Nutrient and Temperature Limitation of Bacterioplankton Growth in Temperate Lakes

Limitation of bacterioplankton production by nutrients and temperature was investigated in eight temperate lakes in summer. Six of the lakes were resampled in autumn. The lakes differ in nutrient content, water color, and concentration of dissolved organic carbon. Nutrients (phosphorus, nitrogen, and organic carbon) were added alone and in all possible combinations to filtered lake water inoculated with bacteria from the lake. After incubation for 36–40 h at in situ temperatures (ranging from 7 to 20°C), the response in bacterioplankton production was determined. The effect of increased temperature on bacterioplankton growth was also tested. Bacterioplankton production was often limited by phosphorus alone, organic carbon alone, or the two in combination. Phosphorus limitation of bacterioplankton production was more common in the summer, whereas limitation by organic carbon was more frequently observed in the autumn. There was a close balance between limitation by phosphorus and organic carbon in the epilimnion in the summer. In the hypolimnion in the summer, bacterioplankton growth was primarily phosphorus-limited. The effect of phosphorus additions decreased with increasing phosphorus concentrations in the lakes. However, there were no correlations between the effect of added organic carbon and water color, dissolved organic carbon concentration, or phosphorus concentration. When temperature was low (in the hypolimnion in the summer, and throughout the water column in the autumn) temperature also limited bacterioplankton production. Thus, temperature and inorganic nutrients or organic compounds can limit bacterioplankton growth both alone and simultaneously. However, at low temperatures, temperature is the most important factor influencing bacterioplankton growth.     

Nutrient limitation of bacterioplankton and phytoplankton in humic lakes in northern Sweden

1. Two small humic lakes in northern Sweden with concentrations of dissolved organic carbon (DOC) between 15 and 20 mg L^–1 were fertilized with inorganic phosphorus (P) and inorganic nitrogen (N), respectively. A third lake was unfertilized and served as a control. In addition to this lake fertilization experiment, data from different regional surveys were used to assess the role of different limiting factors.
2. The P fertilization had no effects on bacterioplankton or phytoplankton, while phytoplankton were significantly stimulated by N fertilization. Inorganic nutrient limitation of bacterioplankton was a function of DOC concentration in water of the investigated region and nutrient-limited bacteria were found only in lakes with DOC concentrations less than around 15 mg L^–1
3. The fertilization experiments demonstrated that the DOC-rich experimental lakes contained a bioavailable pool of P that was not utilized to its full potential under natural conditions. The overall mobilization of energy (bacterioplankton plus phytoplankton) in the experimental lakes was restricted by lack of inorganic N.     

Purple Sulfur Bacteria Control the Growth of Aerobic Heterotrophic Bacterioplankton in a Meromictic Salt Lake

In meromictic Mahoney Lake, British Columbia, Canada, the heterotrophic bacterial production in the mixolimnion exceeded concomitant primary production by a factor of 7. Bacterial growth rates were correlated neither to primary production nor to the amount of chlorophyll a. Both results indicate an uncoupling of bacteria and phytoplankton. In the chemocline of the lake, an extremely dense population of the purple sulfur bacterium Amoebobacter purpureus is present year round. We investigated whether anoxygenic phototrophs are significant for the growth of aerobic bacterioplankton in the overlaying water. Bacterial growth rates in the mixolimnion were limited by inorganic phosphorus or nitrogen most of the time, and the biomass of heterotrophic bacteria did not increase until, in autumn, 86% of the cells of A. purpureus appeared in the mixolimnion because of their reduced buoyant density. The increase in heterotrophic bacterial biomass, soluble phosphorus concentrations below the detection limit, and an extraordinarily high activity of alkaline phosphatase in the mixolimnion indicate a rapid liberation of organically bound phosphorus from A. purpureus cells accompanied by a simultaneous incorporation into heterotrophic bacterioplankton. High concentrations of allochthonously derived dissolved organic carbon (mean, 60 mg of C(middot)liter(sup-1)) were measured in the lake water. In Mahoney Lake, liberation of phosphorus from upwelling purple sulfur bacteria and degradation of allochthonous dissolved organic carbon as an additional carbon source render heterotrophic bacterial production largely independent of the photosynthesis of phytoplankton. A recycling of inorganic nutrients via phototrophic bacteria also appears to be relevant in other lakes with anoxic bottom waters.     

