Microb Ecol (1994) 28:295-298
Controls of the Microbial Loop: Nutrient Limitations
MICROBIAL ECOLOGYInc. © 1994Springer-Verlag New York
Inorganic Nutrients, Bacteria, and the Microbial Loop D.A. Caron Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
Abstract. The realization that natural assemblages of planktonic bacteria may acquire a significant fraction of their nitrogen and phosphorus via the uptake of dissolved inorganic nutrients has modified our traditional view of these microorganisms as nutrient remineralizers in plankton communities. Bacterial uptake of inorganic nitrogen and phosphorus may place bacteria and phytoplankton in competition for growth-limiting nutrients, rather than in their traditional roles as the respective "source" and "sink" for these nutrients in the plankton. Bacterial nutrient uptake also implies that bacterivorous protozoa may play a pivotal role in the remineralization of these elements in the microbial loop. The overall contribution of bacterial utilization of inorganic nutrients to total nutrient uptake in the ocean is still poorly understood, but some generalizations are emerging with respect to the geographical areas and community physiological conditions that might elicit this behavior. As pointed out clearly by Kirchman [9], there now is strong evidence that heterotrophic unicellular organisms (heterotrophic bacteria and phagatrophic protists) are responsible for the bulk of the nutrient remineralization in many planktonic ecosystems. This conclusion can be argued convincingly on an allometric basis [7], and it is supported by the results of a variety of field and laboratory studies. The details of this process, however, are still somewhat unclear. In particular, the traditional role of bacteria as remineralizers of nitrogen and phosphorus contained in dissolved and particulate detrital material may not hold true in many natural communities. Numerous investigators have demonstrated the uptake of dissolved inorganic nutrients by planktonic bacterial assemblages. Based on recent evidence, it now appears certain that the uptake of dissolved inorganic nutrients by natural assemblages of bacteria is much more common than has been believed previously (references in [9]). Moreover, experimental studies have demonstrated that bacteria can compete successfully for these materials [6]. I concur with Kirchman that the evidence for net bacterial uptake in numerous, natural planktonic communities is strong. The competition of bacteria with phytoplankton in some environmental situations for dissolved inorganic nutrients raises at least four interesting questions: (1) What controls the contribution of bacteria to nutrient remineralization/uptake in the plankton? (2) If bacteria are utilizing rather than releasing dissolved inorganic nutrients in the plankton, then what organisms are responsible for most of the
296
D.A. Caron
remineralization? (3) Can we define the environmental situations that determine nutrient uptake by bacteria in nature? (4) Could bacterial growth rates in natural communities ever be limited by nutrient availability rather than the traditional view of carbon (energy) limitation? Kirchman [9] has provided insights into the answers to these questions that are based on recent experimental studies. From a mechanistic perspective, the firstorder consideration for determining whether bacteria are taking up or remineralizing nitrogen and phosphorus contained in their substrate(s) is dependent on the stoichiometric ratios of the elements in the bacteria and the substrates that they are using for growth, and the growth efficiencies of the bacteria. While physiological studies of cultured bacteria have provided information on bacterial elemental composition and growth, still relatively little is known about the overall chemical composition of the natural substrates used by planktonic bacteria. Some recent work has indicated, however, that the C:N ratio of the utilizable fraction of dissolved organic matter in some marine environments may be high enough to promote nitrogen uptake by the bacteria [1]. These observations are suggestive that net transport of dissolved inorganic nitrogen may be into, rather than out of, bacterial cells. Some NH4 ÷ release may still take place even when net transport of NH4 + is into the bacteria in an assemblage, but this behavior may simply indicate the utilization of different chemical constituents of the organic pool by different species of bacteria. If these different constituents possess different C:N:P ratios, then one might expect some bacteria in an assemblage to be remineralizing nitrogen while other bacterial species are taking it up. If bacteria are net consumers of dissolved inorganic nutrients in the plankton, then it is clear that some other heterotrophic group(s) must produce these compounds. Based on an analysis of the weight-specific metabolic rates and the elemental compositions of the major groups of microorganisms, it has been argued that small protozoa are likely candidates for playing an important role in nutrient regeneration in natural plankton assemblages [3]. Bacterivorous protozoa, in particular, may play a pivotal role in nutrient cycles by consuming nitrogen-rich and phosphorus-rich bacterial biomass, and by releasing phytoplankton from competition with the bacteria for limiting inorganic nutrient [2, 4, 8]. Once again, I concur with Kirchman that the weight of the evidence at this time seems to favor small protists as important agents in the remineralization of nitrogen and phosphorus in the plankton. Kirchman [9] has proposed a plausible hypothesis that indicates the conditions that might lead to bacterial utilization of dissolved inorganic nutrients along an environmental gradient from estuaries to the open ocean (Fig. 3 in [9]). He hypothesizes that the organic compounds used as substrates for bacterial growth in highly oligotrophic environments (such as the open ocean) have predominantly high C:N ratios. This predominance is a consequence of nutrient limitation of the phytoplankton community in oligotrophic environments, which in turn leads to the production of organic compounds with high C:N ratios (e.g., storage carbohydrates). The utilization by the bacteria of nitrogen-poor organic compounds results in an increased reliance on inorganic nitrogen for bacterial growth demands. By comparison, organic material produced by algae in eutrophic coastal environments (with higher nutrient concentrations) would be expected to have C:N ratios that are relatively low, thereby meeting a larger percentage of the nitrogen growth demand of the bacteria utilizing these compounds.
