Marine Microbial Processes

Outline

• size-structured food webs

• brief history of the development of our current understanding of microbially dominated food webs

• carbon cycling in marine food webs

• evolving concepts

1 Definitions Autotroph: carbon and energy for growth comes from non-organic sources. For example, phytoplankton are

autotrophs because they use CO2 for their carbon source and use sunlight for their energy source Heterotroph: carbon and energy for growth comes from pre-formed organic material. For example, herbivorous zooplankton are heterotrophs because they 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

2 Optimal Prey Size of Pelagic Animals

Marine Food Webs are Size-Structured

Our conceptualization of marine food webs is built on the general rule that preferred prey size is approximately 1/10 consumer size

3 Traditional Food Chain Concept (early1970’s)

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

Use of Epifluorescent Microscopy and Fluorescent DNA Stains Became Widespread Between 1975 and 1985

• dramatically increased estimates of bacterial concentrations in the ocean

• Allowed easy distinction between autotrophic and heterotrophic cells (i.e., chlorophyll containing or chlorophyll lacking)

5 Bacterial Concentrations Before (Red Fill) and After (Blue Fill) 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-micron diameter cells) that are heterotrophic

What is the carbon and energy source for all this newly discovered heterotrophic bacteria?

6 Heterotrophic Bacteria are growing on dissolved organic matter released from phytoplankton by steady leakage,sudden cell senescence or sloppy feeding by zooplankton

The term Microbial Loop is coined by Azam et al. (1983) to describe the role that microbes play in the marine ecosystem carbon cycle

7 Most carbon entering the heterotrophic bacteria is eventually respired back to carbon dioxide

Summary: Early 1970’s versus Early 1980’s

8 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

9 Prochlorococcus abundance is similar in magnitude to that of heterotrophic bacteria

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

10 Relative Importance of Prochlorococcus and Heterotrophic Bacteria in Oligotrophic Systems

The Role of Microbes in Material Flow Through Marine Ecosystems…

11 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 community shifts to smaller forms as nutrient concentration is reduced.

12 Role of Microbes in Carbon Cycling in the Ocean…

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.

13 Seasonal Variation in the Global Biosphere

14 Magnitude of CO2 flux between Land and Ocean Reservoirs

Conclusions • Heterotrophic and autotrophic bacteria make up a significant 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 microbial carbon is respired back to CO2

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

– the microbial community forms a direct trophic link between primary production and higher trophic levels

15 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

Evolving Concepts of Microbial Food Webs…

16 High Nutrient Low Chlorophyll Regions (HNLC)

Iron Cycling in HNLC Regions

17 Station Aloha - Subtropical North Pacific

Station Aloha

Time Series of N:P Ratio for Total Dissolved, Suspended Particulates in the North Pacific Subtropical Gyre (from Karl 1999)

18 Conclusion

• Iron limits primary production in high nutrient low chlorophyll (HNLC) regions of the Subarctic Pacific, Equatorial Pacific and Southern Ocean.

• North Pacific subtropical gyre seems to be moving toward phosphorus limitation due to added inputs of nitrogen to the system via nitrogen fixation. This is probably a climate change response

19 Good Luck to Those Students Who Are Going out on Seneca Lake Today!

will be like this?… …or this?

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