Primary production and phytoplankton composition in relation to DOC input and bacterioplankton production in humic Lake Örträsket

1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers.
2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket.
3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton.     

Effects of Additions of DOC on Pelagic Biota in a Clearwater System: Results from a Whole Lake Experiment in Northern Sweden

An oligotrophic clearwater lake, initially characterized by a pronounced dominance of autotrophic phytoplankton and mostly by one species, the green alga Botryococcus, was subject to additions of dissolved organic carbon in the form of white sugar (sucrose) during two consecutive years. The hypothesis tested was that it is organic carbon per se, and not other possible effects of humic substances, that determines the differences in structure of the planktonic ecosystem between humic and clearwater lakes. The additions of DOC resulted in a significant increase in bacterial biomass and a decrease in the biomass of autotrophic phytoplankton. The biomass of mixotrophic and heterotrophic flagellates instead increased significantly, whereas no effects were found to propagate to higher trophic levels. As a result of the changes among biota, total planktonic biomass also decreased to a level typical of nearby humic lakes. We suggest that it is the carbon component of humic material and its utilization by bacterioplankton that determines the structure and function of the pelagic food web in humic lakes.     

Interactions between metazoans,autotrophs, mixotrophs and bacterioplankton in nutrient‐depleted high DOC environments: a long‐term experiment

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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.     

Biophysical aspects of resource acquisition and competition in algal mixotrophs

Mixotrophic organisms combine autotrophic and heterotrophic nutrition and are abundant in both freshwater and marine environments. Recent observations indicate that mixotrophs constitute a large fraction of the biomass, bacterivory, and primary production in oligotrophic environments. While mixotrophy allows greater flexibility in terms of resource acquisition, any advantage must be traded off against an associated increase in metabolic costs, which appear to make mixotrophs uncompetitive relative to obligate autotrophs and heterotrophs. Using an idealized model of cell physiology and community competition, we identify one mechanism by which mixotrophs can effectively outcompete specialists for nutrient elements. At low resource concentrations, when the uptake of nutrients is limited by diffusion toward the cell, the investment in cell membrane transporters can be minimized. In this situation, mixotrophs can acquire limiting elements in both organic and inorganic forms, outcompeting their specialist competitors that can utilize only one of these forms. This advantage can be enough to offset as much as a twofold increase in additional metabolic costs incurred by mixotrophs. This mechanism is particularly relevant for the maintenance of mixotrophic populations and productivity in the highly oligotrophic subtropical oceans.     

Flagellate grazing on bacteria in a small dystrophic lake

Fluorescent beads were used to determine the grazing on bacteria by heterotrophic and mixotrophic flagellates in a highly humic (water colour 300–600 mg Pt l^–1) small lake. In summer phagotrophic flagellates constituted about three quarters of the numbers of phytoplankton (including heterotrophic or mixotrophic flagellates) in the uppermost epilimnion. Due to their small size their respective contribution to the biomass was about one quarter. The most important phagotrophic species were Ochromonas sp., and Chromulina spp. which ingested 75–203% of their body carbon per day from bacteria.In view of the abundance and biomass of phagotrophic and mixotrophic flagellates and their very high growth potential, they clearly play a significant role in the food chains of this lake. In addition to providing energy, bacteriovory also represents an important supply of inorganic and organic nutrients under nutrient limiting conditions.     

Microbial Community Structure and Dynamics in the Largest Natural French Lake (Lake Bourget)

