Global biodiversity patterns of marine phytoplankton and zooplankton

Lack of congruence in species diversity indices and community structures of planktonic groups based on local environmental factors

The importance of analyzing the determinants of biodiversity and community composition by using multiple trophic levels is well recognized; however, relevant data are lacking. In the present study, we investigated variations in species diversity indices and community structures of the plankton taxonomic groups-zooplankton, rotifers, ciliates, and phytoplankton-under a range of local environmental factors in pond ecosystems. For each planktonic group, we estimated the species diversity index by using linear models and analyzed the community structure by using canonical correspondence analysis. We showed that the species diversity indices and community structures varied among the planktonic groups and according to local environmental factors. The observed lack of congruence among the planktonic groups may have been caused by niche competition between groups with similar trophic guilds or by weak trophic interactions. Our findings highlight the difficulty of predicting total biodiversity within a system, based upon a single taxonomic group. Thus, to conserve the biodiversity of an ecosystem, it is crucial to consider variations in species diversity indices and community structures of different taxonomic groups, under a range of local conditions.

Can we detect oceanic biodiversity hotspots from space?

Understanding the variability of marine biodiversity is a central issue in microbiology. Current observational programs are based on in situ studies, but their implementation at the global scale is particularly challenging, owing to the ocean extent, its temporal variability and the heterogeneity of the data sources on which compilations are built. Here, we explore the possibility of identifying phytoplanktonic biodiversity hotspots from satellite. We define a Shannon entropy index based on patchiness in ocean color bio-optical anomalies. This index provides a high resolution (1 degree) global coverage. It shows a relation to temperature and mid-latitude maxima in accordance with those previously evidenced in microbiological biodiversity model and observational studies. Regional maxima are in remarkable agreement with several known biodiversity hotspots for plankton organisms and even for higher levels of the marine trophic chain, as well as with some in situ planktonic biodiversity estimates (from Atlantic Meridional Transect cruise). These results encourage to explore marine biodiversity with a coordinated effort of the molecular, ecological and remote sensing communities.

HOW MANY SPECIES OF ALGAE ARE THERE?

Algae have been estimated to include anything from 30,000 to more than 1 million species. An attempt is made here to arrive at a more accurate estimate using species numbers in phyla and classes included in the on-line taxonomic database AlgaeBase (http://www.algaebase.org). Despite uncertainties regarding what organisms should be included as algae and what a species is in the context of the various algal phyla and classes, a conservative approach results in an estimate of 72,500 algal species, names for 44,000 of which have probably been published, and 33,248 names have been processed by AlgaeBase to date (June 2012). Some published estimates of diatom numbers are of over 200,000 species, which would result in four to five diatom species for every other algal species. Concern is expressed at the decline and potential extinction of taxonomists worldwide capable of improving and completing the necessary systematic studies.

Vertical niche partitioning between cryptic sibling species of a cosmopolitan marine planktonic protist

A large portion of the surface-ocean biomass is represented by microscopic unicellular plankton. These organisms are functionally and morphologically diverse, but it remains unclear how their diversity is generated. Species of marine microplankton are widely distributed because of passive transport and lack of barriers in the ocean. How does speciation occur in a system with a seemingly unlimited dispersal potential? Recent studies using planktonic foraminifera as a model showed that even among the cryptic genetic diversity within morphological species, many genetic types are cosmopolitan, lending limited support for speciation by geographical isolation. Here we show that the current two-dimensional view on the biogeography and potential speciation mechanisms in the microplankton may be misleading. By depth-stratified sampling, we present evidence that sibling genetic types in a cosmopolitan species of marine microplankton, the planktonic foraminifer Hastigerina pelagica, are consistently separated by depth throughout their global range. Such strong separation between genetically closely related and morphologically inseparable genetic types indicates that niche partitioning in marine heterotrophic microplankton can be maintained in the vertical dimension on a global scale. These observations indicate that speciation along depth (depth-parapatric speciation) can occur in vertically structured microplankton populations, facilitating diversification without the need for spatial isolation.