Nutrients in the Microbial Loop
297
One can propose a scenario similar to the estuarine-oceanic gradient along a vertical profile in any vertically-stratified water column. Nutrient-limited phytoplankton in surface waters could produce predominantly high C:N and high C:P organic compounds which, when eventually utilized by the bacteria, result in competition by the bacteria for limiting nitrogen or phosphorus. In contrast, light limitation (and nutrient sufficiency) in the deeper euphotic zone of a stratified water column could result in the production of organic compounds with relatively low C:N or C:P ratios. Bacteria utilizing the organic material in this latter situation may act as net nutrient remineralizers. Based on this reasoning, one could envision the possibility of the significant uptake of dissolved inorganic nutrients by bacteria in the surface waters of estuaries during periods of strong seasonal stratification. The experimental data of Suttle et al. [10] and Wheeler and Kirchman [11] tend to support this possibility. Expanding on the concept proposed by Kirchman [9], one can generalize the interactions involving bacterial nutrient uptake or release, and the factors affecting this process in oligotrophic and eutrophic environments. As described above, severe nutrient limitation could result in the production (and eventual release) of organic material with high C:N and C:P ratios. Subsequent utilization of these materials by planktonic bacteria would create an intracellular nitrogen (and/or phosphorus) demand, thereby stimulating uptake of dissolved inorganic nutrients by the bacteria. If the standing stock of bacteria was capable of maintaining the phytoplankton in a state of limitation, then the production of nutrient-deplete organic compounds by the phytoplankton might continue to promote this situation. Under these conditions, it seems plausible that labile, nutrient-deplete organic compounds could accumulate in the water, and that the short-term bacteria growth rates could be limited by nitrogen or phosphorus. This hypothesis is contrary to the conclusion of Kirchman [9] that bacteria are capable of acquiring sufficient nitrogen and phosphorus to meet all their growth requirements, and that bacterial growth rates are limited by organic carbon (energy) availability even in situations where they are taking up dissolved inorganic nutrients. As Kirchman notes, total bacterial productivity in the plankton is ultimately limited by the amount of organic carbon available to bacteria (ignoring the potentially confounding effects of lability and temperature) [5]. This fact, however, does not dictate the rate at which this production takes place. The instantaneous growth rate is determined by the rate of supply of a limiting substance (which may or may not be organic carbon) relative to the size of the bacterial standing stock. One must take into account the biomass of the bacteria relative to the supply rates of nitrogen, phosphorus, and organic carbon. The question of whether or not bacterial growth rates are ever limited by nitrogen or phosphorus can be addressed by establishing whether or not significant quantities of labile, nutrient-deplete organic compounds exist in natural waters, or by determining if adding nitrogen and/or phosphorus will stimulate bacterial growth rates (i.e., demonstrating N or P limitation). Unfortunately, both topics are technically difficult to investigate. The addition of inorganic nitrogen and phosphorus to natural water samples often stimulates bacterial growth, but it is always difficult to identify artifacts due to incubation in bottles, or a "cascade effect" caused by the release of the phytoplankton from severe nutrient stress and their subsequent productivity.
