Small Algae Research Articles

Towards clarifying the presence of alien algae in inland waters — can we predict places of their occurrence?

Small algae are the trophic basis in both marine and freshwater ecosystems. The identification of tiny microorganisms and place of their origin is laborious but necessary. This paper consists of a literature review of 17 species of planktonic algae, with a discussion of taxonomic problems. We also clarify whether these 17 species are non-native, invasive or cryptogenic species, with an indication whether they had been recognised as ‘alien’ in Europe. According to our observations, areas colonized by small and alien algal species, were anthropogenically altered. There were: systems with heated waters (‘heated islands’), which imitated tropical conditions; highly eutrophic to hypereutrophic water ecosystems, easily colonized by alien species, fishponds with intense fish cultivation, where alien species of fish are/had been introduced, which carried also other alien organisms; and inland water ecosystems with high salinity or high conductivity e.g. pits inundated by mine waters, imitating sea or brackish conditions acting as hubs of migration of alien species adapted to brackish or saline waters. We have prepared a map showing areas inhabited by alien species, both of documented places and hypothetical ones, where we would expect alien species to occur.

Effect of melting Antarctic sea ice on the fate of microbial communities studied in microcosms

Although algal growth in the iron-deficient Southern Ocean surface waters is generally low, there is considerable evidence that winter sea ice contains high amounts of iron and organic matter leading to ice-edge blooms during austral spring. We used field observations and ship-based microcosm experiments to study the effect of the seeding by sea ice microorganisms, and the fertilization by organic matter and iron on the planktonic community at the onset of spring/summer in the Weddell Sea. Pack ice was a major source of autotrophs resulting in a ninefold to 27-fold increase in the sea ice-fertilized seawater microcosm compared to the ice-free seawater microcosm. However, heterotrophs were released in lower numbers (only a 2- to 6-fold increase). Pack ice was also an important source of dissolved organic matter for the planktonic community. Small algae (<10 μm) and bacteria released from melting sea ice were able to thrive in seawater. Field observations show that the supply of iron from melting sea ice had occurred well before our arrival onsite, and the supply of iron to the microcosms was therefore low. We finally ran a “sequential melting” experiment to monitor the release of ice constituents in seawater. Brine drainage occurred first and was associated with the release of dissolved elements (salts, dissolved organic carbon and dissolved iron). Particulate organic carbon and particulate iron were released with low-salinity waters at a later stage.

Implications of elevated CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach

Abstract. Recent studies on the impacts of ocean acidification on pelagic communities have identified changes in carbon to nutrient dynamics with related shifts in elemental stoichiometry. In principle, mesocosm experiments provide the opportunity of determining temporal dynamics of all relevant carbon and nutrient pools and, thus, calculating elemental budgets. In practice, attempts to budget mesocosm enclosures are often hampered by uncertainties in some of the measured pools and fluxes, in particular due to uncertainties in constraining air–sea gas exchange, particle sinking, and wall growth. In an Arctic mesocosm study on ocean acidification applying KOSMOS (Kiel Off-Shore Mesocosms for future Ocean Simulation), all relevant element pools and fluxes of carbon, nitrogen and phosphorus were measured, using an improved experimental design intended to narrow down the mentioned uncertainties. Water-column concentrations of particulate and dissolved organic and inorganic matter were determined daily. New approaches for quantitative estimates of material sinking to the bottom of the mesocosms and gas exchange in 48 h temporal resolution as well as estimates of wall growth were developed to close the gaps in element budgets. However, losses elements from the budgets into a sum of insufficiently determined pools were detected, and are principally unavoidable in mesocosm investigation. The comparison of variability patterns of all single measured datasets revealed analytic precision to be the main issue in determination of budgets. Uncertainties in dissolved organic carbon (DOC), nitrogen (DON) and particulate organic phosphorus (POP) were much higher than the summed error in determination of the same elements in all other pools. With estimates provided for all other major elemental pools, mass balance calculations could be used to infer the temporal development of DOC, DON and POP pools. Future elevated pCO2 was found to enhance net autotrophic community carbon uptake in two of the three experimental phases but did not significantly affect particle elemental composition. Enhanced carbon consumption appears to result in accumulation of dissolved organic carbon under nutrient-recycling summer conditions. This carbon over-consumption effect becomes evident from mass balance calculations, but was too small to be resolved by direct measurements of dissolved organic matter. Faster nutrient uptake by comparatively small algae at high CO2 after nutrient addition resulted in reduced production rates under future ocean CO2 conditions at the end of the experiment. This CO2 mediated shift towards smaller phytoplankton and enhanced cycling of dissolved matter restricted the development of larger phytoplankton, thus pushing the system towards a retention type food chain with overall negative effects on export potential.