Nutrient control of bacterioplankton and phytoplankton dynamics

To determine whether positive correlations between phytoplankton and bacterioplankton growth in nutrient addition experiments are due to growth coupling or growth stimulation by the same nutrients, we examined phyto- and bacterioplankton growth in a series of eleven nutrient addition (N × P) and light/dark experiments. In mesotrophic Castle Lake, the phyto- and bacterioplankton growth responses to phosphorus (P) addition were strongly correlated (r²=0.59), while only a weak correlation (r²=0.10) was observed for the nitrogen addition treatments. After normalizing the N + P treatments for the growth stimulation observed in the respective P treatments, we found a substantial stimulation of the phytoplankton (e.g., costimulation by N + P) and no stimulation of the bacterioplankton. Bacteria growth rates were similar in both light and dark incubated P treatments. In these experiments, we found clear evidence suggesting the dynamics of bacteria and phytoplankton were correlated because they are often limited by the same resource (mainly inorganic phosphorus). We found only limited evidence that bacterioplankton growth coupling to algal dynamics was occurring in these experiments. However, we did not consider several factors such as dissolved organic nutrient availability, bacterivory, availability of physical substrates, and temperature which are also thought to influence the nature of bacterial/phytoplankton interactions. Based on the results of our experiments, we conclude the biomass of the bacterio- and phytoplankton covaried because they were stimulated by the same nutrients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Climate‐related changes of soil characteristics affect bacterial community composition and function of high altitude and latitude lakes

Lakes at high altitude and latitude are typically unproductive ecosystems where external factors outweigh the relative importance of in‐lake processes, making them ideal sentinels of climate change. Climate change is inducing upward vegetation shifts at high altitude and latitude regions that translate into changes in the pools of soil organic matter. Upon mobilization, this allochthonous organic matter may rapidly alter the composition and function of lake bacterial communities. Here, we experimentally simulate this potential climate‐change effect by exposing bacterioplankton of two lakes located above the treeline, one in the Alps and one in the subarctic region, to soil organic matter from below and above the treeline. Changes in bacterial community composition, diversity and function were followed for 72 h. In the subarctic lake, soil organic matter from below the treeline reduced bulk and taxon‐specific phosphorus uptake, indicating that bacterial phosphorus limitation was alleviated compared to organic matter from above the treeline. These effects were less pronounced in the alpine lake, suggesting that soil properties (phosphorus and dissolved organic carbon availability) and water temperature further shaped the magnitude of response. The rapid bacterial succession observed in both lakes indicates that certain taxa directly benefited from soil sources. Accordingly, the substrate uptake profiles of initially rare bacteria (copiotrophs) indicated that they are one of the main actors cycling soil‐derived carbon and phosphorus. Our work suggests that climate‐induced changes in soil characteristics affect bacterioplankton community structure and function, and in turn, the cycling of carbon and phosphorus in high altitude and latitude aquatic ecosystems.     

Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake

Bacterial growth and grazing mortality were estimated from Mayto October in two south Swedish oligotrophic lakes, one beinga clearwater lake (water colour 5–10 mg Pt l^–1 DOC2.9–3.4 mg l^–1, Secchi disk depth 5.0–9.4m) and the other a humic, brownwater lake (water colour 105–165mg Pt l^–1, DOC 13.7–22.7mg l^–1, Secchi diskdepth 1.3–2.1 m). Specific rates of growth and grazingmortality were generally similar for both lakes. However, theabundance of bacteria was consistently 2–3 times higherin the water of the humic lake, suggesting that the total productionand consumption of bacterial cells were also higher than inthe dearwater lake. The ratio of bacterial secondary productionto primary production was higher in the humic lake than in theclearwater lake, indicating that the bacterioplankton of thehumic lake utilize allochthonous substrates, in addition tosubstrates originating from autochthonous primary production.Most of the bacterial loss in both lakes could be attributedto small protozoan grazers. This implies that allochthonousand autochthonous organic carbon fixed by bacterioplankton isless important in terms of carbon flow to higher trophic levelsthan would be expected if macrozooplankton were the dominantbacterivores, providing a more direct and efficient transferof carbon to larger organisms.     

Seasonality of chlorophyll and nutrients in Lake Erken – effects of weather conditions

The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.     