The nutrient-load hypothesis: patterns of resource limitation and community structure driven by competition for nutrients and light

Resource competition theory predicts that the outcome of competition for two nutrients depends on the ratio at which these nutrients are supplied. Yet there is considerable debate whether nutrient ratios or absolute nutrient loads determine the species composition of phytoplankton and plant communities. Here we extend the classical resource competition model for two nutrients by including light as additional resource. Our results suggest the nutrient-load hypothesis, which predicts that nutrient ratios determine the species composition in oligotrophic environments, whereas nutrient loads are decisive in eutrophic environments. The underlying mechanism is that nutrient enrichment shifts the species interactions from competition for nutrients to competition for light, which favors the dominance of superior light competitors overshadowing all other species. Intermediate nutrient loads can generate high biodiversity through a fine-grained patchwork of two-species and three-species coexistence equilibria. Depending on the species traits, however, competition for nutrients and light may also produce multiple alternative stable states, suppressing the predictability of the species composition. The nutrient-load hypothesis offers a solution for several discrepancies between classical resource competition theory and field observations, explains why eutrophication often leads to diversity loss, and provides a simple conceptual framework for patterns of biodiversity and community structure observed in nature.

Large-scale biodiversity patterns in freshwater phytoplankton

Our planet shows striking gradients in the species richness of plants and animals, from high biodiversity in the tropics to low biodiversity in polar and high-mountain regions. Recently, similar patterns have been described for some groups of microorganisms, but the large-scale biogeographical distribution of freshwater phytoplankton diversity is still largely unknown. We examined the species diversity of freshwater phytoplankton sampled from 540 lakes and reservoirs distributed across the continental United States and found strong latitudinal, longitudinal, and altitudinal gradients in phytoplankton biodiversity, demonstrating that microorganisms can show substantial geographic variation in biodiversity. Detailed analysis using structural equation models indicated that these large-scale biodiversity gradients in freshwater phytoplankton diversity were mainly driven by local environmental factors, although there were residual direct effects of latitude, longitude, and altitude as well. Specifically, we found that phytoplankton species richness was an increasing saturating function of lake chlorophyll a concentration, increased with lake surface area and possibly increased with water temperature, resembling effects of productivity, habitat area, and temperature on diversity patterns commonly observed for macroorganisms. In turn, these local environmental factors varied along latitudinal, longitudinal, and altitudinal gradients. These results imply that changes in land use or climate that affect these local environmental factors are likely to have major impacts on large-scale biodiversity patterns of freshwater phytoplankton.

Quantifying the deep-sea rock and fossil record bias using coccolithophores

While many studies show a correlation between observed taxonomic richness and various measures of geological sampling, all have been based on the same record of terrestrial and marine sediments collected from the land. Here we present the first analyses of how rock and fossil records vary in the deep-sea. We have developed a novel database of species occurrences of coccolithophores sampled during major drilling programs of the North Atlantic, including the Mediterranean and Caribbean. Our sampling proxy, the number of deep-sea sites sampled – perhaps the most direct measure of sampling used so far – shows an exponential rise towards the Recent. Over the same period species-richness has grown in an approximately linear fashion, but genus-level richness shows a sharp initial increase followed by a much slower decline. However, correlations between both richness measures and sampling are extremely strong and a model assuming true diversity to be constant accurately predicts much of observed richness. We conclude that the deep-sea fossil record, like its land-based counterpart, bears a rock record bias.

A holistic approach to marine eco-systems biology

The structure, robustness, and dynamics of ocean plankton ecosystems remain poorly understood due to sampling, analysis, and computational limitations. The Tara Oceans consortium organizes expeditions to help fill this gap at the global level.

Productivity-diversity relationships in lake plankton communities

One of the most intriguing environmental gradients connected with variation in diversity is ecosystem productivity. The role of diversity in ecosystems is pivotal, because species richness can be both a cause and a consequence of primary production. However, the mechanisms behind the varying productivity-diversity relationships (PDR) remain poorly understood. Moreover, large-scale studies on PDR across taxa are urgently needed. Here, we examined the relationships between resource supply and phyto-, bacterio-, and zooplankton richness in 100 small boreal lakes. We studied the PDR locally within the drainage systems and regionally across the systems. Second, we studied the relationships between resource availability, species richness, biomass and resource ratio (N:P) in phytoplankton communities using Structural Equation Modeling (SEM) for testing the multivariate hypothesis of PDR. At the local scale, the PDR showed variable patterns ranging from positive linear and unimodal to negative linear relationships for all planktonic groups. At the regional scale, PDRs were significantly linear and positive for phyto- and zooplankton. Phytoplankton richness and the amount of chlorophyll a showed a positive linear relationship indicating that communities consisting of higher number of species were able to produce higher levels of biomass. According to the SEM, phytoplankton biomass was largely related to resource availability, yet there was a pathway via community richness. Finally, we found that species richness at all trophic levels was correlated with several environmental factors, and was also related to richness at the other trophic levels. This study showed that the PDRs in freshwaters show scale-dependency. We also documented that the PDR complies with the multivariate model showing that plant biomass is not mirroring merely the resource availability, but is also influenced by richness. This highlights the need for conserving diversity in order to maintain ecosystem processes in freshwaters.