Phytoplankton Cell Size: Intra- and Interspecific Effects of Warming and Grazing

Decreasing body size has been suggested as the third universal biological response to global warming after latitudinal/altitudinal range shifts and shifts in phenology. Size shifts in a community can be the composite result of intraspecific size shifts and of shifts between differently sized species. Metabolic explanations for the size shifts dominate in the literature but top down effects, i.e. intensified size-selective consumption at higher temperatures, have been proposed as alternative explanation. Therefore, we performed phytoplankton experiments with a factorial combination of warming and consumer type (protist feeding mainly on small algae vs. copepods mainly feeding on large algae). Natural phytoplankton was exposed to 3 (1st experiment) or 4 (2nd experiment) temperature levels and 3 (1st experiment: nano-, microzooplankton, copepods) or 2 (2nd experiment: microzooplankton, copepods) types of consumers. Size shifts of individual phytoplankton species and community mean size were analyzed. Both, mean cell size of most of the individual species and mean community cell size decreased with temperature under all grazing regimes. Grazing by copepods caused an additional reduction in cell size. Our results reject the hypothesis, that intensified size selective consumption at higher temperature would be the dominant explanation of decreasing body size. In this case, the size reduction would have taken place only in the copepod treatments but not in the treatments with protist grazing (nano- and microzooplankton).

Pre-treating algae-laden raw water by silver carp during Microcystis-dominated and non-Microcystis-dominated periods

Performance of pre-treating algae-laden raw water by silver carp during a non-Microcystis-dominated period (period I) and a Microcystis-dominated period (period II) was investigated in terms of algae cell concentration, total phosphorus content, chlorophyll a and phytoplankton species structure. During period I the ineffective filter-feeding for small green algae resulted in the increase of small single algae, which led to the negative removal of chlorophyll a, and when the biomass was higher, the negative was more significant. However, due to the effective filter-feeding of silver carp for Microcystis flos-aquae, the average removal efficiency exceeded 50% at all stocking biomass levels (20-120 g/m(3)) used in experiments during period II. Total phosphorus removal efficiencies could exceed 50% at silver carp biomass stocking levels of 60-80 g/m(3) during both period I and period II. The experimental results indicated that silver carp stocking contributed to the removal of colony-forming cyanobacteria, but led to the increase of single-cell algae (mainly green algae and diatoms) during both period I and period II. The initial phytoplankton community structure and the control of nutrient level were important factors in the choice of silver carp stocking biomass when used to purify algae-loaded water.

Mixotrophic basis of Atlantic oligotrophic ecosystems

Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO(2) fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO(2) fixers in the oligotrophic gyres.

Initial size structure of natural phytoplankton communities determines the response to Daphnia diel vertical migration

Diel vertical migration (DVM) is a common behavior of many pelagic herbivorous zooplankton species in response to predation pressure. It is characterized by a twice daily habitat shift of the zooplankton species: staying in the epilimnion only during night time and migrating down in the crack of dawn in deeper water layers, staying there during the day time. This causes a discontinuous grazing regime and previous studies have shown that the direction and strength of phytoplankton community responses to zooplankton DVM most probably depends on the size of phytoplankton species. To examine the influence of zooplankton DVM on different sized phytoplankton communities, we designed an experiment where we manipulated the size distribution of a natural phytoplankton community a priori in field mesocosms. We investigated the influence of DVM of the cladoceran Daphnia hyalina on two different phytoplankton communities, by the use of deep (10 m) field enclosures. Epilimnetic lake water, containing a summer phytoplankton community, was filtered with two different mesh sizes (11 mm and 64 mm). The 11 mm phytoplankton community (“small”) contained mainly small algal species, while the 64 mm community (“large”) had a wider range of phytoplankton sizes. To simulate zooplankton DVM, D. hyalina were placed in mesh cages that were lowered or raised (“migration”) as dictated by the study design; a “no migration” (representing absence of DVM) treatment was also tested. Phytoplankton abundance was measured using chlorophyll-a and biovolume; size distribution of the algae and nutrient availability was also determined in each treatment. The results indicated that DVM had contrasting effects on the two evaluated phytoplankton communities. Comparison of “migration” and “no migration” zooplankton treatments showed that nutrient availability and total phytoplankton biovolume was higher in (1) “no migration” treatments with phytoplankton communities comprising mainly small algae and (2) “migration” treatments with phytoplankton communities of a broader size spectrum of algae. Hence our study showed two different mechanisms of how zooplankton DVM may influence the phytoplankton community dynamics. Nutrient cycling was an important factor in phytoplankton communities of mainly small algae, whereas the refuge effect was the main driver of phytoplankton dynamics in phytoplankton communities of a large size spectrum of algae.