Influence of phytoplankton on the response of bacterioplankton growth to nutrient enrichment

1. Laboratory experiments were conducted to test the effect of nutrient enrichment on bacterioplankton growth in the presence and absence of phytoplankton. 2. In one series of experiments, bacterioplankton growth in terms of specific activity [³H-thymidine incorporation (cell number)^?1] was greater in whole lake water samples than in samples from which phytoplankton had been removed by filtration (1.0 μm), regardless of the nutrient enrichments (control, NH⁺₄ plus PO^3-₄ and mannitol). Organic C enhanced bacterioplankton growth in both whole and filtered lake water. 3. In another series of experiments (with the same nutrient enrichments as in the first experiment except that glucose replaced mannitol), bacterioplankton growth in whole lake water enriched with PO^3-₄ plus NH⁺₄ and incubated in the light was greater than in two treatments designed to inhibit photosynthetic activity (+DCMU and dark). Bacterioplankton response to nutrient addition was greatest in the PO^3-₄ plus NH⁺₄ enrichment under all three conditions (light +DCMU, and dark). 4. These results indicate that bacterioplankton growth could be directly limited by inorganic P and N when these elements are in short supply. Enhancement of bacterioplankton growth by phytoplankton occurs only under PO^3-₄ and NH⁺₄ replete environments.     

Regulation of bacterioplankton production and biomass in an oligotrophic cleanvater lake--the importance of the phytoplankton community

Estimations of bacterioplankton production and biomass werecarried out in enclosure experiments during two consecutiveyears (1989 and 1990) in oligotrophic clearwater Lake Njupfatet.The lake was limed in November 1989, and the experiments werecarried out both in 1989 (unlimed) and in 1990 (limed). Bags(3001) were manipulated with inorganic phosphorus and nitrogen,organic carbon, and metazoan zooplankton abundance. Both years,bacterial production was stimulated by inorganic nutrients aloneand in combination with organic carbon. However, the increasein bacterial production when inorganic nutrients were addedalone was much stronger in 1990 than in 1989. In 1989. bacterialproduction increased strongly only when inorganic nutrientsand organic carbon were added together. The phytoplankton communitywas dominated by the cyanobacterium Merismopedia tenuis-simaduring 1989, and the phytoplankton biomass increased only slightlywhen receiving inorganic nutrients. In 1990, when the lake hadbeen limed. M.tenuissima had completely disappeared and thephytoplankton community, dominated by Chrysophyceae and Chlorophyceae,responded strongly to additions of inorganic nutrients. Theincreased phytoplankton productivity in 1990 may have resultedin increased release of organic carbon, and this in turn thatthe carbon limitation of bacterioplankton production decreasedfrom 1989 to 1990. Zooplankton had a positive effect on bacterioplanktonproduction in 1989, but no effect in 1990. The loss of bacterialbiomass approximated 60% of the bacterial production in 1989,while in 1990 it almost equalled the bacterioplankton production.     

Effects of Resources and Trophic Interactions on Freshwater Bacterioplankton Diversity

Abstract In a study of bacterioplankton in an oligotrophic lake in northern Wisconsin, a community fingerprinting technique, automated ribosomal intergenic spacer analysis (ARISA), was used to determine the effect of resources and trophic interactions on bacterioplankton diversity. Inorganic nitrogen and phosphorus (NP), carbon in the form of glucose (G) or dissolved organic matter extracted from peat (DOM), and carbon and NP in combination were added to two types of experimental systems. Ten-liter mesocosms contained all components of the original aquatic community except for large zooplankton. One-liter dilution cultures were prepared so that the effects of grazers and phytoplankton were removed. During a 3-day incubation, bacterial production showed the greatest response to the carbon plus NP treatment in both experimental systems, but bacterial diversity was strikingly different between them. In the mesocosms, the number of ARISA-PCR fragments averaged 41 per profile, whereas the dilution culture communities were highly reduced in complexity, dominated in most cases by a single PCR fragment. Further analysis of the mesocosm data suggested that whereas the NPDOM addition caused the greatest aggregate bacterial growth response, the addition of NP alone caused the largest shifts in community composition. These results suggest that the measurement of aggregate responses, such as bacterial production, alone in studies of freshwater bacterial communities may mask the effects of resources on bacterioplankton. Received: 24 January 2000; Accepted: 10 May 2000; Online Publication: 28 August 2000     

Effects of Viruses on Nutrient Turnover and Growth Efficiency of Noninfected Marine Bacterioplankton

