Mucilaginous Colonies Research Articles

Morphological diversity and phylogeny of the palmelloid chrysophyte genera Chrysotilos and Globulochrysis, gen. nov

ABSTRACT Chrysophytes are a diverse group of stramenopile protists comprising a variety of taxa with different cell organizations. Here, we morphologically and genetically characterize a novel chrysophyte genus isolated from three freshwater localities in the Czech Republic, described as Globulochrysis compacta gen. nov. sp. nov. The genus is well characterized by the frequent formation of closely packed cells, developing into large, spherical sporangia. This alga also forms solitary cells, mucilaginous colonies, amoebae, and teardrop-shaped zoids. We also investigated the morphologically similar genus Chrysotilos, isolated from its type locality in the Austrian Alps. In culture, Chrysotilos showed characteristic neustonic growth, which was, however, not observed in Globulochrysis. Phylogenetic analyses based on 18S rRNA and rbcL genes showed that these two algae belong to two distinct lineages. Whereas Chrysotilos was inferred within the clade Hibberdiales, Globulochrysis formed a completely new lineage within the order Hydrurales.

Spatio-temporal connectivity of a toxic cyanobacterial community and its associated microbiome along a freshwater-marine continuum

Due to climate changes and eutrophication, blooms of predominantly toxic freshwater cyanobacteria are intensifying and are likely to colonize estuaries, thus impacting benthic organisms and shellfish farming representing a major ecological, health and economic risk. In the natural environment, Microcystis form large mucilaginous colonies that influence the development of both cyanobacterial and embedded bacterial communities. However, little is known about the fate of natural colonies of Microcystis by salinity increase. In this study, we monitored the fate of a Microcystis dominated bloom and its microbiome along a French freshwater-marine gradient at different phases of a bloom. We demonstrated changes in the cyanobacterial genotypic composition, in the production of specific metabolites (toxins and compatible solutes) and in the heterotrophic bacteria structure in response to the salinity increase. In particular M. aeruginosa and M. wesenbergii survived salinities up to 20. Based on microcystin gene abundance, the cyanobacteria became more toxic during their estuarine transfer but with no selection of specific microcystin variants. An increase in compatible solutes occurred along the continuum with extensive trehalose and betaine accumulations. Salinity structured most the heterotrophic bacteria community, with an increased in the richness and diversity along the continuum. A core microbiome in the mucilage-associated attached fraction was highly abundant suggesting a strong interaction between Microcystis and its microbiome and a likely protecting role of the mucilage against an osmotic shock. These results underline the need to better determine the interactions between the Microcystis colonies and their microbiome as a likely key to their widespread success and adaptation to various environmental conditions.

Invasive diatom, Cymbella janischii adhering on body surface of native aquatic beetles, Grouvellinus nitidus (Elmidae) and Ectopria opaca (Psephenidae) in Japan

AbstractCymbella janischii, an invasive species of diatom, has been broadening its geographical range in Japan since 2004, flourishing in substantial colonies in river beds. As thick and mucilaginous colonies of this diatom often cover large areas of river beds, this elicits concerns regarding the potential alteration of sedimentary bottom structures and resultant effects on the native ecosystems. The presence of the diatoms growing on the body surface of two aquatic beetles, Grouvellinus nitidus (Elmidae) and Ectopria opaca (Psephenidae) in a Japanese river was confirmed. This constitutes the inaugural documentation of invasive diatoms having a specific relationship with native aquatic insects at an individual level.

Impacts of small dams on the phytoplankton and invertebrates in an Amazonian river

AbstractThe construction of small hydroelectric plants (SHPs) has increased greatly worldwide, and the impacts of these impoundments can cause changes in the aquatic biota. Here, we investigated the ecological attributes (abundance, richness, and species diversity) of three communities (phytoplankton, invertebrates in the water column, and benthos) and the functional groups of phytoplankton, before and after the construction of five SHPs in an Amazonian river. The communities were sampled at two sites, one upstream and another downstream of each dam or construction site, before the dam closure. The dam did not change the limnological factors, and the acid waters and low electrical conductivity, typical of “cerrado” rivers, were characteristic of both the pre‐ and post‐dam phases. For aquatic communities, the phytoplankton abundance increased and the diversity of groups decreased in the post‐dam phase. Regarding the morphology‐based functional groups (MBFG), there was an increase in groups I (small algae with a high surface/volume ratio) and V (phytoflagellates), with a reduction in groups VI (diatoms) and VII (large mucilaginous colonies) in the post‐dam phase. The relative abundance of zooplankton species increased in the post‐dam phase, and the opposite was observed for the benthic community, with reductions in abundance, richness, and group diversity in the post‐dam phase. The structure of the three communities differed between the pre‐ and post‐dam phases, but no difference in communities' similarity was observed between the upstream and downstream sites in both phases. The results show that the change in hydrodynamics, from lotic to semi‐lentic, in the reservoirs of the SHPs, was the main driving force for the alterations in the three communities. For phytoplankton, SHPs are favorable in the sense of decreasing the incidence of algal blooms, particularly of cyanobacteria.