Strain-related physiological and behavioral effects of Skeletonema marinoi on three common planktonic copepods

Three strains of the chain-forming diatom Skeletonema marinoi, differing in their production of polyunsaturated aldehydes (PUA) and nutritional food components, were used in experiments on feeding, egg production, hatching success, pellet production, and behavior of three common planktonic copepods: Acartia tonsa, Pseudocalanus elongatus, and Temora longicornis. The three different diatom strains (9B, 1G, and 7J) induced widely different effects on Acartia tonsa physiology, and the 9B strain induced different effects for the three copepods. In contrast, different strains induced no or small alterations in the distribution, swimming behavior, and turning frequency of the copepods. 22:6(n-3) fatty acid (DHA) and sterol content of the diet typically showed a positive effect on either egg production (A. tonsa) or hatching success (P. elongatus), while other measured compounds (PUA, other long-chain polyunsaturated fatty acids) of the algae had no obvious effects. Our results demonstrate that differences between strains of a given diatom species can generate effects on copepod physiology, which are as large as those induced by different algae species or groups. This emphasizes the need to identify the specific characteristics of local diatoms together with the interacting effects of different mineral, biochemical, and toxic compounds and their potential implications on different copepod species.

The Humpbacked Species Richness-Curve: A Contingent Rule for Community Ecology

Functional relationships involving species richness may be unimodal, monotonically increasing, monotonically decreasing, bimodal, multimodal, U-shaped, or with no discernable pattern. The unimodal relationships are the most interesting because they suggest dynamic, nonequilibrium community processes. For that reason, they are also contentious. In this paper, we provide a wide-ranging review of the literature on unimodal (humpbacked) species richness-relationships. Though not as widespread as previously thought, unimodal patterns of species richness are often associated with disturbance, predation and herbivory, productivity, spatial heterogeneity, environmental gradients, time, and latitude. These unimodal patterns are contingent on organism and environment; we examine unimodal species richness-curves involving plants, invertebrates, vertebrates, plankton, and microbes in marine, lacustrine, and terrestrial habitats. A goal of future research is to understand the contingent patterns and the complex, interacting processes that generate them.

Size-structured planktonic ecosystems: constraints, controls and assembly instructions

Here we present a nutrient-phytoplankton-zooplankton (NPZ) model that has arbitrary size-resolution within the phytoplankton- and zooplankton-state variables. The model assumes allometric scaling of biological parameters. This particular version of the model (herbivorous zooplankton only) has analytical solutions that allow efficient exploration of the effects of allometric dependencies of various biological processes on the model's equilibrium solutions. The model shows that there are constraints on the possible combinations of allometric scalings of the biological rates that will allow ecosystems to be structured as we observe (larger organisms added as the total biomass increases). The diversity (number of size classes occupied) of the ecosystem is the result of simultaneous bottom-up and top-down control: resources determine which classes can exist; predation determines which classes do exist. Thus, the simultaneous actions of bottom-up and top-down controls are essential for maintaining and structuring planktonic ecosystems. One important conclusion from this model is that there are multiple, independent ways of obtaining any given biomass spectrum, and that the spectral slope is not, in and of itself, very informative concerning the underlying dynamics. There is a clear need for improved size-resolved field measurements of biological rates; these will both elucidate biological processes in the field, and allow strong testing of size-structured models of planktonic ecosystems.