298
D.A. Caron
Technical approaches to addressing this question must be developed, however, because the consequences of whether or not nitrogen or phosphorus can limit bacterial growth rates in nature are substantial. Competition for, and removal of, dissolved inorganic nutrients by the bacteria would maintain the phytoplankton assemblage in a physiological state where a significant fraction of the total primary production might be channeled into nutrient-deplete organic compounds. If the bacteria compete successfully with the phytoplankton and other organisms for the growth-limiting nutrient in this situation then, in essence, much of the primary production is funneled directly into the microbial loop. In high-light, nutrient-poor environments such as the surface waters of subtropical oceanic gyres, this scenario seems particularly plausible. If this situation does exist in oligotrophic oceanic environments, then the presence and grazing activities of the bacterivorous protozoa may be pivotal in nutrient cycling in these ecosystems, and in determining the partitioning of nitrogen and phosphorus between the bacteria and phytoplankton. References 1. Benner R, Pakulski JD, McCarthy M, Hedges JI, Hatcher PG (1992) Bulk chemical characteristics of dissolved organic matter in the ocean. Science 255:1561-1564 2. Bratbak G, Thingstad TF (1985) Phytoplankton-bacteria interactions: an apparent paradox? Analysis of a model system with both competition and commensalism. Mar Ecol Prog Ser 25:23-30 3. Caron DA (1991) Evolving role of protozoa in aquatic nutrient cycles. In: Reid PC, Turley CM. Burkill PH (ed) Protozoa and their role in marine processes, vol 25. Springer-Verlag, Berlin, pp 387--415 4. Caron DA, Goldman JC, Dennett MR (1988) Experimental demonstration of the roles of bacteria and bacteriovorous protozoa in plankton nutrient cycles. Hydrobiologia 159:27-40 5. Cole JJ, Findlay S, Pace ML (1988) Bacterial production in fresh and saltwater ecosystems: a cross-system overview. Mar Ecol Prog Ser 43:1-10 6. Currie DJ, Kalff J (1984) A comparison of the abilities of freshwater algae and bacteria to acquire and retain phosphorus. Limnol Oceanogr 29:298-310 7. Fenchel T, Finlay BJ (1983) Respiration rates in heterotrophic, free-living protozoa. Microb Ecol 9:99-122 8. Jfirgens K, Gfide H (1990) Incorporation and release of phosphorus by planktonic bacteria and phagotrophic flagellates. Mar Ecol Prog Ser 59:271-284 9. Kirchman D (1994) The uptake of inorganic nutrients by heterotrophic bacteria. Microb Ecol 28:255-271 10. Suttle CA, Fuhrman JA, Capone DG (1990) Rapid ammonium cycling and concentration-dependent partitioning of ammonium and phosphate: implications for carbon transfer in planktonic communities. Limnol Oceanogr 35:424-433 11. Wheeler PA, Kirchman DL (1986) Utilization of inorganic and organic nitrogen by bacteria in marine systems. Limnol Oceanogr 31:998-1009
Controls of the Microbial Loop: Nutrient Limitations
MICROBIAL ECOLOGYInc. © 1994Springer-Verlag New York
Inorganic Nutrients, Bacteria, and the Microbial Loop D.A. Caron Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
Abstract. The realization that natural assemblages of planktonic bacteria may acquire a significant fraction of their nitrogen and phosphorus via the uptake of dissolved inorganic nutrients has modified our traditional view of these microorganisms as nutrient remineralizers in plankton communities. Bacterial uptake of inorganic nitrogen and phosphorus may place bacteria and phytoplankton in competition for growth-limiting nutrients, rather than in their traditional roles as the respective "source" and "sink" for these nutrients in the plankton. Bacterial nutrient uptake also implies that bacterivorous protozoa may play a pivotal role in the remineralization of these elements in the microbial loop. The overall contribution of bacterial utilization of inorganic nutrients to total nutrient uptake in the ocean is still poorly understood, but some generalizations are emerging with respect to the geographical areas and community physiological conditions that might elicit this behavior. As pointed out clearly by Kirchman [9], there now is strong evidence that heterotrophic unicellular organisms (heterotrophic bacteria and phagatrophic protists) are responsible for the bulk of the nutrient remineralization in many planktonic ecosystems. This conclusion can be argued convincingly on an allometric basis [7], and it is supported by the results of a variety of field and laboratory studies. The details of this process, however, are still somewhat unclear. In particular, the traditional role of bacteria as remineralizers of nitrogen and phosphorus contained in dissolved and particulate detrital material may not hold true in many natural communities. Numerous investigators have demonstrated the uptake of dissolved inorganic nutrients by planktonic bacterial assemblages. Based on recent evidence, it now appears certain that the uptake of dissolved inorganic nutrients by natural assemblages of bacteria is much more common than has been believed previously (references in [9]). Moreover, experimental studies have demonstrated that bacteria can compete successfully for these materials [6]. I concur with Kirchman that the evidence for net bacterial uptake in numerous, natural planktonic communities is strong. The competition of bacteria with phytoplankton in some environmental situations for dissolved inorganic nutrients raises at least four interesting questions: (1) What controls the contribution of bacteria to nutrient remineralization/uptake in the plankton? (2) If bacteria are utilizing rather than releasing dissolved inorganic nutrients in the plankton, then what organisms are responsible for most of the