Single-colony sequencing reveals microbe-by-microbiome phylosymbiosis between the cyanobacterium Microcystis and its associated bacteria

BackgroundCyanobacteria from the genus Microcystis can form large mucilaginous colonies with attached heterotrophic bacteria—their microbiome. However, the nature of the relationship between Microcystis and its microbiome remains unclear. Is it a long-term, evolutionarily stable association? Which partners benefit? Here we report the genomic diversity of 109 individual Microcystis colonies—including cyanobacteria and associated bacterial genomes—isolated in situ and without culture from Lake Champlain, Canada and Pampulha Reservoir, Brazil.ResultsWe identified 14 distinct Microcystis genotypes from Canada, of which only two have been previously reported, and four genotypes specific to Brazil. Microcystis genetic diversity was much greater between than within colonies, consistent with colony growth by clonal expansion rather than aggregation of Microcystis cells. We also identified 72 bacterial species in the microbiome. Each Microcystis genotype had a distinct microbiome composition, and more closely related genotypes had more similar microbiomes. This pattern of phylosymbiosis could be explained by co-phylogeny in only two out of the nine most prevalent associated bacterial genera, Roseomonas and Rhodobacter. These phylogenetically associated genera could enrich the metabolic repertoire of Microcystis, for example by encoding the biosynthesis of complementary carotenoid molecules. In contrast, other colony-associated bacteria showed weaker signals of co-phylogeny, but stronger evidence of horizontal gene transfer with Microcystis. These observations suggest that acquired genes are more likely to be retained in both partners (Microcystis and members of its microbiome) when they are loosely associated, whereas one gene copy is sufficient when the association is physically tight and evolutionarily long-lasting.ConclusionsWe have introduced a method for culture-free isolation of single colonies from nature followed by metagenomic sequencing, which could be applied to other types of microbes. Together, our results expand the known genetic diversity of both Microcystis and its microbiome in natural settings, and support their long-term, specific, and potentially beneficial associations.A9SU3YME_Rf3MWzGs5jm1TVideo

Using morphofunctional characteristics as a model of phytoplankton dynamics in a tropical reservoir

The study of phytoplankton morphological characteristics, using the morphology-based functional classification (MBFG), reflects the different abilities for absorption of light and nutrients, growth, mechanisms for preventing sedimentation and predation in the aquatic ecosystem and can be a model in simplification of the phytoplankton community dynamics in the reservoirs. This study that was developed in a tropical reservoir aimed to evaluate which phytoplankton morphofunctional characteristics are selected by the spatial and temporal heterogeneity in the reservoir, and how these characteristics are related to the environmental conditions. For this, phytoplankton and environmental variables were sampled in different sites along the spatial extent of the reservoir during dry and rainy periods. The lotic region showed the greatest environmental dissimilarity in relation to the other regions, both in the dry and rainy periods. Differences in density of all MBFG were verified along the spatial extent of the reservoir, with higher values in the lentic region, while differences between hydrological periods were verified for MBFG I (small organisms with a high surface/volume ratio), MBFG IV (medium-sized organisms without specialized traits) and MBFG VII (large mucilaginous colonies). The conditions of pH, temperature and turbidity were important in the selection of the morphofunctional characteristics and in MBFG dynamics. Therefore, the use of MBFG can be a form of simplification of the relationship between phytoplankton and environmental heterogeneity, reflecting the reservoir functioning.

