Gitai Yahel The School of Marine Sciences Ruppin Academic Center Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Biological Oceanography 22 Microbial food webs IV Mortality factors (viruses and protists) closing the microbial loop Gitai Yahel The School of Marine Sciences Ruppin Academic Center Yahel@Ruppin.ac.il | Tel. 0522918007 | Skype gitaiyahel | Web http://Moodle.Ruppin.ac.il
Classical view of marine food web (chain) Heterotrophs Phototrophs 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm Plankton size Ciliates Dinoflagellates Diatoms Nano phytoplankton Dissolved nutrients Saturday, May 18, 2019
Classic Food Chain P: Phytoplankton (e.g., Diatoms) The classic view of aquatic food webs was the linear food chain from phytoplankton to fish. Although bacteria were known to exist, they were not thought to be significant consumers of carbon or energy. A Standard NPZF oceanographic Model P: Phytoplankton (e.g., Diatoms) Z: Zooplankton (e.g., Copepods) F: Fish (both planktivors and piscivors) Saturday, May 18, 2019
What ecological roles do bacteria play? Saturday, May 18, 2019
What ecological roles do bacteria play? Interactions with phytoplankton Predators (pathogens) Compete for nutrients (autotrophs) Recycle nutrients (heterotrophs) Interactions with zooplankton Pathogens Prey Recycling detritus (POM -> DOM)? ? “Traditionally, bacteria have been regarded as remineralisers, responsible for converting organic matter to inorganic and recycling nutrients to primary producers. We have considered the questions: Is this true? If so, how does it occur? “ “Bacteria consume carbon as a source of energy while scavenging nitrogen for protein synthesis”. Azam et al. 1983 This is also true for other nutrients – GY Saturday, May 18, 2019
Classical view of marine food web (chain) Heterotrophs Phototrophs 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm Plankton size Ciliates Dinoflagellates Diatoms Nano phytoplankton Dissolved nutrients Saturday, May 18, 2019
The addition of the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm POM Diatoms Dinoflagellates Plankton size Ciliates Nano phytoplankton HNF Bact Pico eukaryotic algae Dissolved nutrients (NH4) Synechococcus Prochlorococcus DOM DOM Saturday, May 18, 2019
Total carbon (~28 mg liter-1) Organic carbon Living organisms Relative distribution of carbon species in the ocean Water overlying Eilat coral-reef, August 1998 The vast majority (>95%) of organic carbon in the sea is dissolved (passing through 0.2 or 0.7 µm filters) Planktonic carbon <0.5% Total carbon (~28 mg liter-1) Organic carbon Living organisms D, dissolved; P, particulate; IC, inorganic carbon; OC, Organic carbon; Det=detritus; Zoo, zooplankton; Pro, Prochlorococcus, Syn, Synechococcus; Euk, Eukaryotic algae; Bact, non-photosynthetic bacteria Zooplankton and PIC data curtsy of R. Yahel Gitai Yahel (sgitai@vms.huji.ac.il)
Feeding on dissolved organic matter Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Feeding on dissolved organic matter Over 90% of the organic carbon in the sea is dissolved Is DOC an important food source for animals? The Pütter hypothesis - a century old debate Oxygen demand >> available particulate food In vitro most invertebrates possess transporters for DOC uptake (e.g., FAA, small sugars) Evidence for the in situ removal of bulk DOC by metazoans other than high microbial abundance sponges (see Yahel et al. L&O 2003 and follower) are lacking The vast majority of organic carbon in the world oceans is found in the dissolved pool. A centaury ago, an apparent discrepancy between the oxygen demand and the available particulate organic matter has led Putter to proposed that DOC should account for a significant portion of the diet of some suspension feeders. This idea (some times referred to as “the Putter Hypothesis”) is still debated. Indeed, most soft bodied invertebrates posses specific, highaffinity integumental transporters for simple organic molecules such as free amino acids and simple sugars. Surprisingly, direct evidence for the removal of bulk DOC by adults metazoans are still lacking, and thus, bacteria are considered the sole consumer of DOC in the ocean. Bacteria are considered the principal DOC consumers (the microbial loop) Saturday, May 18, 2019
