Oligotrophic Systems

The Role of Microbes in Material Flow Through Marine

Ecosystems…

The Changing Role of Marine Microbes Along a Nutrient Gradient

Microbes are Recyclers ----------------------------------------> Microbes are Direct Trophic Link

The role of marine microbes as recyclers in eutrophic waters versus a direct trophic link in oligotrophic waters derives solely from the concept that the dominant cell size in the phytoplankton communiy shifts to smaller forms as nutrient concentration is reduced

Role of Microbes in Nitrogen and Carbon Cycling in the Ocean…

Nitrogen Cycling

Primary

Production

fueled mostly by

Nitrate from the

deep ocean

Primary

Production

fueled mostly

from Recycled

Ammonia

Upwelled nitrate from the deep ocean is the dominant source of nitrogen for phytoplankton growth in eutrophic waters. Recycled ammonia is the dominant source nitrogen in oligotrophic waters.

Carbon Cycling

When the dominant phytoplankton cells are large, the dominant grazers are large and the large fecal material easily sinks to the deep ocean taking organic carbon with it - this forms an efficient biological carbon pump. The opposite is true when the dominant phytoplankton is small and the biological pump is more inefficient.

Conclusions

• Heterotrophic bacteria make up a large percentage of the total community biomass in the ocean

• In eutrophic systems the microbial community acts as a sink for organic carbon - i.e. most carbon is respired

• In oligotrophic systems, Prochlorococcus is an extremely important component of the phytoplankton

• In oligotrophic systems the microbial community forms a direct trophic link between primary production and the highest trophic levels

Conclusions• As nutrient concentration is reduced the

competitive growth advantage shifts to small phytoplankton cells

• Small phytoplankton cells enhance the importance of microbial grazers and increases the level of nitrogen recycling in the upper ocean

• Small phytoplankton cells also enhance the percentage of organic carbon that is respired back to carbon dioxide and consequently is not pumped to the deep ocean