Microb Ecol (1994) 28:299-301
Controls of the Microbial Loop: Nutrient Limitations
MICROBIAL ECOLOGYInc. © 1994Springer-Verlag New York
Limitation of Primary Production by Heterotrophic Assimilation and Transformation of Inorganic Nutrients N.P. Revsbech Department of Microbial Ecology, Institute of Biological Sciences, Bd. 540, University of Aarhus, DK-8000 Aarhus C, Denmark
Abstract. The planktonic environment is usually characterized by nonsteady state conditions with events of phytoplankton blooms and sedimentation. Inorganic nutrients are stripped from the water column by sedimentation and end up in the sediments where they may be permanently deposited, or nitrogen may be liberated as nitrogen gas by denitrification. A major part of the denitrification activity is a coupled process of nitrification and denitrification which is dependent on a good supply of oxygen to the sediment. Urea may constitute a major part of the total outflux of dissolved N compounds from the sediment. The apparent competition for low-molecular weight dissolved N-compounds between photosynthetic primary producers and heterotrophic bacteria is often a matter of definitions. The scientific community needed a couple of decades of intensive research on planktonic productivity before it became evident for most scientists involved that the ~4C-based methods used to determine primary production gave only a momentary (and often very inaccurate [14]) picture of the amount of dissolved inorganic carbon fixed at any particular time. The estimates obtained had limited relevance for those working with fisheries, etc., as a major part of the assimilated carbon could be remineralized within the photic zone without an interaction with any higher trophic level. "New primary production" therefore had to be invented as a term indicating how much was available for export from the photosynthetic community. In the same way there may also be a rapid recycling of N and P within the planktonic community, and as both autotrophs and heterotrophs assimilate N and P, it is easy to demonstrate an apparent short-term competition for these nutrients. However, as pointed out by Kirchman [6], this uptake of N and P by heterotrophs should not result in a limitation of primary production, as primary producers, by their carbon-fixation, determine the population size of the heterotrophs (i.e., the heterotrophs are energy limited). This argument may, however, not be valid under non-steady state conditions. The transient accumulation of nutrients in heterotrophs due to mineralization of a spring bloom may, for example, have a significant impact on nutrient availability. Also, the uptake of dissolved nutrients into a particulate pool does result in a downward export of nutrients away from the photic zone. The sinking rate of individual heterotrophic bacteria is negligible, but due to aggregation and formation of "marine snow" [1], sedimentation of nutrients
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in bacterial biomass may nevertheless be important. Heterotrophic binding of available nutrients and subsequent sedimentation may be particularly important in estuaries where sedimentation rates of flocculating humic materials can be very high. Once in the sediments, a large fraction of the ammonia liberated by mineralization may become nitrified and subsequently denitrified by the coupled process of nitrification-denitrification within the sediment [9]. Sedimentation of N-containing organic material may thus be the first step in a sequence of reactions where a significant fraction of the combined N is removed. When nitrogen is supplied to near-shore sediments as particulate organic nitrogen (PON), denitrification is most efficient when the overlying water is well oxygenated and when oxygen penetrates a few millimeters into the sediment [4]. Above a certain threshold of organic matter input to the sediment, the nitrifiers are out-competed by heterotrophic bacteria, and denitrification decreases due to reduced availability of NO3- [3]. The effect of the mechanisms outlined above is an initially moderate eutrophication of coastal areas by nutrient input, but above a certain threshold the self-cleaning capacity of the system (by denitrification) breaks down, and eutrophication increases rapidly. Photosynthetic activity by benthic microalgae seems to lower rates of coupled nitrification--denitrification in most environments, probably due to a competition for ammonia (L.P. Nielsen, pets. comm.), whereas the presence of a dense infaunal population with irrigated burrows stimulates the process [11]. The export of biologically available N from sediments is, however, often underestimated as most investigations focus only on ammonia and nitrate, although most of the export may actually be as DON [5], of which urea is particularly important [7]. Even when permanent deposition of N in deeper sediment layers is taken into account, the rate of denitrification can thus not be estimated by subtracting net rates of NO3- plus NH4 + outflux from the rate of PON input to the sediment. The nature of DON in seawater is largely unknown; quantified components of offshore seawater have shown that only about 25% of the DON could be accounted for, and it has been reported [10] that