The effects of virus infection and lysis of a marine Vibrio sp. on C, N, and P turnover and the growth efficiency of noninfected bacterioplankton were studied in a series of dilution cultures. The cultures were enriched with various sources of organic matter and N and P. The growth of the Vibrio host and the growth of the natural bacterioplankton were measured by immunofluorescence and 4(prm1),6-diamidino-2-phenylindole staining methods, respectively. Lysis products resulting from infection of the Vibrio sp. caused an increase in metabolic activity and cell production by the noninfected bacterioplankton. In P-limited cultures, the addition of viruses increased the uptake of dissolved organic carbon by 72% and the potential alkaline phosphatase activity by 89% compared with control cultures without viruses. Our data suggest that input of available phosphorus through virus-induced Vibrio lysates occurred, which caused an increase in the bacterial nutrient uptake. The growth efficiency of noninfected bacteria was reduced in the presence of viruses compared with the control without viruses (growth efficiencies, 0.08 (plusmn) 0.03 and 0.24 (plusmn) 0.02, respectively). We suggest that the decrease in growth efficiency may be explained by an increase in bacterial energy demand associated with extracellular degradation of polymeric organic nitrogen and phosphorus in cell lysates.     

The phosphorus economy of a hypertrophic seepage lake in Scania,south Sweden groundwater influence

The phosphorus dynamics and economy of Lake Bysjön, a hypertrophic seepage lake in Scania, southern Sweden, were investigated during 1973–1977. The mean dissolved inorganic phosphorus concentration (1973–1977) was 580 µg · l^–1. There were no correlations between dissolved inorganic P, total organic P, dissolved organic P, particulate P and phytoplankton biomass. Groundwater inflow and lake water outflow through the ground are the most important factors for maintaining a constant water volume. Groundwater seepage is also important for water quality. Groundwater inflow, together with planktonic activity, keeps the P concentration high in the lake water.     

Temporal and Vertical Difference in Factors Limiting Growth Rate of Heterotrophic Bacteria in Lake Biwa

Abstract Dilution bioassays were performed to examine the seasonal and vertical difference in the relative importance of factors limiting growth of heterotrophic bacteria in Lake Biwa. The lake water diluted by 0.2 μm lake filtrate (1:6.6) was enriched either with glucose (C), inorganic phosphorus (P), ammonium nitrogen (N), amino acids (AA), or a combination of these, and incubated for 2 days at the depths where lake water was collected (2.5, 20 and 30 m depths). Experiments showed that at 2.5 m, P was the most deficient resource for bacterial growth, but the magnitude of P limitation depended on water temperature. Among others, amino acids showed a slight but significant stimulation of bacterial growth rates during the fall. At 20 and 30 m, however, growth stimulation by resource addition was rarely detected. Vertically reciprocal translocation experiments revealed that the growth rate was limited by low temperature rather than resource supply at the greater depths. The results support a simple view that bacterial growth rate is basically regulated by water temperature, but high growth rate is not realized in summer because of resource depletion. The present study suggests that both temperature and P supply play a crucial role in biogeochemical cycling of organic matter in Lake Biwa through the bacterial growth rate. Received: 10 March 1999; Accepted: 14 May 1999     

Differential response of high-elevation planktonic bacterial community structure and metabolism to experimental nutrient enrichment

Nutrient enrichment of high-elevation freshwater ecosystems by atmospheric deposition is increasing worldwide, and bacteria are a key conduit for the metabolism of organic matter in these oligotrophic environments. We conducted two distinct in situ microcosm experiments in a high-elevation lake (Emerald Lake, Sierra Nevada, California, USA) to evaluate responses in bacterioplankton growth, carbon utilization, and community structure to short-term enrichment by nitrate and phosphate. The first experiment, conducted just following ice-off, employed dark dilution culture to directly assess the impact of nutrients on bacterioplankton growth and consumption of terrigenous dissolved organic matter during snowmelt. The second experiment, conducted in transparent microcosms during autumn overturn, examined how bacterioplankton in unmanipulated microbial communities responded to nutrients concomitant with increasing phytoplankton-derived organic matter. In both experiments, phosphate enrichment (but not nitrate) caused significant increases in bacterioplankton growth, changed particulate organic stoichiometry, and induced shifts in bacterial community composition, including consistent declines in the relative abundance of Actinobacteria. The dark dilution culture showed a significant increase in dissolved organic carbon removal in response to phosphate enrichment. In transparent microcosms nutrient enrichment had no effect on concentrations of chlorophyll, carbon, or the fluorescence characteristics of dissolved organic matter, suggesting that bacterioplankton responses were independent of phytoplankton responses. These results demonstrate that bacterioplankton communities in unproductive high-elevation habitats can rapidly alter their taxonomic composition and metabolism in response to short-term phosphate enrichment. Our results reinforce the key role that phosphorus plays in oligotrophic lake ecosystems, clarify the nature of bacterioplankton nutrient limitation, and emphasize that evaluation of eutrophication in these habitats should incorporate heterotrophic microbial communities and processes.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