There are three “types” of DOC that Occur in seawater: Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, DOC availability DOC is the presumed substrate that supports bacterial metabolism in seawater. There are three “types” of DOC that Occur in seawater: Labile* DOC - supports bacterial Production Semi labile DOC that is produced and accumulates in surface seawater and may be lost by convective mixing Refractory DOC (Non reactive) - most DOC that is going no where * Labile, easily broken down Santinelli C, Nannicini L, Seritti A (2010) DOC dynamics in the meso and bathypelagic layers of the Mediterranean Sea. Deep Sea Res Part II Top Stud Oceanogr 57:1446–1459 Abstract Seven years (2001–2008) of dissolved organic carbon (DOC) vertical profiles were examined in order to assess the main processes determining DOC concentration and distribution in the meso- and bathypelagic layers of the Mediterranean Sea. As expected, DOC showed high and highly variable concentrations in the surface layer of 57–68 μM (average values between 0 and 100 m), with a decrease to 44–53 μM between 200 and 500 m. Deep DOC distribution was strongly affected by deep-water formation, with a significant increase to values of 76 μM in recently ventilated deep waters, and low concentrations, comparable to those observed in the open oceanic waters (34–45 μM), where the oldest, deep waters occurred. In winter 2004/2005 a deep-water formation event was observed and the consequent DOC export at depth was estimated to range between 0.76–3.02 Tg C month–1. In the intermediate layer, the main path of the Levantine Intermediate Water (LIW) was followed in order to estimate the DOC consumption rate in its core. Multiple regression between DOC, apparent oxygen utilization (AOU), and salinity indicated that 38% of the oxygen consumption was related to DOC mineralization when the effect of mixing was removed. In deep waters of the southern Adriatic Sea a DOC decrease of 6 μM, together with an AOU increase of 9 μM, was observed between the end of January 2008 and the end of June 2008 (5 months). These data indicate a rate of microbial utilization of DOC of about 1.2 μM C month−1, with 92% of the oxygen consumption due to DOC mineralization. These values are surprisingly high for the deep sea and represent a peculiarity of the Mediterranean Sea Fig. 8. Two conceptual models for the distribution of refractory (cyan and blue), semi-labile (yellow) and labile (green) fractions of DOM in the Mediterranean Sea. The refractory fraction was divided in two broad pools based on DOC gradient observed in the Mediterranean deep waters, in the same way as reported by Carlson (2002) for the open ocean. The lowest concentration of DOC (34 μM) was observed in the tEMDW (estimated age of 126 years), while concentrations of 44–48 μM were found in the old WMDW (residence time of 10–20 years). (A) The model proposed for the open ocean (Carlson, 2002) and supposedly valid for some Mediterranean regions; (B) a model adapted for specific areas of the Mediterranean Sea impacted by deep water formation, in which high amounts of semi-labile DOC are exported to depth (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Saturday, May 18, 2019
Deep DOC is refractory (~35-45 μM) Surface DOC is refractory + labile Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, DOC availability -III Deep DOC is refractory (~35-45 μM) Surface DOC is refractory + labile In culture, phytoplankton release about 10% of total PP as DOC. Most of this DOC is considered to be very labile Deep-ocean gradients in the concentration of dissolved organic carbon Labile DOC is metabolized by bacteria in a few days Large fraction of PP is funneled though DOC to bacteria (> 10%) (based on Global estimates of bacterial carbon demand) Non reactive DOC is removed by photo-oxidation, adsorption (onto sinking particles), and flocculation There is as much carbon in dissolved organic material in the oceans as there is CO2 in the atmosphere1, but the role of dissolved organic carbon (DOC) in the global carbon cycle is poorly understood. DOC in the deep ocean has long been considered to be uniformly distributed2,3 and hence largely refractory to biological decay4. But the turnover of DOC, and therefore its contribution to the carbon cycle, has been evident from radiocarbon dating studies5,6. Here we report the results of a global survey of deep-ocean DOC concentrations, including the region of deep-water formation in the North Atlantic Ocean, the Circumpolar Current of the Southern Ocean, and the Indian and Pacific oceans. DOC concentrations decreased by 14 micromolar from the northern North Atlantic Ocean to the northern North Pacific Ocean, representing a 29% reduction in concentration. We evaluate the spatial patterns in terms of source/sink processes. Inputs of DOC to the deep ocean are identifiable in the mid-latitudes of the Southern Hemisphere, but the mechanisms have not been identified with certainty. Hansell and Carlson | NATURE | 395 | 17 SEPTEMBER 1998 Saturday, May 18, 2019