Extirpation of macroalgae (Sargassum spp.) on the subtropical east Australian coast.

Populations of large brown algae of the Laminariales and Fucales (Phaeophyta) have declined or been extirpated from many locations on temperate coasts worldwide. We conducted field surveys and a literature review, and examined herbarium specimens, through which we discovered previously unreported extirpations of large brown algal species from a tropical and subtropical coastline. Sargassum amaliae, S. aquifolium, S. carpophyllum, S. polycystum, and S. spinifex were common habitat-forming macroalgae that supported diverse assemblages of invertebrates and smaller algae before urbanization began in 1970 along the 45-km length of Sunshine Coast in Queensland, Australia. Causes of these extirpations are not known, but are consistent with losses of other large brown algal species from coastal areas undergoing urbanization or eutrophication. Sargassum spp. do not have the characteristics thought to protect marine species from extinction (large geographical ranges, occurrence on many different substrata, long-distance dispersal). Some local Sargassum spp. are endemic to eastern Australia. Abundance of Sargassum is limited by suitable substrata on the sandy southern Queensland coast (370 km). These substrata are 12 rocky headlands separated by long (5-105 km) sandy beaches. Most multicellular propagules (the only motile stage in Sargassum) settle within 1-3 m of parental thalli, which restricts long-distance dispersal needed to maintain connectivity among populations and to recolonize areas of the headlands from which populations have been extirpated. Local Sargassum spp. could be categorized as data deficient by the International Union for Conservation of Nature (IUCN), but the IUCN vulnerable category is more accurate given extirpations, limited habitat, and the lack of connectivity among populations.

Endogenous regulation of the growth-rate responses of a spring-dwelling strain of the freshwater alga, Chlorella minutissima, to light and temperature

The paper presents a laboratory investigation of the temperature- and light- dependence of autotrophic growth of the alga Chlorella minutissima. It was isolated from the storage basin of a spring in the mountains of the Massif Central, France. The alga was grown at temperatures between (10 and 35°C) and under irradiances from 30 to 550μmolm−2s−1, under a light/dark cycle. The results were fitted to selected descriptive models, seeking to express, as far as possible, the observed physiological behaviour of the strain and the minimum irradiance required to sustain net growth. At all temperatures, the maximum rates of growth observed are strikingly slower than those of other Chlorella strains and of other small algae, reported in the literature, even when correction is made for continuous light. The Q10 statistic for growth at temperatures >20°C rates is also noticeably lower than in other species, while the apparent threshold of any growth is about 8°C. Growth rates are readily light-saturated at all temperatures but with little evidence of adaptation of photosynthesis to low photon-flux rates. No short-term flexibility in these properties (over a time-scale of days) was demonstrated during the course of our experiments. We deduce that the algal strain had become genetically adapted to the relatively constant, even-temperature and low-light conditions of the spring-water habitat whence it was originally isolated.

Macrolgae as an indicator of the environmental health of the Pirangi reefs, Rio Grande do Norte, Brazil

The coral reefs of Pirangi beach have suffered the impact of anthropic actions, mainly those related to tourism. To evaluate these effects, algal samples were collected at nine stations, distributed along the reef fringe. The macrobenthic community (algae/coral) was identified through photographic methods. A total of thirty species of algae, five species of coral, and one species of mollusk were identified. In areas of intense tourist activity, small algae were predominant, while in areas without human interference, foliose algae were predominant. Cluster analysis of the organisms revealed a pattern in spatial distribution into five zones: (1) a submerged zone with very diverse flora, (2) a zone with a predominance of Caulerpa racemosa, (3) a zone with high coverage of Sargassum vulgare, (4) a trampled zone with bare spaces, small algae and Zoanthus sociatus, and (5) a zone with predominance of Palythoa caribaeroum. The results show that human disturbances of the natural order can result in a different distribution model for benthic organisms in reefs. Moreover, these results allow us to infer that the area studied has undergone changes resulting from human activities and that the differences in biological composition can be used as an important indicator of the health of the Pirangi reef.