Composition and genetic diversity of picoeukaryotes in subtropical coastal waters as revealed by 454 pyrosequencing

Information on genetic diversity of picoeukaryotes (<2-3 microm) comes mainly from traditional gene cloning and sequencing, but this method suffers from cloning biases and limited throughput. In this study, we explored the feasibility of using the cloning-independent and massively parallel 454 pyrosequencing technology to study the composition and genetic diversity of picoeukaryotes in the coastal waters of the subtropical western Pacific using the hypervariable V4 region of the 18S rRNA gene. Picoeukaryote assemblages between two sites with different hydrography and trophic status were also compared. The approach gave a high coverage of the community at genetic difference > or =5% but still underestimated the total diversity at a genetic difference < or =2%. Diversity of picoeukaryotes was higher in an oligomesotrophic bay than in a eutrophic bay. Stramenopiles, dinoflagellates, ciliates and prasinophytes were the dominant groups comprising approximately 27, 19, 11 and 11%, respectively, of the picoeukaryotes. Water samples collected from the two bays contained different high-level taxonomic groups and phylotype operational taxonomic units of picoeukaryotes. Our study represents one of the first and most comprehensive examinations of marine picoeukaryotic diversity using the 454 sequencing-by-synthesis technology.

MOLECULAR DIVERSITY OF MARINE PHYTOPLANKTON COMMUNITIES BASED ON KEY FUNCTIONAL GENES(1)

The phylogeny and diversity of two key functional genes were investigated as the basis for improved understanding of the community structure of natural phytoplankton assemblages in marine environments. New partial NR (encoding eukaryotic assimilatory nitrate reductase) and rbcL (encoding LSU of RUBISCO) sequences from 10 cultured phytoplankton strains are reported. Phytoplankton community composition from Monterey Bay (MB), a coastal upwelling site on the California coast, and the Western English Channel (EC), a North Atlantic spring bloom environment, was elucidated based on NR and rbcL sequences. Diatoms were by far the most frequently detected group in both environments, consistent with their importance as a major bloom-forming group. Both NR and rbcL libraries contained sequences representing cosmopolitan types such as Emiliania huxleyi (Lohmann) W. W. Hay et H. P. Mohler, Phaeocystis, and Pseudo-nitzschia. The NR and rbcL libraries also contained sequences from other chromophytic algal groups and the Dinophyceae (alveolates). Sequences showing identity with key bloom-forming organisms including E. huxleyi, Phaeocystis pouchetii (Har.) Lagerh., Pseudo-nitzschia sp., and Thalassiosira sp. in the rbcL libraries confirm previous studies from these environments based on traditional approaches. Diversity/pattern analyses detected significant compositional differences among the libraries, which were consistent with patterns identified by phylogenetic analysis, but these patterns were not strongly correlated with obvious environmental variables such as temperature and nitrate concentration. Many new and divergent NR and rbcL sequences are reported, but the extent to which they represent unknown types cannot be determined until greater effort is made to sequence the existing culture collections.

Macroevolutionary turnover through the Ediacaran transition: ecological and biogeochemical implications

Ecological and evolutionary principles are often context-dependent, particularly where the context is biologically defined. Organ-grade animals (eumetazoans) are particularly powerful contextual agents, with a unique capacity to drive escalatory co-evolution and build multi-tiered food-webs. The evolution of eumetazoans through the Ediacaran and early Cambrian fundamentally altered macroecological and macroevolutionary dynamics, including the structure and function of the marine carbon cycle. Pelagic eumetazoans can be held responsible for driving the evolution of relatively large eukaryotic phytoplankton, thereby shifting the system from a turbid, stratified, cyanobacteria-dominated stable state to the clear-water, well-oxygenated, algae-dominated condition typical of the Phanerozoic. Intermittent return to the pre-Ediacaran state during Phanerozoic extinctions and oceanic anoxic events suggests that the widespread anoxia detected in pre-Ediacaran deep-marine sequences may be a consequence of this alternate biological pump rather than a reflection of fundamentally lower levels of atmospheric oxygen. The transition between the pre- and post-Ediacaran states is also associated with the oldest commercially exploitable hydrocarbons, a possible by-product of invading animals and their top-down impact on the biological pump.

Interannual variability in species composition explained as seasonally entrained chaos

The species composition of plankton, insect and annual plant communities may vary markedly from year to year. Such interannual variability is usually thought to be driven by year-to-year variation in weather conditions. Here we examine an alternative explanation. We studied the effects of regular seasonal forcing on a multi-species predator-prey model consisting of phytoplankton and zooplankton species. The model predicts that interannual variability in species composition can easily arise without interannual variability in external conditions. Seasonal forcing increased the probability of chaos in our model communities, but squeezed these irregular species dynamics within the seasonal cycle. As a result, the population dynamics had a peculiar character. Consistent with long-term time series of natural plankton communities, seasonal variation led to a distinct seasonal succession of species, yet the species composition varied from year to year in an irregular fashion. Our results suggest that interannual variability in species composition is an intrinsic property of multi-species communities in seasonal environments.