296
D.A. Caron
remineralization? (3) Can we define the environmental situations that determine nutrient uptake by bacteria in nature? (4) Could bacterial growth rates in natural communities ever be limited by nutrient availability rather than the traditional view of carbon (energy) limitation? Kirchman [9] has provided insights into the answers to these questions that are based on recent experimental studies. From a mechanistic perspective, the firstorder consideration for determining whether bacteria are taking up or remineralizing nitrogen and phosphorus contained in their substrate(s) is dependent on the stoichiometric ratios of the elements in the bacteria and the substrates that they are using for growth, and the growth efficiencies of the bacteria. While physiological studies of cultured bacteria have provided information on bacterial elemental composition and growth, still relatively little is known about the overall chemical composition of the natural substrates used by planktonic bacteria. Some recent work has indicated, however, that the C:N ratio of the utilizable fraction of dissolved organic matter in some marine environments may be high enough to promote nitrogen uptake by the bacteria [1]. These observations are suggestive that net transport of dissolved inorganic nitrogen may be into, rather than out of, bacterial cells. Some NH4 ÷ release may still take place even when net transport of NH4 + is into the bacteria in an assemblage, but this behavior may simply indicate the utilization of different chemical constituents of the organic pool by different species of bacteria. If these different constituents possess different C:N:P ratios, then one might expect some bacteria in an assemblage to be remineralizing nitrogen while other bacterial species are taking it up. If bacteria are net consumers of dissolved inorganic nutrients in the plankton, then it is clear that some other heterotrophic group(s) must produce these compounds. Based on an analysis of the weight-specific metabolic rates and the elemental compositions of the major groups of microorganisms, it has been argued that small protozoa are likely candidates for playing an important role in nutrient regeneration in natural plankton assemblages [3]. Bacterivorous protozoa, in particular, may play a pivotal role in nutrient cycles by consuming nitrogen-rich and phosphorus-rich bacterial biomass, and by releasing phytoplankton from competition with the bacteria for limiting inorganic nutrient [2, 4, 8]. Once again, I concur with Kirchman that the weight of the evidence at this time seems to favor small protists as important agents in the remineralization of nitrogen and phosphorus in the plankton. Kirchman [9] has proposed a plausible hypothesis that indicates the conditions that might lead to bacterial utilization of dissolved inorganic nutrients along an environmental gradient from estuaries to the open ocean (Fig. 3 in [9]). He hypothesizes that the organic compounds used as substrates for bacterial growth in highly oligotrophic environments (such as the open ocean) have predominantly high C:N ratios. This predominance is a consequence of nutrient limitation of the phytoplankton community in oligotrophic environments, which in turn leads to the production of organic compounds with high C:N ratios (e.g., storage carbohydrates). The utilization by the bacteria of nitrogen-poor organic compounds results in an increased reliance on inorganic nitrogen for bacterial growth demands. By comparison, organic material produced by algae in eutrophic coastal environments (with higher nutrient concentrations) would be expected to have C:N ratios that are relatively low, thereby meeting a larger percentage of the nitrogen growth demand of the bacteria utilizing these compounds.
Nutrients in the Microbial Loop
297
One can propose a scenario similar to the estuarine-oceanic gradient along a vertical profile in any vertically-stratified water column. Nutrient-limited phytoplankton in surface waters could produce predominantly high C:N and high C:P organic compounds which, when eventually utilized by the bacteria, result in competition by the bacteria for limiting nitrogen or phosphorus. In contrast, light limitation (and nutrient sufficiency) in the deeper euphotic zone of a stratified water column could result in the production of organic compounds with relatively low C:N or C:P ratios. Bacteria utilizing the organic material in this latter situation may act as net nutrient remineralizers. Based on this reasoning, one could envision the possibility of the significant uptake of dissolved inorganic nutrients by bacteria in the surface waters of estuaries during periods of strong seasonal stratification. The experimental data of Suttle et al. [10] and Wheeler and Kirchman [11] tend to support this possibility. Expanding on the concept proposed by Kirchman [9], one can generalize the interactions involving bacterial nutrient uptake or release, and the factors affecting this process in oligotrophic and eutrophic environments. As