The addition of the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm POM Diatoms Dinoflagellates Plankton size Ciliates Nano phytoplankton HNF Bact Pico eukaryotic algae Dissolved nutrients (NH4) Synechococcus Prochlorococcus DOM DOM Saturday, May 18, 2019
The addition of the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm POM Diatoms Dinoflagellates Plankton size Ciliates Nano phytoplankton HNF Bact Pico eukaryotic algae NH4 Synechococcus Prochlorococcus NO3 DOM DOM Saturday, May 18, 2019
The addition of the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm POM Diatoms Dinoflagellates Plankton size Ciliates Nano phytoplankton HNF Bact Pico eukaryotic algae Dissolved nutrients Synechococcus Prochlorococcus Dissolved nutrients DOM DOM Saturday, May 18, 2019
The addition of the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm POM Diatoms Dinoflagellates Plankton size Ciliates Nano phytoplankton HNF Bact Pico eukaryotic algae NH4 Synechococcus Prochlorococcus NO3 DOM DOM Saturday, May 18, 2019
Bacterial vs Phytoplankton Productivity Development of epi-fluorescence reveals a large number of bacteria (106 ml-1) Development of bacterial productivity assay shows that large fraction of NPP is processed by bacteria (50%?). NPP From: Cole et al. MEPS (1988). P CO2 BP B DOM ? There is a general trend for increasing bacterial numbers and biomass with increasing primary productivity (Azam et al. 1983) Saturday, May 18, 2019
Typical Efficiencies: Link or Sink? Uptake= Production + Respiration Efficiency U P B = P U Typical Efficiencies: 0.1 0.6 R How much bacterial C makes it to zooplankton via the microbial loop? NPP P CO2 Z(zooplankton) A significant amount of research focuses on measuring growth efficiencies and feeding rates 100 100 B nF C DOM Z Assumed efficiencies: BnF nFC CZ 10% 10 1 0.1 50% 50 25 12.5 Saturday, May 18, 2019
Virus infections can terminate blooms and derive species succession Bratbak et al. 1998 MEPS 93: 39-48 Nature Cultures virus control control virus control virus control Algae virus control virus virus Saturday, May 18, 2019
Viral loop added to the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm ? Plankton size Dinoflagellates Ciliates Diatoms HNF Nano phytoplankton Pico eukaryotic algae DIN Bact Synechococcus Prochlorococcus Virus DOM Saturday, May 18, 2019
Viral loop added to the microbial loop 20 nm 2 µm 0.2 µm 20 µm 200 µm 2mm 2 cm POM Diatoms Dinoflagellates Plankton size Ciliates Nano phytoplankton HNF Bact Pico eukaryotic algae Dissolved nutrients Synechococcus Prochlorococcus Virus DOM DOM Saturday, May 18, 2019
The dominance of the microbial loop depends on: Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, The dominance of the microbial loop depends on: Rate on “New” production (external nutrients input) Size of primary producers Rate that DOC is produced Rate that bacteria convert DOC into biomass Rate of bacterial grazing Saturday, May 18, 2019
Virus short-circuit the microbial loop Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Virus short-circuit the microbial loop Enhance phytoplankton growth rate Enhance phytoplankton photosynthetic efficiency Lysis release large quantities of DOM Fig. 1. Carbon flow experiment. (a) Carbon flow and population dynamics in 2 parallel cultures of Phaeocystis pouchetii infected with PpVOl virus at Day 8. (b) Carbon flow and populatlon dynamlcs In 2 parallel non-infected (control) cultures. A background DOC concentration in the medium of 2.65 mgC 1.' was subtracted from all DOC values to focus on net changes. Dotted vertical line marks the time of virus addition Bratbak et al. 1998 AME 16: 11-16 Saturday, May 18, 2019
Boost primary production Viruses summary 106-109 per ml 0.002-0.2 micron Responsible for ~20% of bacterial mortality Reduce algal production by up to 78% (in cultures) Increase respiration & remineralization in the food web 106-109 per ml 0.002-0.2 micron Responsible for ~20% of bacterial mortality Reduce algal production by up to 78% (in cultures) Increase respiration & remineralization in the food web Terminate some phytoplankton blooms Drive species successions Enhance microbial diversity Mediate transduction Change bacterial phenotype through lysogenic conversion Boost primary production Nutrients Saturday, May 18, 2019
