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

Definitions Autotroph: Grows on non-organic forms of carbon and energy. For example, phytoplankton are autotrophs - they use CO 2 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 Chain Concept (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 )

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 U pwelled 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 W hen 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