The global scientific literature on applications and trends in the use of functional morphological groups in phytoplankton studies

Abstract: Aim The application of deconstructive approaches in aquatic ecology has been increasing recently. Especially for phytoplankton, some functional classifications summarize similar traits of a group of species to understand organisms’ response to landscape variability. One of these approaches deals with phytoplankton functional classification based on morphology (MBFG - Morphologically Based Functional Groups). Focusing on this approach, we systematic mapping the scientific literature to reveal this functional framework´s applications for freshwater phytoplankton. Methods For this study, we selected from the Thomson ISI Web of Science database all articles published between 2010 and 2018 dealing with MBFG. We recorded 179 manuscripts citing the phytoplankton functional classification based on morphology and, among them, we excluded three due to lack of access to information. Results A clear temporal trend occurred with an increase in citations involving the morphological approach, with Brazil, Uruguay, and China as the countries with the highest number of studies. Of the total records, 60 manuscripts applied morphological classification in their studies, of which 23 manuscripts comprised comparative studies with other functional approaches. Most applications were for phytoplankton in lakes, with biomass being the most used metric for framing taxa in MBFG. The most often recorded groups are MBFG IV (medium-sized organisms without specialization), VII (large mucilaginous colonies), and III (large filamentous organisms with aerotopes). Conclusion This study showed an increasing trend in the number of studies that used the functional approach based on MBFG. We believe that deconstructive approaches, such as MBFG, help assess issues of interest in phytoplankton ecology.

Habitat preferences in freshwater benthic macroinvertebrates: Algae as substratum and food resource in high mountain rivers from Mexico

The diversity of benthic macroinvertebrates in lotic systems is closely related to the availability and heterogeneity of habitats. These habitats may be of inorganic origin, such as the rocky substratum associated with the river bed, or organic such as macroscopic algae. The objective of this study was to determinate the importance of five species of algae that differ in their morphological type as a substratum and food resource regarding the associated establishment of macroinvertebrate assemblages taking account the climatic seasonality (warm dry, cool dry and rainy). We then evaluate the differences in macroinvertebrates assemblages with respect to the inorganic substratum by sampling high mountain rivers in central Mexico. The mucilaginous colonies of Nostoc parmelioides and Placoma regulare, the pseudoparenchymatous bambusiform thallus of Paralemanea mexicana and the laminate thallus of Prasiola mexicana had the highest densities of macroinvertebrates, represented by the genera Cricotopus, Paramerina, Simulium and Tanytarsini tribe. The relationship between algal morphological type and the richness and diversity of macroinvertebrates was positively related to specific conductivity, total dissolved solids and discharge variables. The dominant taxa associated with the inorganic substratum belonged mainly to the Trichoptera, Diptera and Ephemeroptera orders. Water temperature, discharge and concentration of orthophosphates were the main environmental variables able to explain the diversity of macroinvertebrates on this substratum. The dominance of detritivorous macroinvertebrates in these mountain rivers suggests the contribution of allochthonous organic matter possibly of anthropogenic origin. The assemblages of macroinvertebrates on inorganic substratum did not significantly differ among sites or climatic seasons.

A multilevel trait-based approach to the ecological performance of Microcystis aeruginosa complex from headwaters to the ocean

The Microcystis aeruginosa complex (MAC) clusters cosmopolitan and conspicuous harmful bloom-forming cyanobacteria able to produce cyanotoxins. It is hypothesized that low temperatures and brackish salinities are the main barriers to MAC proliferation. Here, patterns at multiple levels of organization irrespective of taxonomic identity (i.e. a trait-based approach) were analyzed. MAC responses from the intracellular (e.g. respiratory activity) to the ecosystem level (e.g. blooms) were evaluated in wide environmental gradients. Experimental results on buoyancy and respiratory activity in response to increased salinity (0–35) and a literature review of maximum growth rates under different temperatures and salinities were combined with field sampling from headwaters (800km upstream) to the marine end of the Rio de la Plata estuary (Uruguay-South America). Salinity and temperature were the major variables affecting MAC responses. Experimentally, freshwater MAC cells remained active for 24h in brackish waters (salinity=15) while colonies increased their flotation velocity. At the population level, maximum growth rate decreased with salinity and presented a unimodal exponential response with temperature, showing an optimum at 27.5°C and a rapid decrease thereafter. At the community and ecosystem levels, MAC occurred from fresh to marine waters (salinity 30) with a sustained relative increase of large mucilaginous colonies biovolume with respect to individual cells. Similarly, total biomass and, specific and morphological richness decreased with salinity while blooms were only detected in freshwater both at high (33°C) and low (11°C) temperatures. In brackish waters, large mucilaginous colonies presented advantages under osmotic restrictive conditions. These traits values have also been associated with higher toxicity potential. This suggest salinity or low temperatures would not represent effective barriers for the survival and transport of potentially toxic MAC under likely near future scenarios of increasing human impacts (i.e. eutrophication, dam construction and climate change).

