Microbial Food Webs

Bruce Monger ([email protected]) Department of Earth and Atmospheric Science, 4134 Snee Hall Outline

• size-structured food webs

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

• nitrogen and carbon cycling in marine food webs

• evolving concepts Definitions Autotroph: 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 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

Traditional Food Chain Concept (early1970’s) 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) 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 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 and Heterotrophic Bacteria 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 community shifts to smaller forms as nutrient concentration is reduced Role of Microbes in Nitrogen and Carbon Cycling in the Ocean… Nitrogen Cycling

Primary Primary Production Production fueled mostly fueled mostly from by Nitrate Recycled from the deep Ammonia ocean

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 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

• 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 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… High Nutrient Low Chlorophyll Regions (HNLC) Dust (Iron) Induced Phytoplankton Blooms Downwind of the Galapagos Islands Iron Cycling in HNLC Regions Station Aloha - Subtropical North Pacific

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

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