BACTERIA AND THE MICROBIAL LOOP

I. Characteristics of bacteria
    A. Bacterial species; what is in nature?
        1. Two major groups
            a) Eubacteria
            b) Archaea (archaebacteria)

        2. Small prokaryotic cells – generally 0.2 to 1 um in length
        3. Tolerate a wide range of conditions
        4. Generation times: 20 minutes or in viable resting stages for centuries!
        5. Locomotion – some have a rotary motor (100 rev/sec)
        6. Cyanobacteria – oxygen in atmosphere
        7. Bacterial species
            a) Morphology
            b) New molecular techniques

    B. Microbial Ecology
        1. Enumeration – How many are there?
            a) Underestimates of old plating techniques
            b) Modern epifluorescent techniques
            c) 106/ml in natural waters
            d) 109/ml in sediments
            e) Some variation with productivity
            f) Mysterious consistency of bacterial values – balance of growth and losses
        2. Activity – how fast are they doing things?
            a) Most often measured with radio labeled tracers
            b) Which bacteria are the most active?  Specific molecular probes.
            c) 'Ghost cells' can be from 20-90% of total countable cells

II. Role of bacteria in the lake
    A. Potential ways to make a living
 
Classification Energy Source for generating ATP Source of carbon for building cell components
Photoautotroph Light CO2
Chemoautotroph Inorganic compounds CO2
Photoheterotroph Light CO2, organic matter
Heterotroph Organic matter Organic matter

    B. Autotrophs produce organic matter
    C. Decomposers (mineralizers)
        1. DOM often up to 20X more abundant than POM
        2. Importance of oxygen

    D. Fix nitrogen from atmosphere into useable form
    E. Pathogenic
 

III. Controls of bacterial growth
    A. Temperature
    B. Acquisition of nutrients
        1. types of cellular processes
            a) Assimilative
            b) Dissimilative
        2. DOM sources (DOC, DON, DOP)
            a) C is often limiting
            b) Bacteria and algae compete for phosphorus and inorganic nutrients (bacteria are usually better competitors), but
            c) There is occasional nutrient limitation -- especially P
            d) Best bacterial growth when DOM contains C, N and P
            e) 'Quality' of DOC
                a. Labile
                b. Refractory
                c. UV light can help to break down DOC to more usable forms, but also directly inhibits bacterial growth
            f) Sources of DOC - algae, macrophytes, watershed
    C. Correlation of bacterial numbers with source of DOM
        1. As chlorophyll a increases, bacterial numbers increase

direct positive correlation between chlorophyll a and bacterial numbers

        2. Still a small range of bacterial numbers
        3. Up to 50% of the C fixed by phytoplankton is exuded and used by bacteria
    D. Correlation of bacterial productivity with source of DOM

positive correlation between net primary production and bacterial numbers

        1. bacterial productivity is often about 25% of net primary productivity
        2. bacterial productivity is generally 2X that of zooplankton

IV. Controls of bacterial attrition
    A. Grazing – bacteria are fed on by protozoans
        1. Protozoans – eukaryotic, heterotrophic, phagotrophic
            a) Amoebas
            b) Ciliates
            c) Flagellates
        2. Can ingest 100-1000 bacterial cells per flagellate per day
        3. Predator-prey cycles

        4. Selective grazing
        5. Tests of grazer limitation of bacterial numbers

    B. Viruses (Fuhrman, SUNY Stony Brook, Suttle, Univ. British Columbia)
        1. Cyanobacteria bloom crashes – historical anecdotal reports; attempted control of blooms
        2. Very recent developments in technology
        3. 10-100 million viruses per mL
        4. diversity of forms
        5. may cause half of bacterial mortality; lyse 10-20% of bacteria daily

V. The microbial food web – how important are bacteria to overall lake productivity?
    A. Questions
        1. Are bacteria food for higher trophic levels?
        2. Are bacteria nutrient regenerators or nutrient sinks?
    B. Microbial loop

microbial loop and its relationship to the traditional 'grazer chain'

    C. Experimental evidence
        1. First real test - Ducklow (1986) - only 2% of the C taken up by bacteria ended up in higher pools

               14C-glucose -> bacteria -> protozoans/larger zooplankton

        3. However, these bacteria were only given DOC for growth, with little P or N -- were slow growing
        4. The study was repeated with additional nutrients -- higher transfer rates, but variable
    D. General current thinking
        1. Often a small amount of material from the microbial loop is utilized by higher trophic levels (<20%)
        2. Bacteria can compete with algae for inorganic nutrients, and thus may actually be a 'sink' rather than a remineralization source
        3. Protozoans and viruses may be more important in remineralization
        4. Bacteria are very important in mediating chemical reactions and in decomposing organic material
 

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