Chloroparva pannonica gen. et sp. nov. (Trebouxiophyceae, Chlorophyta) – a new picoplanktonic green alga from a turbid, shallow soda pan

Somogyi B., Felföldi T., Solymosi K., Makk J., Homonnay Z.G., Horváth G., Turcsi E., Böddi B., Márialigeti K. and Vörös L. 2011. Chloroparva pannonica gen. et sp. nov. (Trebouxiophyceae, Chlorophyta) – a new picoplanktonic green alga from a turbid, shallow soda pan. Phycologia 50: 1–10. DOI: 10.2216/10-08.1We describe Chloroparva pannonica Somogyi, Felföldi & Vörös gen. et sp. nov., a new trebouxiophycean picoplanktonic alga isolated from a turbid, shallow soda pan in Hungary. The cells are spherical to oval, less than 2 µm in diameter, with simple ultrastructure typical to small green algae. Cells divide by autosporulation, forming two daughter cells per autosporangium. Cell wall structure consists of an outer trilaminar layer, an inner microfibrillar layer and an electron-transparent layer covering the plasma membrane. The trilaminar layer of the mother cell wall often persists around the autospores. Typical chlorophyte pigments have been found, including chlorophyll a and b and lutein as the dominant carotenoid. The main fatty acid was oleic acid. The phylogenetic position of the new chlorophyte confirms the proposal of a new genus within the Trebouxiophyceae. Based on its 18S rRNA gene sequence, this isolate is distantly related to Nannochloris eucaryotum UTEX 2502, Chlorella minutissima C-1.1.9 and C. minutissima SAG 1.80 (≤ 97.6% 18S rRNA gene pairwise similarities).

Spawning substrata are important for breeding habitat selection but do not determine premating reproductive isolation in three sympatric Hexagrammos species

Habitat use and spawning substrata were surveyed to characterize the contribution of habitat divergence to reproductive isolation in greenling Hexagrammos species. The spawning substrata and microhabitat in breeding territories differed amongst the three Hexagrammos species studied: H. octogrammus, H. agrammus and H. otakii used small red algae, surfgrass and bryozoans, respectively, as spawning substrata, and breeding territories were established in areas where those substrata were abundant. In contrast, non-territorial individuals were observed in a comparatively wider range of habitats than conspecific territories. Consequently, the distributions of non-territorial individuals of the three species partially overlapped. Since hybrids have been frequently collected, the difference in spawning substrata and the subsequent microhabitat divergence in breeding territories do not prevent females from encountering males of other species. Thus, in addition to habitat divergence, other factors such as behavioural differentiation may be needed to complete premating reproductive isolation amongst these three Hexagrammos species.

Successful Sargassum Bed Formation in a “Isoyake” Area by Net Transplantation of Juveniles with Small Algae

Transplanted juveniles of the Sargassum spp were used to try to create Sargassum beds in the Isoyake Area of Nasa Bay in Tokushima Prefecture between 1996-2001. There were no juveniles of the next generation in the area until 1997 because of the high population of grazing herbivores. From 1998, a different method of transplantation, involving young Sargassum and other small annual and perennial algae was used. Moreover, two types of protective netting were settled over the transplanted area to prevent grazing by herbivores. One type net was a roofless cage net that was used from 1997, the other type was a cover net that was used from 1999. Consequently, Sargassum juveniles appeared among the small annual and perennial algae on the substratum of the sea bottom. This fact indicated that the existence of small algae protected the young Sargassum algae from grazing by herbivores. Thereafter, Sargassum beds regenerated and spread throughout the adjacent area. The new method employed in this study, combining the transplantation of small algae with Sargassum and setting protective netting to prevent by grazing herbivores were effective in the production of Sargassum bed formation in the Isoyake Area.