海水富营养化对海洋细菌影响的研究进展

综述了海水富营养化对海洋细菌影响的研究进展。随着海水富营养化程度的增加,海洋细菌数量或生物量增加;反硝化细菌、大肠菌群尤其是厌氧性的硫酸盐还原菌和产甲烷菌等典型细菌生理群数量增加;浮游细菌群落结构随富营养化递增趋于简单,物种多样性降低;富营养化也明显导致细菌群落正常功能活性的紊乱。海水富营养化对细菌群落的结构和功能有着深远的影响。     

太湖夏季浮游细菌群落多样性的空间格局

为了研究太湖夏季浮游细菌群落多样性与水体营养盐的关系,在太湖全湖范围内开展了一次大规模浮游细菌采样调查,分析了太湖不同湖区浮游细菌丰度和多样性组成。研究发现,浮游细菌丰度在不同湖区中存在明显的空间差异,从北部和西部湖区沿湖流向东南方向至湖心和南部沿岸再到东部湖区呈下降趋势,这与太湖水体营养水平从高到低变化趋势一致。浮游细菌丰度与营养盐浓度回归分析结果显示,总磷（TP）与细菌丰度存在较好的正相关（R2=0.6392,n=29,P0.01）,而总氮（TN）与细菌丰度无显著相关（R2=0.0663,n=29,P0.05）。因此,磷是太湖夏季浮游细菌生长的限制因子。不同湖区营养盐与浮游细菌群落多样性也具有显著的正相关,随着营养水平的升高,浮游细菌多样性增加。此外,细菌群落的组成在不同湖区间亦具有明显的空间异质性,与不同湖区营养水平空间变化一致。研究结果将有助于人们更好地理解淡水湖泊中微生物循环和生态系统功能。       

Members of a readily enriched beta-proteobacterial clade are common in surface waters of a humic lake

Humic lakes are systems often characterized by irregular high input of dissolved organic carbon (DOC) from the catchment. We hypothesized that specific bacterial groups which rapidly respond to changes in DOC availability might form large populations in such habitats. Seasonal changes of microbial community composition were studied in two compartments of an artificially divided bog lake with contrasting DOC inputs. These changes were compared to community shifts induced during short-term enrichment experiments. Inocula from the two compartments were diluted 1:10 into water from the more DOC-rich compartment, and inorganic nutrients were added to avoid microbial N and P limitation. The dilutions were incubated for a period of 2 weeks. The microbial assemblages were analyzed by cloning and sequencing of 16S rRNA genes and by fluorescence in situ hybridization with specific oligonucleotide probes. beta-Proteobacteria from a cosmopolitan freshwater lineage related to Polynucleobacter necessarius (beta II) were rapidly enriched in all treatments. In contrast, members of the class Actinobacteria did not respond to the enhanced availability of DOC by an immediate increase in growth rate, and their relative abundances declined during the incubations. In lake water members of the beta II clade seasonally constituted up to 50% of all microbes in the water column. Bacteria from this lineage annually formed a significantly higher fraction of the microbial community in the lake compartment with a higher allochthonous influx than in the other compartment. Actinobacteria represented a second numerically important bacterioplankton group, but without clear differences between the compartments. We suggest that the pelagic microbial community of the studied system harbors two major components with fundamentally different growth strategies.     

Occurrence of mixotrophic flagellates in relation to bacterioplankton production, light regime and availability of inorganic nutrients in unproductive lakes with differing humic contents