described above, severe nutrient limitation could result in the production (and eventual release) of organic material with high C:N and C:P ratios. Subsequent utilization of these materials by planktonic bacteria would create an intracellular nitrogen (and/or phosphorus) demand, thereby stimulating uptake of dissolved inorganic nutrients by the bacteria. If the standing stock of bacteria was capable of maintaining the phytoplankton in a state of limitation, then the production of nutrient-deplete organic compounds by the phytoplankton might continue to promote this situation. Under these conditions, it seems plausible that labile, nutrient-deplete organic compounds could accumulate in the water, and that the short-term bacteria growth rates could be limited by nitrogen or phosphorus. This hypothesis is contrary to the conclusion of Kirchman [9] that bacteria are capable of acquiring sufficient nitrogen and phosphorus to meet all their growth requirements, and that bacterial growth rates are limited by organic carbon (energy) availability even in situations where they are taking up dissolved inorganic nutrients. As Kirchman notes, total bacterial productivity in the plankton is ultimately limited by the amount of organic carbon available to bacteria (ignoring the potentially confounding effects of lability and temperature) [5]. This fact, however, does not dictate the rate at which this production takes place. The instantaneous growth rate is determined by the rate of supply of a limiting substance (which may or may not be organic carbon) relative to the size of the bacterial standing stock. One must take into account the biomass of the bacteria relative to the supply rates of nitrogen, phosphorus, and organic carbon. The question of whether or not bacterial growth rates are ever limited by nitrogen or phosphorus can be addressed by establishing whether or not significant quantities of labile, nutrient-deplete organic compounds exist in natural waters, or by determining if adding nitrogen and/or phosphorus will stimulate bacterial growth rates (i.e., demonstrating N or P limitation). Unfortunately, both topics are technically difficult to investigate. The addition of inorganic nitrogen and phosphorus to natural water samples often stimulates bacterial growth, but it is always difficult to identify artifacts due to incubation in bottles, or a "cascade effect" caused by the release of the phytoplankton from severe nutrient stress and their subsequent productivity.
298
D.A. Caron
Technical approaches to addressing this question must be developed, however, because the consequences of whether or not nitrogen or phosphorus can limit bacterial growth rates in nature are substantial. Competition for, and removal of, dissolved inorganic nutrients by the bacteria would maintain the phytoplankton assemblage in a physiological state where a significant fraction of the total primary production might be channeled into nutrient-deplete organic compounds. If the bacteria compete successfully with the phytoplankton and other organisms for the growth-limiting nutrient in this situation then, in essence, much of the primary production is funneled directly into the microbial loop. In high-light, nutrient-poor environments such as the surface waters of subtropical oceanic gyres, this scenario seems particularly plausible. If this situation does exist in oligotrophic oceanic environments, then the presence and grazing activities of the bacterivorous protozoa may be pivotal in nutrient cycling in these ecosystems, and in determining the partitioning of nitrogen and phosphorus between the bacteria and phytoplankton. References 1. Benner R, Pakulski JD, McCarthy M, Hedges JI, Hatcher PG (1992) Bulk chemical characteristics of dissolved organic matter in the ocean. Science 255:1561-1564 2. Bratbak G, Thingstad TF (1985) Phytoplankton-bacteria interactions: an apparent paradox? Analysis of a model system with both competition and commensalism. Mar Ecol Prog Ser 25:23-30 3. Caron DA (1991) Evolving role of protozoa in aquatic nutrient cycles. In: Reid PC, Turley CM. Burkill PH (ed) Protozoa and their role in marine processes, vol 25. Springer-Verlag, Berlin, pp 387--415 4. Caron DA, Goldman JC, Dennett MR (1988) Experimental demonstration of the roles of bacteria and bacteriovorous protozoa in plankton nutrient cycles. Hydrobiologia 159:27-40 5. Cole JJ, Findlay S, Pace ML (1988) Bacterial production in fresh and saltwater ecosystems: a cross-system overview. Mar Ecol Prog Ser 43:1-10 6. Currie DJ, Kalff J (1984) A comparison of the abilities of freshwater algae and bacteria to acquire and retain phosphorus. Limnol Oceanogr 29:298-310 7. Fenchel T, Finlay BJ (1983) Respiration rates in heterotrophic, free-living protozoa. Microb Ecol 9:99-122 8. Jfirgens K, Gfide H (1990) Incorporation and release of phosphorus by planktonic bacteria and phagotrophic flagellates. Mar Ecol Prog Ser 59:271-284 9. Kirchman D (1994) The uptake of inorganic nutrients by heterotrophic bacteria. Microb Ecol 28:255-271 10. Suttle CA, Fuhrman JA, Capone DG (1990) Rapid ammonium cycling and concentration-dependent partitioning of ammonium and phosphate: implications for carbon transfer in planktonic communities. Limnol Oceanogr 35:424-433 11. Wheeler PA, Kirchman DL (1986) Utilization of inorganic and organic nitrogen by bacteria in marine systems. Limnol Oceanogr 31:998-1009