Boost primary production Viruses summary 106-109 per ml 0.002-0.2 micron Responsible for ~20% of bacterial mortality Reduce algal production by up to 78% (in cultures) Increase respiration & remineralization in the food web Viruses cause cell Iysis and divert the particulate production of their hosts into dissolved organic material. They do not add any new processes or connections to the food web, but they may change the relative importance of particulate and dissolved production 106-109 per ml 0.002-0.2 micron Responsible for ~20% of bacterial mortality Reduce algal production by up to 78% (in cultures) Increase respiration & remineralization in the food web Terminate some phytoplankton blooms Drive species successions Enhance microbial diversity Mediate transduction Change bacterial phenotype through lysogenic conversion Boost primary production Nutrients Saturday, May 18, 2019
“Modern” Observations Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, “Modern” Observations Biomass is distributed as a power law with respect to size (i.e. there are many more small cells than large) Prokaryotes are present in large concentrations Small phytoplankton (<20μm) account for >90% of primary production Dissolved organic matter (DOM) is present in significant concentrations and released by phytoplankton and zooplankton Microbes are responsible for a significant fraction (>50%) of respiration Viruses tweak the loop in many subtle ways (e.g., enhancing host productivity) but they normally boost it. (compiled by Pomeroy, 1974) Saturday, May 18, 2019
Modern view of oceanic food web (simplified) Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Modern view of oceanic food web (simplified) Major fluxes of carbon and energy are delineated by continuous lines; Fluxes usually of lesser magnitude are delineated by broken lines. Mucus-net feeders (salps and other microphages) are separated from other mesozooplankton because of their different feeding mode. Other than the mesozooplankton (including mucus-net feeders) and fishes (all blue boxes), the boxes represent organisms that are a part of the microbial loop (green = photosynthetic and yellow = heterotrophic) Pomeroy et al. 2007 The Microbial Loop Major fluxes of energy and carbon Minor fluxes of energy and carbon Major fluxes of inorganic N and P Saturday, May 18, 2019
High bacteriovory by picophytoplankton – yet another twist Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, High bacteriovory by picophytoplankton – yet another twist Planktonic algae <5 m in size are major fixers of inorganic carbon in the ocean1. They dominate phytoplankton biomass in post-bloom, stratified oceanic temperate waters2. Traditionally, large and small algae are viewed as having a critical growth dependence on inorganic nutrients, which the latter can better acquire at lower ambient concentrations owing to their higher surface area to volume ratios3, 4. Nonetheless, recent phosphate tracer experiments in the oligotrophic ocean5 have suggested that small algae obtain inorganic phosphate indirectly, possibly through feeding on bacterioplankton. There have been numerous microscopy-based studies of algae feeding mixotrophically6, 7 in the laboratory8, 9, 10 and field11, 12, 13, 14, as well as mathematical modelling of the ecological importance of mixotrophy15. However, because of methodological limitations16 there has not been a direct comparison of obligate heterotrophic and mixotrophic bacterivory. Here we present direct evidence that small algae carry out 40–95% of the bacterivory in the euphotic layer of the temperate North Atlantic Ocean in summer. A similar range of 37–70% was determined in the surface waters of the tropical North-East Atlantic Ocean, suggesting the global significance of mixotrophy. This finding reveals that even the smallest algae have less dependence on dissolved inorganic nutrients than previously thought, obtaining a quarter of their biomass from bacterivory. This has important implications for how we perceive nutrient acquisition and limitation of carbon-fixing protists as well as control of bacterioplankton in the ocean. Saturday, May 18, 2019