1. Field data from five unproductive Swedish lakes were used to investigate the occurrence of mixotrophic flagellates in relation to bacterioplankton, autotrophic phytoplankton, heterotrophic flagellates and abiotic environmental factors. Three different sources of data were used: (i) a 3‐year study (1995–97) of the humic Lake Örträsket, (ii) seasonal measurements from five lakes with widely varying dissolved organic carbon (DOC) concentrations, and (iii) whole lake enrichment experiments with inorganic nutrients and organic carbon. 2. Mixotrophic flagellates usually dominated over autotrophic phytoplankton in Lake Örträsket in early summer, when both bacterial production and light levels were high. Comparative data from the five lakes demonstrated that the ratio between the biomasses of mixotrophic flagellates and autotrophic phytoplankton (the M/A‐ratio) was positively correlated to bacterioplankton production, but not to the light regime. Whole lake carbon addition (white sugar) increased bacterial biomass, and production, reduced the biomass of autotrophs by a factor of 16, and increased the M/A‐ratio from 0.03 to 3.4. Collectively, the results indicate that the dominance of mixotrophs among phytoplankton was positively related to bacterioplankton production. 3. Whole lake fertilisation with nitrogen (N) and phosphorus (P) demonstrated that the obligate autotrophic phytoplankton was limited by N. N‐addition increased the biomass of the autotrophic phytoplankton but had no effect on mixotrophic flagellates or bacteria, and the M/A‐ratio decreased from 1.2 to 0.6 after N‐enrichment. Therefore, we suggest that bacteria under natural conditions, by utilising allochthonous DOC as an energy and carbon source, are able to outcompete autotrophs for available inorganic nutrients. Consequently, mixotrophic flagellates can become the dominant phytoplankters when phagotrophy permits them to use nutrients stored in bacterial biomass. 4. In Lake Örträsket, the biomass of mixotrophs was usually higher than the biomass of heterotrophs during the summer. This dominance could not be explained by higher grazing rates among the mixotrophs. Instead, ratios between mixotrophic and heterotrophic biomass (the M/H‐ratio) were positively related to light availability. Therefore, we suggest that photosynthesis can enable mixotrophic flagellates to outcompete heterotrophic flagellates.     

Effects of terrigenous organic substrates and additional phosphorus on bacterioplankton metabolism and exoenzyme stoichiometry

1. Bamboo, as a pioneer vegetation, often forms forests on bare lands after catastrophic landslides. Compared to evergreen forest soil, bamboo forest soil is much more labile, with a higher percentage of microbially derived organic carbon (OC), lower molecular weight, and lower humic acid content. We hypothesised that different terrigenous organic matter (tOM) sources with varying lability and phosphorus (P) availability select for bacterioplankton with distinct metabolic pathways.
2. We incubated natural bacterioplankton assemblages with tOM leached from bamboo forest soil (BOM) and evergreen forest soil (EOM) and compared these to a lake water control. To test if microbial metabolism would be limited by OC or P availability of each tOM treatment, we used acetate as an extra labile OC source and phosphate as an inorganic P source. Bacterial metabolism was measured by analysing respiration via O₂ consumption and production via tritiated thymidine (TdR) assimilation.
3. Bacterioplankton metabolism is limited by the availability of P in BOM substrates. When using BOM, bacteria had higher enzymatic activities for phosphatase. The nutrients required for bacterial biomass seemed to be derived from organic matter. Under BOM treatment, bacterial production (BP) (0.92 ± 0.13 μg C L⁻¹ hr⁻¹) and cell specific TdR assimilation rates (0.015 ± 0.002 10^–18 M TdR cell⁻¹ hr⁻¹) were low. Adding P enhanced BP (BOM_+P 1.52 ± 0.31 and BOM_+C+P 2.25 ± 0.37 μg C L⁻¹ hr⁻¹) while acetate addition had no significant effect on BOM treatment.
4. This indicated that the bacteria switched to using added inorganic P to respire a P-limited BOM substrate, which increased total BP and abundance, resulting in even more active respiration and lower growth efficiency. We also found higher activities for chitin-degrading enzyme β-N-acetylglucosaminidase, which is associated with N mining from aminosaccharides.
5. Microbes using EOM, however, did not change metabolic strategies with additional acetate or/and inorganic P. This is due to higher concentrations of organic P in EOM substrates and the presence of inorganic N in the EOM leachates an alternative nutrient source. Bacteria produced β-glucosidase and leucyl-aminopeptidase in order to utilise the humic substances, which sustained greater bacterial abundance, higher BP (2.64 ± 0.39 μg C L⁻¹ hr⁻¹), and lower cell-specific respiration. This yielded a much higher bacterial growth efficiency (15 ± 9.2%) than the lake water control.
6. Our study demonstrated the aquatic metabolic discrepancy between tOM of different forest types. Bacterioplankton in BOM and EOM exhibit distinct metabolic responses. Bacterial metabolic strategy when using BOM implied that the supposedly stabilised biomass OM might be efficiently used by aquatic bacterioplankton. As the labile and nutrient-deficient BOM is more susceptible to the influence of additional nutrients, fertiliser residues in bamboo forest catchments might have a stronger effect on aquatic bacterial metabolic pathways. Thus, it is important to take tOM differences into consideration when building models to estimate soil carbon turnover rates along a terrestrial–aquatic continuum.