Environmental dissimilarity over time in a large subtropical shallow lake is differently represented by phytoplankton functional approaches

The aim of the present study was to understand how different phytoplankton functional approaches responded to environmental variability in a large shallow lake, namely Lake Mangueira, in southern Brazil. This coastal lake has a maximum depth of 7m and is ~90km long and ~3–10km wide. Physical and chemical variables, maximum linear dimension, life forms, functional and morphofunctional groups of phytoplankton were analysed. The results showed that the phytoplankton were primarily comprised Cyanobacteria (63.9%), followed by middle-sized algae (21–50μm; 46.7%) and colonial non-flagellated taxa (63%). The highest percentage of total biomass was accounted for by the functional group ‘K’ (as classified by Reynolds et al. 2002; 36.3%), large mucilaginous colonies (46.79%) according to morphologically based functional classification and other large colonies (mostly non-vacuolated; 36.7%) according to morphofunctional classification. Dissimilarity analysis indicated a significant correlation between abiotic data and functional approaches (P=0.001). The dissimilarity in the functional compositions of phytoplankton was related to nutrient and light conditions, especially to silicon content and water transparency, in all functional approaches. The highest correlation with abiotic variables was seen for morphologically based functional group community structure, although the functional group composition of phytoplankton sensu Reynolds et al. seemed to be the most effective system in describing environmental variability in Lake Mangueira over the long term.

Why predation is not a controlling factor of phytoplankton in a Neotropical shallow lake: a morpho-functional perspective

This work explores the reasons why predation plays a minor role in structuring phytoplankton composition in a Neotropical shallow lake. Phytoplankton, zooplankton, and fish were sampled from a shallow lake over the course of a year, and the stomach contents of 80 individuals of the dominant omnivorous–planktivorous fish species were analyzed. The field study was complemented with a 5-day microcosm experiment in which the predation effects of micro, meso, and macrozooplankton were measured. Stomach content analysis revealed that fish predation was high, and primarily comprised by Cladocera (Ivlev’s index >0.75). Both meso and macrozooplankton fractions are able to feed on colonial cyanobacteria, silica cell-wall organisms ( 35 µm). Macrozooplankton, however, can feed on single cells, mucilaginous colonies, non-mucilaginous colonies, and silica cell-wall organisms (>35 µm) (P < 0.05 in all cases). Our results support the idea that absence of predation on phytoplankton is mainly mediated by fish predation on zooplankton and morpho-functional characteristics of algae which prevent zooplankton predation; showing fish a lack of direct predation effect on phytoplankton.

Mexican Strains ofHirsutellaIsolated fromDiaphorina citri(Hemiptera: Liviidae): Morphologic and Molecular Characterization

Diaphorina citri Kuwayama (Hemiptera: Liviidae), the vector of huanglongbing, has been found in several citrus areas of Mexico, apparently attacked by the fungus Hirsutella citriformis Speare (Hypocreales: Ophiocordycipitaceae). As this entomopathogen could represent a potential agent of D. citri biological control in the country, we performed this study to characterize 7 strains of the fungus isolated from this insect. For the molecular characterization we used internal transcribed spacer sequences 1 and 2 (ITSs 1 and 2) and 28S rDNA. In the morphological description we determined size of reproductive structures in the strains as well as their blastospore production capacity. BLAST analysis of the ITSs sequences showed less than 91% and 72% of identity and coverage with Ophiocordyceps sinensis or H. citriformis; meanwhile, gen 28S sequences showed high identity (98–99%) and coverage (98–100%) with O. elongata (Petch) G.H. Sung, J.M. Sung, Hywel-Jones & Spatafora and O. coccidiicola (Kobayasi) G.H. Sung, J.M. Sung, Hywel-Jones & Spatafora. The phylogenetic analysis using 28S sequences demonstrated that the Mexican strains are closely related with H. citriformis; however, size of phialide and conidial diameter showed differences with dimensions reported for the species in the original description. Under dark conditions, 6 strains were able to produce mucilaginous colonies that contained blastospores; these structures caused 30.8–41.2% mortality in the target insect. The results suggest that Hirsutella strains isolated from D. citri in Mexico, correspond to H. citriformis. We found dimorphic capacity and variability in size of the reproductive structures of the fungus. This is the first report of ITSs and 28S sequences of H. citriformis obtained from strains isolated from D. citri

The Northernmost Populations of Tetraspora Gelatinosa (Chlorophyta) from Spitsbergen