We investigated the dynamics and diversity of heterotrophic bacteria, autotrophic and heterotrophic flagellates, and ciliates from March to July 2002 in the surface waters (0–50 m) of Lake Bourget. The heterotrophic bacteria consisted mainly of “small” cocci, but filaments (>2 μm), commonly considered to be grazing-resistant forms under increased nanoflagellate grazing, were also detected. These elongated cells mainly belonged to the Cytophaga-Flavobacterium (CF) cluster, and were most abundant during spring and early summer, when mixotrophic or heterotrophic flagellates were the main bacterial predators. The CF group strongly dominated fluorescent in situ hybridization–detected cells from March to June, whereas clear changes were observed in early summer when Beta-proteobacteria and Alpha-proteobacteria increased concomitantly with maximal protist grazing pressures. The analysis of protist community structure revealed that the flagellates consisted mainly of cryptomonad forms. The dynamics of Cryptomonas sp. and Dinobryon sp. suggested the potential importance of mixotrophs as consumers of bacteria. This point was verified by an experimental approach based on fluorescent microbeads to assess the potential grazing impact of all protist taxa in the epilimnion. From the results, three distinct periods in the functioning of the epilimnetic microbial loop were identified. In early spring, mixotrophic and heterotrophic flagellates constituted the main bacterivores, and were regulated by the availability of their resources mainly during April (phase 1). Once the “clear water phase” was established, the predation pressure of metazooplankton represented a strong top-down force on all microbial compartments. During this period only mixotrophic flagellates occasionally exerted a significant bacterivory pressure (phase 2). Finally, the early summer was characterized by the highest protozoan grazing impact and by a rapid shift in the carbon pathway transfer, with a fast change-over of the main predators contribution, i.e., mixotrophic, heterotrophic flagellates and ciliates in bacterial mortality. The high abundance of ciliates during this period was consistent with the high densities of resources (heterotrophic nanoflagellates, algae, bacteria) in deep layers containing the most chlorophyll. Bacteria, as ciliates, responded clearly to increasing phytoplankton abundance, and although bacterial grazing impact could vary largely, bacterial abundance seemed to be primarily bottom-up regulated (phase 3).     

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.     

Mixotrophy everywhere on land and in water: the grand écart hypothesis

There is increasing awareness that many terrestrial and aquatic organisms are not strictly heterotrophic or autotrophic but rather mixotrophic. Mixotrophy is an intermediate nutritional strategy, merging autotrophy and heterotrophy to acquire organic carbon and/or other elements, mainly N, P or Fe. We show that both terrestrial and aquatic mixotrophs fall into three categories, namely necrotrophic (where autotrophs prey on other organisms), biotrophic (where heterotrophs gain autotrophy by symbiosis) and absorbotrophic (where autotrophs take up environmental organic molecules). Here we discuss their physiological and ecological relevance since mixotrophy is found in virtually every ecosystem and occurs across the whole eukaryotic phylogeny, suggesting an evolutionary pressure towards mixotrophy. Ecosystem dynamics tend to separate light from non‐carbon nutrients (N and P resources): the biological pump and water stratification in aquatic ecosystems deplete non‐carbon nutrients from the photic zone, while terrestrial plant successions create a canopy layer with light but devoid of non‐carbon soil nutrients. In both aquatic and terrestrial environments organisms face a grand écart (dancer's splits, i.e., the need to reconcile two opposing needs) between optimal conditions for photosynthesis vs. gain of non‐carbon elements. We suggest that mixotrophy allows adaptation of organisms to such ubiquist environmental gradients, ultimately explaining why mixotrophic strategies are widespread.     

Trophic coupling of the microbial and the classical food web in Lake Baikal,Siberia

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Pico- and Nanoplankton in Aquatic Ecosystems in the Valley of the Lakes and Great Lakes Depression (Mongolia)

The abundance and biomass of bacterioplankton, phototrophic picoplankton, and heterotrophic nanoflagellates has been determined in lakes, rivers, and reservoirs located in the Valley of the Lakes and Great Lakes Depression (Mongolia). The species richness of the heterotrophic flagellates and their consumption of bacteria are estimated. Pico- and nanoplankton are the most abundant in shallow mineral lakes Orog and Tatsyn and in the freshwater Durgun Reservoir. Heterotrophic nanoflagellates consume 26–92% (on average 66%) of the daily bacterioplankton production. Thus, flagellates are important in the transfer of bacterial carbon to the higher levels of planktonic trophic webs. A total of 30 species and their forms of heterotrophic flagellates from 14 large taxa are identified. The highest species diversity of these protists are found in the Durgun and Taishyr reservoirs.     

Mixotrophic organisms become more heterotrophic with rising temperature

The metabolic theory of ecology predicts that temperature affects heterotrophic processes more strongly than autotrophic processes. We hypothesized that this differential temperature response may shift mixotrophic organisms towards more heterotrophic nutrition with rising temperature. The hypothesis was tested in experiments with the mixotrophic chrysophyte Ochromonas sp., grown under autotrophic, mixotrophic and heterotrophic conditions. Our results show that (1) grazing rates on bacterial prey increased more strongly with temperature than photosynthetic electron transport rates, (2) heterotrophic growth rates increased exponentially with temperature over the entire range from 13 to 33 °C, while autotrophic growth rates reached a maximum at intermediate temperatures and (3) chlorophyll contents during mixotrophic growth decreased at high temperature. Hence, the contribution of photosynthesis to mixotrophic growth strongly decreased with temperature. These findings support the hypothesis that mixotrophs become more heterotrophic with rising temperature, which alters their functional role in food webs and the carbon cycle.     