Importance of mixotrophic bacterivory Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Importance of mixotrophic bacterivory classic (misrepresentation) Revaluated Fig 1. Schematic showing the classic misrepresentation of the functional classification of planktonic protists as contributors to primary production (on the right) or to secondary production (on the left). Only some dinoflagellates, forams, radiolaria and acantheria are accorded a mixotrophic status, and these are down-played (usually ignored) in models. Fig. 2. Schematic of the functional classification of planktonic protists as contributors to primary production (on the right) and/or to secondary production (on the left). Note that in contrast to the classic misrepresentation (Fig. 1), the potential for individual organisms to contribute both to primary and secondary production is now acknowledged. References in support of this description are as follows: Acantharia (Caron et al., 1995; Stoecker et al., 1996); Chrysophyceae (Kristiansen, 2005) with mixotrophy common in Chrysophyceae sensu stricto, i.e. excluding the photolithophic Synurophyceae (Bhatti and Colman, 2008); Ciliates (Bernard and Rassoulzadegan, 1994; Stoecker et al., 1996, 2009; Perez et al., 1997; Esteban et al., 2010); Cryptophyta (Laybourn-Parry et al., 2005; Callieri et al., 2006). Dinophyta (Callieri et al., 2006; Park et al., 2010; Jeong et al., 2010b; Hansen, 2011; Minnhagen et al., 2011); Foraminifera (Caron et al., 1995; Stoecker et al., 1996, 2009); Pavlovophyceae (Callieri et al., 2006); Prasinophyceae (Bell and Laybourn-Parry, 2003), though mixotrophy appears rare in the Prasinophyceae; Prymnesiophyceae (Hansen and Hjorth, 2002; Carvalho and Granéli, 2010; Granéli et al., 2012Rokitta et al., 2011), though mixotrophy is very rare in coccolithophores, occurring only in some non-calcified species, it is common in the sister class Pavlovophyceae); Radiolaria (Caron et al., 1995). Also, not shown in the figure: mixotrophy occurs in the Chlorarachniophyta (Calderon-Saenz and Schnetter, 1989) and Raphidophyceae (Jeong et al., 2010a; Jeong, 2011). For an overview of the phylogenetic occurrence of mixotrophy, see Table II of Raven et al. (Raven et al., 2009). Saturday, May 18, 2019
High bacteriovory by picophytoplankton – yet another twist Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, High bacteriovory by picophytoplankton – yet another twist Planktonic algae <5 m in size are major fixers of inorganic carbon in the ocean1. They dominate phytoplankton biomass in post-bloom, stratified oceanic temperate waters2. Traditionally, large and small algae are viewed as having a critical growth dependence on inorganic nutrients, which the latter can better acquire at lower ambient concentrations owing to their higher surface area to volume ratios3, 4. Nonetheless, recent phosphate tracer experiments in the oligotrophic ocean5 have suggested that small algae obtain inorganic phosphate indirectly, possibly through feeding on bacterioplankton. There have been numerous microscopy-based studies of algae feeding mixotrophically6, 7 in the laboratory8, 9, 10 and field11, 12, 13, 14, as well as mathematical modelling of the ecological importance of mixotrophy15. However, because of methodological limitations16 there has not been a direct comparison of obligate heterotrophic and mixotrophic bacterivory. Here we present direct evidence that small algae carry out 40–95% of the bacterivory in the euphotic layer of the temperate North Atlantic Ocean in summer. A similar range of 37–70% was determined in the surface waters of the tropical North-East Atlantic Ocean, suggesting the global significance of mixotrophy. This finding reveals that even the smallest algae have less dependence on dissolved inorganic nutrients than previously thought, obtaining a quarter of their biomass from bacterivory. This has important implications for how we perceive nutrient acquisition and limitation of carbon-fixing protists as well as control of bacterioplankton in the ocean. Pomeroy et al. 2007 Major fluxes of energy and carbon Minor fluxes of energy and carbon Saturday, May 18, 2019
Gitai Yahel (Yahel@Ruppin. ac Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Hot spots of microbial activity are believed to facilitate the observed rates of the microbial community metabolism Rassoulzadegan, Fereidoun (Lead Author); Jean-Pierre Gattuso (Topic Editor). 2007. "Marine microbial loop." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [Published in the Encyclopedia of Earth June 4, 2007; Retrieved May 21, 2008]. <http://www.eoearth.org/article/Marine_microbial_loop> The Encyclopedia of Earth 2007 Saturday, May 18, 2019
Food webs and trophic structure of the pelagic community in cold temperate waters 6 5 4 3 2 1 Monday, 26-Feb-2015
1 2 3 4 5 6 7 10 9 8 7 6 5 Ciliates 4 1 3 2 Food webs and trophic structure of the pelagic community tropical waters 1 2 3 4 5 6 7 10 9 8 7 6 5 Ciliates 4 1 3 Monday, 26-Feb-2015 2