Abstract This article describes the morphological characteristics of the populations of green alga, Tetraspora gelatinosa, growing in the stressful Arctic conditions (77°00’22” N, 015°32’54.33” E). We present the first detailed morphological characteristics of this spe-cies from such a high latitude. Populations from both stagnant and flowing waters were studied. Depending on the type of habitat, their mucilaginous colonies (thalli) have differ-ent shapes, but the structure, size and the placement of the vegetative cells, akinetes and ameboid forms, as well as the pseudocilia morphology of both populations, were very simi-lar. Literature data on the distribution of T. gelatinosa indicate that it is a cosmopolitan spe-cies. Our data are compared with some characteristic features of this species growing in dif-ferent geographical and climatic zones. No significant differences were found in the morphology of the colonies compared, nor in the location and the inner structure of cells. How-ever, there were slight differences in cell size between the populations from warm and cold zones.

Edible Cyanobacteria (&lt;em&gt;Nostochopsis&lt;/em&gt; spp.) from Glass House, Queen Sirikit Botanical Garden, Thailand

<em>Nostochopsis</em> spp. are edible and rare cyanobacteria which form thick mucilaginous colonies, 0.1-8 cm in size, attached on the rocks or cobbles in transparent shallow streams or rivers. They are classified in the Order Nostocales, Family Hapalosiphonaceae. The objective of this study was to investigate the colonial growth, pigments and quality of water in the ponds at 3 glass houses in the Queen Sirikit Botanic Garden, Chiang Mai Province in which these cyanobacteria were growing during April-June 2012. The three glass houses were; the Aquatic house, the Bromeliad house and Orchids and Fern house. Ten colonies from each sampling site were measured once a week. The average increase in colonial size was found to be 0.17±0.06, 0.30±0.08 and 0.15±0.08 cm/week respectively. Chlorophyll, phycocyanin, allophycocyanin and carotenoid were highest in the samples from Aquatic house as 16.22±4.28, 11.95±8.55, 73.62±4.07 and 12.70±1.54 mg/g.dw, respectively. These cyanobacteria grew at 22-30°C, pH 6.17-8.75 and conductivity 112-171 μs/cm. The water quality was clean-moderate and in oligo- mesotrophic status.

Morphofunctional traits reflect differences in phytoplankton community between rivers of contrasting flow regime

A morphofunctional traits approach has been adopted to identify how environmental factors shape the phytoplankton community. This approach has been applied in two rivers where hydrodynamical conditions are expected to be the main factor acting on the phytoplankton community. Hence, morphological traits (motility, shape, size, mucilage, and silica) related to sedimentation resistance have been chosen. We have shown that differences in flow regulation through differences in flow velocities induce shifts in phytoplankton community. These shifts depend mostly on shape, buoyancy regulation, and motility of phytoplankton cells. Elongated shapes are the characteristic of unregulated sites with high flow velocities, while cells able to regulate actively their position or to reduce their density (mucilaginous colonies) are found in regulated sites with low flow velocities. Flattened shapes are also the characteristic of sites with rather low flow velocities. These results highlight the key role of flow velocity as a driving factor controlling the structure of phytoplankton community. In this study, flow velocity also structures phytoplankton community according to location rather than seasonality.

A new freshwater chrysophyte, &lt;i&gt;Chrysomorula cohaerens gen. et sp. nov.&lt;/i&gt; (Chrysophyceae, Chrysocapsaceae) from North America

A new freshwater chrysophyte genus and species, Chrysomorula cohaerens, is described from the United States. Chrysomorula cohaerens is a distinct taxon of non-motile heterokont chrysophyte. It forms macroscopic mucilaginous colonies attached to aquatic macrophytes, filamentous algae, and other substrata. Each colony is initially held together forming a hollow sphere by the confluence of its cell walls. The colony is at first a sphere, becoming ovoid to irregular with growth in size. Thus, it differs from other similar chrysophyte genera such as Heimiochrysis and Chalkopyxis. Cells bear pseudocilia.

Mass developments of the cyanobacteria Anabaenopsis and Cyanospira (Nostocales) in the soda lakes of Kenya: ecological and systematic implications