Bacterial grazing by phagotrophic phytoflagellates in a deep humic lake in northern Sweden

Bacterial grazing was measured from June to August 1995 in Lake Ortrasket, a deep brown-water lake in northern Sweden. Mixotrophic chrysophytes were the dominating bacterivores at all times, grazing 3-14% of bacterial standing stock daily. The effects of altered nutrient supply and light availability on grazing activity and growth were studied in two mesocosm experiments. Incubation in the dark did not stimulate phagotrophy, which would otherwise be expected if bacteria were mainly being used as an energy source. Furthermore, clearance rates were not reduced after alleviation of nutrient limitation conditions. Rather, phagotrophy may work as a relatively fixed attribute of the mixotrophic community in this lake. When availability of dissolved nutrients is restricted, phagotrophy permits the mixotrophs to outcompete other phytoplankton, but they become less competitive at high nutrient concentrations. The relative share of mixotrophs in relation to total phytoplankton decreased considerably after enrichment with nitrogen + phosphorus.      

Comparison of pelagic food webs in lakes along a trophic gradient and with seasonal aspects: influence of resource and predation

Composition and seasonal dynamics of phytoplankton, bacteria,and zooplankton (including heterotrophic flagellates, ciliates,rotifers and crustaceans) were studied in 55 lakes in NorthernGermany with different trophic status, ranging from mesotrophicto hypertrophic. Mean abundance and biomass of all groups increasedsignificantly with trophic level of the lake, but bacteria andmetazooplankton showed only a weak correlation and a slightincrease with chlorophyll concentration. Composition of phytoplanktonshowed a dominance of cyanobacteria in hypertrophic lakes, whereasthe importance of chrysophytes and dinophytes decreased withan increase in trophic status. Protozoans (heterotrophic flagellatesand ciliates) made up 24% (mesotrophic lakes) to 42% (hypertrophiclakes) of total zooplankton biomass on average, and were dominatedby ciliates (62–80% of protozoan biomass). Seasonally,protozoans can build up to 60% of zooplankton biomass in spring,when heterotrophic flagellates can contribute     

On the importance of dissolved organic matter in the nutrition of zooplankton in some lake waters

Summary A steady state, radiotracer technique was used to study the original source of the carbon in zooplankton. The experiments were started in filtered lake water with added inorganic radiocarbon. At the beginning of the experiments, a proportionally insignificant volume of unfiltered water was introduced into the culture, together with some ovigerous zooplankton individuals. Since the radioactivity: carbon ratio in the dissolved inorganic carbon was kept constant, a similar ratio would be expected to develop in the autotrophic phytoplankton. The same ratio would then be expected to develop in the zooplankton, if its sole carbon source was autotrophic phytoplankton.According to the results of this approach dissolved organic matter seems to be an important food resource for zooplankton, particularly in highly humic lakes. This conclusion was confirmed by the finding that zooplankton from these lakes was able to grow and reproduce in experiments started with filtered lake water and conducted in complete darkness.The development of algae was followed over the course of one experiment in highly humic water. The same micro-flagellates reproduced equally well in both light and darkness, which indicates the importance of heterotrophic metabolism in their nutrition. Although there are no direct observations about the food of zooplankton in our experiments, it appears likely that heterotrophic flagellates play an important role as a food of zooplankton in humic waters.The importance of dissolved organic matter in the nutrition of aquatic organisms would seem to be much greater than has generally been recognized. Consequently the prevailing concepts of the structure and functioning of planktonic ecosystem should be thoroughly re-evaluated.     