How can we characterize food webs? Dominant taxa Complexity Number of links Number of “levels” (and degree of isolation) Influence of indirect interactions Productivity/biomass at base Rate of flow of energy/mass Degree of fluctuation (seasonal, annual, decadal scales of time) Resilience (recovery from disturbance) Degree of isolation/openness (spatial scale) Monday, 26-Feb-2015
Importance of the Microbial Food web Prokaryotes are the dominant consumers of DOM. No other organisms compete effectively for it (in pelagic habitats) Prokaryote respiration is the major biogeochemical loss term for oceanic DOM Most bacteria, even in coastal waters, are free living, and thus can be assumed to subsist on DOM. Microbial communities in the sea include a full suite of ecological interactions among organisms less than 5μm: Primary producers, Herbivores, carnivores, parasites Mixotrophs, scavengers, remineralizers The principal effect of microbial food webs is nutrient regeneration: “Bacteria” incorporate organic matter Then serve as food for bacteriovores it is the bacteriovores that regenerate the nutrients. Bacteriovores, mostly mixotrophs and protozoans, are important food sources for meso-zooplankton, few or none of which are strict herbivores Saturday, May 18, 2019
בהצלחה בבחינות! Saturday, May 18, 2019
Saturday, May 18, 2019
Modern view of Marine Food Web (Azam 1998) Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Modern view of Marine Food Web (Azam 1998) Saturday, May 18, 2019
Mass balance and energy transfer Gitai Yahel (Yahel@Ruppin.ac.il) The School of Marine Sciences and Marine Environment Ruppin Academic Center, Mass balance and energy transfer Saturday, May 18, 2019
Techniques for measuring bacterial grazing rates Metabolic inhibitors Use an inhibitor (i.e., antibiotic) specific for eukaryotes. Measure increase in bacterial numbers in the presence and absence of inhibitor. Size fractionation Filter predator out of sample, and measure bacterial growth. Dilution method Measure bacterial growth rates at several sample dilutions Radio-labeled bacteria Feed predators radio-labeled bacterial Fluorescently label particles Feed predators fluorescently labeled particles or bacteria.
Metabolic inhibitors and size fractionation Metabolic Inhibitor Method Size Fractionation Method Unfiltered seawater Eukaryote inhibitor Unfiltered seawater 0.6 mm Filter A B A B Measure bacterial number increases in treatment A’s compared to treatment B’s. Problems: Filtration can cause cell lysis. Inhibitors may not be perfectly selective, and may be consumed by bacteria. Cannot look at species-level grazing. Incubation time is long. Saturday, May 18, 2019
Theoretical bacterial specific growth (no grazers) Dilution Method Dilute with 0.2 mm filtered sea water Measure bacterial concentrations at t0, and again at a later time (one or more days). Calculate apparent bacterial specific growth rate for each experiment 100% Unfiltered SW 80% Unfiltered SW 60% Unfiltered SW Plot m% versus fraction unfiltered water: Theoretical bacterial specific growth (no grazers) 40% Unfiltered SW 20% Unfiltered SW Incubate in situ or at simulated in situ conditions -Slope: grazing mortality Problems: Dilution alters system. Cannot look at species-level grazing. Incubation time is long. Saturday, May 18, 2019
Grazing Rate from Dilution Cultures B C G R Mass balance around bacteria: B Uptake, U, minus respiration, R, equals production, or: Bacterial consumption by grazers, C, could be approximated by: So that Dividing by B gives: where is the apparent bacterial specific growth rate. If we assume that the specific grazing rate depends linearly on G, then where f is the dilution fraction and is the unfiltered specific grazing rate. Saturday, May 18, 2019
Fluorescently or Radiolabeled bacteria or particles Water samples Added FLP or RLP at <20-50% of natural bacterial abundance At specific times, take sample and preserver. Filter sample on 0.8 mm filter to remove unconsumed particles. Either microscopically count abundance of ingested particles per specific group of protozoa, or measure radioactivity. Accounting for bacterial abundance relative to added particles, calculate total number of bacteria consumed per protozoan per unit time. Can also calculate total bacterial removal rate. Advantages: Can obtain species specific grazing rates Short incubation times. Problems: Predators may discriminate against particles. Saturday, May 18, 2019