The food web of the saline–alkaline lakes of East Africa is characterised by a unique interaction between the Lesser Flamingos as consumer birds and the cyanobacterium Arthrospira fusiformis as the primary producer. However, this interaction is disturbed frequently by alterations of the phytoplankton community. During the period 2001–2012, species of the cyanobacteria Anabaenopsis and Cyanospira were observed in four soda lakes of the African Rift Valley and compared to the entire phytoplankton biomass and composition. Their morphology was highly variable. Each species preferred a distinct range of salinity: C. capsulata 30–40 ppt, C. rippkae 25–35 ppt, A. arnoldii and A. abijatae 10–30 ppt, and A. elenkinii 0–15 ppt. Occasional dominance of Anabaenopsis and Cyanospira in the lakes investigated shows that members of these genera are serious competitors of A. fusiformis, the main food for Lesser Flamingos. Furthermore, mass developments of C. capsulata adversely affected food uptake by the flamingos at Lake Bogoria because they formed mucilaginous colonies that clogged the food filter system. From field samples of the three lakes, uncultured Anabaenopsis and Cyanospira spp. clones were obtained and subjected to phylogenetic analyses. The 16S rRNA gene sequencing data put into doubt the differentiation of Anabaenopsis and Cyanospira into separate genera as recently suggested.

Phylogenetic and taxonomic delimitation of the cyanobacterial genus Aphanothece and description of Anathece gen. nov.

The traditional cyanobacterial form-genus Aphanothece was established according to morphological criteria in the middle of the 19th century by Nägeli. It comprises unicellular morphotypes with oval or cylindrical cells arranged irregularly in amorphous mucilaginous colonies. Up to now, more than 80 morphospecies have been described, mostly from various freshwater aquatic habitats. The genus Aphanothece is heterogeneous in the traditional morphological concept as well as according to modern molecular criteria. Therefore, it must be divided into several taxonomic clusters. Generic characteristics, studied by a combination of molecular sequencing of 16S rDNA, ultrastructure, ecology and morphological variation, were investigated in selected strains and populations of type species of the traditional subgenera Aphanothece and Anathece. Distinct differences were found between both of these clusters in their position in the phylogenetic tree, ultrastructure of cells, morphology (size of cells), mucilage structure, life cycles and ecology. They must therefore be separated into two genera, according to both bacteriological and botanical criteria, and placed in different orders (Chroococcales and Synechococcales). Accordingly we describe Anathece, gen. et stat. nov., and transfer to it the type A. clathrata comb. nov. and six other species. Several other morphotypes, originally classified in the genus Aphanothece (but morphologically different from Aphanothece sensu stricto), correspond to the genera Cyanoaggregatum, Cyanogastrum, Gloeobacter, Gloeothece and Halothece. The closest relative of Anathece appears to be Cyanobium.

Resource synergy in stream periphyton communities

Summary 1. Light and nutrients play pivotal roles in determining the growth of autotrophs, yet the potential for synergistic interactions between the two resources in algal communities is poorly understood, especially in stream ecosystems. In this study, light and phosphorus were manipulated in large experimental streams to examine resource colimitation and synergy in stream periphyton. 2. Whole‐stream metabolism was simultaneously limited by light and phosphorus. Increasing the supply of either light or phosphorus resulted in significant increases in primary production and the transformation of the streams from heterotrophy to autotrophy. 3. Resource‐driven changes in periphyton community structure occurred in concert with changes in production. Algal assemblages in highly shaded streams were composed primarily of small diatoms such as Achnanthidium minutissima, whereas larger diatoms such as Melosira varians predominated at higher irradiances. Phosphorus enrichment had relatively little effect on assemblage structure, but it did substantially diminish the abundance of Meridion circulare, a diatom whose mucilaginous colonies were conspicuously abundant in phosphorus‐poor, high‐light streams. Bacterial biomass declined relative to algal biomass with increases in primary productivity, regardless of whether the increases were caused by light or phosphorus. 4. Synergistic effects on primary production appeared to occur because the availability of one resource facilitated the utilization of the other. Light increased the abundance of large diatoms, which are known to convert high concentrations of nutrients into primary production more effectively than smaller taxa. Phosphorus enrichment led to the replacement of Meridion circulare by non‐mucilaginous taxa in phosphorus‐enriched streams, and we hypothesize that this change enabled more efficient use of light in photosynthesis. Higher ratios of chlorophyll a : biomass in phosphorus‐enriched streams may have also led to more efficient photon capture and higher photosynthetic rates. 5. Synthesis. Our results underscore the potential for resource colimitation, even in habitats where a single resource is as strongly limiting as is light in shaded streams. The capacity of autotrophic communities to respond to more than one limiting resource suggests that prevailing single‐resource models of ecosystem productivity are overly simplistic.

COLONY SIZE OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AS INFLUENCED BY ZOOPLANKTON GRAZERS(1).

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer-induced morphological response exists in P.antarctica, we conducted incubation experiments during the austral summer using natural P.antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P.antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial-to-solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P.antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P.antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.