The role of C, N and P in dissolved and particulate organic matter as a nutrient source for phytoplankton growth, including toxic species

Phytoplankton have traditionally been regarded as strictly phototrophic, with a well defined position at the base of pelagic food webs. However, recently we have learned that the nutritional demands of a growing number of phytoplankton species can be met, at least partially, or under specific environmental conditions, through heterotrophy. Mixotrophy is the ability of an organism to be both phototrophic and heterotrophic, in the latter case utilizing either organic particles (phagotrophy) or dissolved organic substances (osmotrophy). This finding has direct implications for our view on algal survival strategies, particularly for harmful species, and energy- and nutrient flow in pelagic food webs. Mixotrophic species may outcompete strict autotrophs, e.g. in waters poor in inorganic nutrients or under low light. In the traditional view of the ‘microbial loop’ DOC is thought to be channeled from algal photosynthesis to bacteria and then up the food chain through heterotrophic flagellates, ciliates and mesozooplankton. Are mixotrophic phytoplankton that feed on bacteria also significantly contributing to this transport of photosynthetic carbon up the food chain? How can we estimate the fluxes of carbon and nutrients between different trophic levels in the plankton food web involving phagotrophic algae? These questions largely remain unanswered. In this review we treat evidence for both osmotrophy and phagotrophy in phytoplankton, especially toxic marine species, and some ecological implications of mixotrophy.     

Net heterotrophy in Faroe Islands clear-water lakes: causes and consequences for bacterioplankton and phytoplankton

1. Five oligotrophic clear‐water lakes on the Faroe Islands were studied during August 2000. Algal and bacterial production rates, community respiration, and CO₂ saturation were determined. In addition, we examined the plankton community composition (phytoplankton and heterotrophic nanoflagellates) and measured the grazing pressure exerted by common mixotrophic species on bacteria. 2. High respiration to primary production (6.6–33.2) and supersaturation of CO₂ (830–2140 μatm) implied that the lakes were net heterotrophic and that the pelagic heterotrophic plankton were subsidised by allochthonous organic carbon. However, in spite of the apparent high level of net heterotrophy, primary production exceeded bacterial production and the food base for higher trophic levels appeared to be mainly autotrophic. 3. We suggest that the observed net heterotrophy in these lakes was a result of the oligotrophic conditions and hence low primary production in combination with an input of allochthonous C with a relatively high availability. 4. Mixotrophic phytoplankton (Cryptomonas spp., Dinobryon spp. and flagellates cf. Ochromonas spp.) constituted a large percentage of the plankton community (17–83%), possibly as a result of their capacity to exploit bacteria as a means of acquiring nutrients in these nutrient poor systems.     

Abundance and biomass of heterotrophic microorganisms in Lake Tanganyika

1. This study focused on heterotrophic microorganisms in the two main basins (north and south) of Lake Tanganyika during dry and wet seasons in 2002. Bacteria (81% cocci) were abundant (2.28–5.30 × 10⁶ cells mL^?1). During the dry season, in the south basin, bacterial biomass reached a maximum of 2.27 g C m^?2 and phytoplankton biomass was 3.75 g C m^?2 (integrated over a water column of 100 m). 2. Protozoan abundance was constituted of 99% of heterotrophic nanoflagellates (HNF). Communities of flagellates and bacteria consisted of very small but numerous cells. Flagellates were often the main planktonic compartment, with a biomass of 3.42–4.43 g C m^?2. Flagellate biomass was in the same range and often higher than the total autotrophic biomass (1.60–4.72 g C m^?2). 3. Total autotrophic carbon was partly sustained by the endosymbiotic zoochlorellae Strombidium. These ciliates were present only in the euphotic zone and usually contributed most of the biomass of ciliates. 4. Total heterotrophic ciliate biomass ranged between 0.35 and 0.44 g C m^?2. In 2002, heterotrophic microorganisms consisting of bacteria, flagellates and ciliates represented a large fraction of plankton. These results support the hypothesis that the microbial food web contributes to the high productivity of Lake Tanganyika. 5. As the sole source of carbon in the pelagic zone of this large lake is phytoplankton production, planktonic heterotrophs ultimately depend on autochthonous organic carbon, most probably dissolved organic carbon (DOC) from algal excretion.     

Stable carbon isotope ratios differentiate autotrophs supporting animal diversity in Lake Baikal

Stable carbon isotopes (δ¹³C) were determined for autotrophic producers and animals from Lake Baikal (eastern Siberia), the deepest, the oldest and largest body of fresh water in the world. The extensive survey shows that the mean carbon isotope compositions of planktonic and benthic autotrophs differ in Lake Baikal by 21.5 ‰ the largest difference ever observed for lakes, thus giving an easy means to distinguish between pelagic and inshore carbon flows. Furthermore, our data give evidence that inshore macrofauna, which contributes greatly to the diversity within Baikalian animals, is supported by less abundant but highly diversified benthic flora, rather than by phytoplankton or terrigenous organic matter that dominate in the global carbon cycle of the lake.     

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.