Top-predator effects of jellyfish Odessia maeotica in Mediterranean salt marshes

Jellyfish can act as planktonic top predators, and their effects may cause drastic changes in the plankton structure of marine and freshwater systems. However, the top predator effects may not necessarily be the same in species-poor habitats as they are in species-rich habitats. The present study analyses the effects of the small lacunae jellyfish Odessia maeotica in a species- poor habitat, Mediterranean salt marshes in the wetlands of Emporda (NE Iberian Peninsula). A field experiment was carried out in March 2008 to assess the direct and indirect effects of O. maeo- tica on plankton composition. Our results show that the presence of O. maeotica changed the plank- ton composition through top-down effects. Changes were strong in zooplankton, because O. maeot- ica can suppress almost the entire trophic level of large zooplankton (> 50 µm). Weak indirect effects on phytoplankton composition were observed as well. When O. maeotica was present, changes in the relative abundance of the phytoplankton species were found, but there was no net increase in phytoplankton biomass. Our results suggest that these weak indirect effects may be the result of trophic cascade effects coupled with the oligotrophic conditions of these salt marshes. Thus, trophic cascade effects lead to an increase in ciliate biomass, and these ciliates would feed on small algae (jellyfish-copepods-ciliates-small algae), while oligotrophic conditions would prevent increases in algal biomass.

Katamysis warpachowskyi Sars, 1877 (Crustacea, Mysida) invaded Lake Constance

The mysid Katamysis warpachowskyi Sars, 1877 originated in the Ponto-Caspian region and the associated river systems. The first evidence of its transgression of the limits of the watersheds of its natural Ponto-Caspian origin was found when three individuals were recorded in October 2009 in eastern Lake Constance (Austria). In March 2010, K. warpachowskyi comprised 10% of the mysid assemblage and was mainly found in rocky habitats. On both sampling dates, breeding females were present. Before this invasion, Limnomysis benedeni Czerniavsky, 1882, was the only mysid in Lake Constance and is also distributed in the Rhine river system. Since the two mysids coexist in Lake Constance, K. warpachowskyi will likely become established in the lake and further expand into the main part of the Rhine. K. warpachowskyi is known as a benthic organism and feeds on detritus and small algae; therefore, the impact on the lake ecosystem should be weaker than that of pelagic mysids.

The Potential for Using Bubble Modification Chemicals in Dissolved Air Flotation for Algae Removal

This paper investigates the potential for using surface modified bubbles in the treatment of algae using dissolved air flotation (DAF) instead of upstream coagulation and flocculation. Bubble modification is attempted by adding either metal coagulant, surfactant or polymers direct to the saturator. In this way, the chemical characteristics most suitable for removing small algae cells using this technique are examined. Optimum removal using metal coagulant, aluminium sulphate, was 60%; however, both a decrease in the magnitude of the zeta potential and microfloc generation occurred concurrently, thus accounting for the improved removal. In contrast, there was no change in system zeta potential and no microfloc generation when using cationic surfactant cetyltrimethyl-ammonium bromide (CTAB), for which 63% removal was achieved. An average of 95% removal was achieved using the cationic polymer, PolyDADMAC, with no change to system zeta potential. The results therefore confirm that there is a potential for adapting the conventional DAF process to operate without upstream coagulation and flocculation. A chemical with both a hydrophobic component in addition to a high molecular weight, hydrophilic, highly charge component is advised for the process.

Theoretical study of small Al–Ga–N ternary clusters

Density-functional theory with generalized gradient approximation for the exchange–correlation potential has been used to calculate the geometric structures, stabilities, and electronic properties of Al x Ga y N z ( x, y, z = 1–3) clusters. The lowest-energy structures of different-sized Al x Ga y N z clusters are given based on the extensive search of the local minima of the potential energy surface. The results indicate that the Al x Ga y N z clusters prefer planar structures except AlGaN 4. The lowest-lying structures of the clusters keep the analogous framework as small Ga m N n clusters by replacing Ga atom(s) with Al. The strong Al–N bond is a dominant factor in the building-up principle of each Al x Ga y N z clusters. The calculated dissociation energies of the Al x Ga y N z ( x, y, z = 1–3) clusters manifest that all of the clusters are stable with respect to dissociation into smaller clusters or atoms except for AlGaN 3. On the basis of the lowest-energy geometries obtained, the electronic properties of the clusters such as HOMO–LUMO gap, Mulliken charge, ionization potential and electron affinity, etc., have been computed and analyzed.