Controls of the Microbial Loop: Nutrient Limitations
MICROBIAL ECOLOGYInc. © 1994Springer-Verlag New York
Limitation of Primary Production by Heterotrophic Assimilation and Transformation of Inorganic Nutrients N.P. Revsbech Department of Microbial Ecology, Institute of Biological Sciences, Bd. 540, University of Aarhus, DK-8000 Aarhus C, Denmark
Abstract. The planktonic environment is usually characterized by nonsteady state conditions with events of phytoplankton blooms and sedimentation. Inorganic nutrients are stripped from the water column by sedimentation and end up in the sediments where they may be permanently deposited, or nitrogen may be liberated as nitrogen gas by denitrification. A major part of the denitrification activity is a coupled process of nitrification and denitrification which is dependent on a good supply of oxygen to the sediment. Urea may constitute a major part of the total outflux of dissolved N compounds from the sediment. The apparent competition for low-molecular weight dissolved N-compounds between photosynthetic primary producers and heterotrophic bacteria is often a matter of definitions. The scientific community needed a couple of decades of intensive research on planktonic productivity before it became evident for most scientists involved that the ~4C-based methods used to determine primary production gave only a momentary (and often very inaccurate [14]) picture of the amount of dissolved inorganic carbon fixed at any particular time. The estimates obtained had limited relevance for those working with fisheries, etc., as a major part of the assimilated carbon could be remineralized within the photic zone without an interaction with any higher trophic level. "New primary production" therefore had to be invented as a term indicating how much was available for export from the photosynthetic community. In the same way there may also be a rapid recycling of N and P within the planktonic community, and as both autotrophs and heterotrophs assimilate N and P, it is easy to demonstrate an apparent short-term competition for these nutrients. However, as pointed out by Kirchman [6], this uptake of N and P by heterotrophs should not result in a limitation of primary production, as primary producers, by their carbon-fixation, determine the population size of the heterotrophs (i.e., the heterotrophs are energy limited). This argument may, however, not be valid under non-steady state conditions. The transient accumulation of nutrients in heterotrophs due to mineralization of a spring bloom may, for example, have a significant impact on nutrient availability. Also, the uptake of dissolved nutrients into a particulate pool does result in a downward export of nutrients away from the photic zone. The sinking rate of individual heterotrophic bacteria is negligible, but due to aggregation and formation of "marine snow" [1], sedimentation of nutrients
300
N.P. Revsbech
in bacterial biomass may nevertheless be important. Heterotrophic binding of available nutrients and subsequent sedimentation may be particularly important in estuaries where sedimentation rates of flocculating humic materials can be very high. Once in the sediments, a large fraction of the ammonia liberated by mineralization may become nitrified and subsequently denitrified by the coupled process of nitrification-denitrification within the sediment [9]. Sedimentation of N-containing organic material may thus be the first step in a sequence of reactions where a significant fraction of the combined N is removed. When nitrogen is supplied to near-shore sediments as particulate organic nitrogen (PON), denitrification is most efficient when the overlying water is well oxygenated and when oxygen penetrates a few millimeters into the sediment [4]. Above a certain threshold of organic matter input to the sediment, the nitrifiers are out-competed by heterotrophic bacteria, and denitrification decreases due to reduced availability of NO3- [3]. The effect of the mechanisms outlined above is an initially moderate eutrophication of coastal areas by nutrient input, but above a certain threshold the self-cleaning capacity of the system (by denitrification) breaks down, and eutrophication increases rapidly. Photosynthetic activity by benthic microalgae seems to lower rates of coupled nitrification--denitrification in most environments, probably due to a competition for ammonia (L.P. Nielsen, pets. comm.), whereas the presence of a dense infaunal population with irrigated burrows stimulates the process [11]. The export of biologically available N from sediments is, however, often underestimated as most investigations focus only on ammonia and nitrate, although most of the export may actually be as DON [5], of which urea is particularly important [7]. Even when permanent deposition of N in deeper sediment layers is taken into account, the rate of denitrification can thus not be estimated by subtracting net rates of NO3- plus NH4 + outflux from the rate of PON input to the sediment. The nature of DON in seawater is largely unknown; quantified components of offshore seawater have shown that only about 25% of the DON could be accounted for, and it has been reported [10] that