Grazing impact and phenology of the freshwater sponge Ephydatia muelleri and the bryozoans Plumatella emarginata and Fredericella sultana under experimental warming

Summary1. Both the clearance rates (CR) and abundances of the freshwater sponge Ephydatia muelleri and the bryozoans Plumatella emarginata and Fredericella sultana were investigated from autumn to spring under different temperature regimes. The experiments were performed in bypass channels of the River Rhine (Cologne, Germany) in which temperature could be manipulated.2. The impact of temperature increase on CRs depends upon the grazer: E. muelleri showed a clear increase in CRs with increasing temperature whereas P. emarginata was not significantly affected by experimental warming.3. Distinct differences in food preference were found for the sponge (which is an efficient grazer of bacteria and small algae) and for the bryozoan P. emarginata (which feeds primarily on large algae, and with no significant grazing on bacteria).4. In contrast to their temperature‐related patterns in CR, respiration of both P. emarginata and E. muelleri increased with temperature between 19 and 32 °C, suggesting that the risk of experiencing energy deficiency at high temperatures due to increased metabolic rates is particularly high for the bryozoan.5. A temperature elevation of 3 °C above the natural Rhine temperature resulted in a delay in the disappearance of active tissue and formation of resting stages for E. muelleri in autumn. This delay ranged from 8 (beginning of gemmulation) to 22 days (termination of gemmulation). In contrast, there was no distinct effect of warming on the disappearance of active zooids of the two bryozoan species in autumn. However, warming can positively affect the maintenance of active zooids during winter in F. sultana. In spring, the appearance of active zooids of P. emarginata was clearly stimulated by temperature elevations, whereas the hatching of both F. sultana and E. muelleri was hardly affected by warming.6. The study demonstrated different patterns in the thermal ecology of the two freshwater bryozoans and the sponge in comparison to other filter feeders, particularly mussels. Such patterns need to be considered when predicting the impact of temperature on pelagic‐benthic coupling in aquatic habitats.

High bacterivory by the smallest phytoplankton in the North Atlantic Ocean

Planktonic algae <5 m in size are major fixers of inorganic carbon in the ocean. They dominate phytoplankton biomass in post-bloom, stratified oceanic temperate waters. Traditionally, large and small algae are viewed as having a critical growth dependence on inorganic nutrients, which the latter can better acquire at lower ambient concentrations owing to their higher surface area to volume ratios. Nonetheless, recent phosphate tracer experiments in the oligotrophic ocean have suggested that small algae obtain inorganic phosphate indirectly, possibly through feeding on bacterioplankton. There have been numerous microscopy-based studies of algae feeding mixotrophically in the laboratory and field as well as mathematical modelling of the ecological importance of mixotrophy. However, because of methodological limitations there has not been a direct comparison of obligate heterotrophic and mixotrophic bacterivory. Here we present direct evidence that small algae carry out 40-95% of the bacterivory in the euphotic layer of the temperate North Atlantic Ocean in summer. A similar range of 37-70% was determined in the surface waters of the tropical North-East Atlantic Ocean, suggesting the global significance of mixotrophy. This finding reveals that even the smallest algae have less dependence on dissolved inorganic nutrients than previously thought, obtaining a quarter of their biomass from bacterivory. This has important implications for how we perceive nutrient acquisition and limitation of carbon-fixing protists as well as control of bacterioplankton in the ocean.

Protozoan grazing in shallow macrophyte- and plankton lakes

The aim of the present study was to describe the role of ciliates and heterotrophic nanoflagellates in macrophyte and phytoplankton dominated shallow lakes and to compare the grazing on pico- and nano-sized particles by these protozoans. We sampled three shallow lakes, seasonally, one dominated by macrophytes, one by phytoplankton, and one with both conditions (two basins of this lake were sampled separately). The role of ciliates as consumers of bacteria and small algae was important in both lake types. In plankton dominated lakes the grazing rate of ciliates on bacteria was higher than in macrophyte dominated ones. But when ciliates were grazing on small algae, the highest rates were found in macrophyte lakes. Our results suggest that the microbial loop is weaker in macrophyte dominated lakes and grows stronger, when a lake becomes more turbid. The dominance of herbivorous ciliates in macrophyte lakes in summer and their high grazing rates on phytoplankton may have contributed to the decrease in phytoplankton abundance in these lakes. Heterotrophic nanoflagellates were relatively less abundant and, due to their low specific filtering rates, grazed only a minor fraction of the bacterioplankton.