Geographical distribution of cryptic genetic types in the planktonic foraminifer Globigerinoides ruber

We present SSU rDNA data resolving the seasonal and geographical distribution of 'cryptic' genetic types of the planktonic foraminifer morphospecies Globigerinoides ruber in the eastern Atlantic Ocean and the Mediterranean Sea. Analysis of 262 sequences revealed the presence of five genetic types belonging to two distinct lineages. Although the morphospecies G. ruber occurs throughout the investigated region, its constituent 'cryptic' genetic types show a pattern of widespread exclusion, which is difficult to reconcile with the concept of ubiquitous dispersal. One of the newly discovered genetic types was exclusively found at stations in the Mediterranean Sea, possibly representing the smallest-scale example of endemism known in planktonic foraminifera. In general, our results suggest that the geographical scale of mutual exclusion between the genotypes is negatively correlated with their phylogenetic relatedness: the most similar and most recently diverged pair of siblings showed the strongest evidence for small-scale competitive exclusion. This pattern is consistent with the concept of niche partitioning, implying decreasing level of competition between genetic types with increasing degree of genetic divergence.

Allelopathy at the land/sea interface: microalgae and Brazilian pepper

Microalgae are significant contributors to biodiversity and primary production in shallow coastal habitats. The invasive Brazilian pepper (Schinusterebinthifolius) can occupy former mangrove habitats in disturbed areas, and its allelopathic capability can affect growth rates of microalgae. The four microalgae tested against dilution extracts of Schinus fruits varied in their growth response, suggesting variable but unpredictable impacts on microalgal biodiversity, primary production, and community structure.

Update of the Diatom EST Database: a new tool for digital transcriptomics

The Diatom Expressed Sequence Tag (EST) Database was constructed to provide integral access to ESTs from these ecologically and evolutionarily interesting microalgae. It has now been updated with 130 000 Phaeodactylum tricornutum ESTs from 16 cDNA libraries and 77 000 Thalassiosira pseudonana ESTs from seven libraries, derived from cells grown in different nutrient and stress regimes. The updated relational database incorporates results from statistical analyses such as log-likelihood ratios and hierarchical clustering, which help to identify differentially expressed genes under different conditions, and allow similarities in gene expression in different libraries to be investigated in a functional context. The database also incorporates links to the recently sequenced genomes of P. tricornutum and T. pseudonana, enabling an easy cross-talk between the expression pattern of diatom orthologs and the genome browsers. These improvements will facilitate exploration of diatom responses to conditions of ecological relevance and will aid gene function identification of diatom-specific genes and in silico gene prediction in this largely unexplored class of eukaryotes. The updated Diatom EST Database is available at http://www.biologie.ens.fr/diatomics/EST3.

A niche-based modeling approach to phytoplankton community assembly rules

Six niche-based models proposed by Tokeshi, based on different assumptions of resource allocation by species, were fitted on phytoplankton relative abundance distributions, and potential environmental and biotic factors supporting the applicability of the fitted models were discussed. Overall 16 assemblages corresponding to different sampling times, various environmental conditions, and resource regimes within a year were fitted to the models. Phytoplankton biovolume was used as a measure of abundance, and a randomization test was applied to compare the model fit to the field data. The majority of the phytoplankton assemblages (11 of 16) were successfully described by the Random Fraction model, which is based on the theoretical assumption that resource is apportioned by the species in a random way. Only a few assemblages (three of 16), characterized by extremes in resource availability or disturbance, were not fitted by any of the models. The Random Fraction model in particular was rejected due to a steep slope during the first ranks, while the rest of the distribution remained relatively even, providing further evidence of resilience in phytoplankton communities. Although larger cells seem to have the potential to develop higher biomass, it seems that other factors, including the surface-to-volume ratio, counterbalance this advantage, resulting in a random-like behaviour in resource acquisition by phytoplankton, irrespective of cell size or species identity.

Diversity predicts stability and resource use efficiency in natural phytoplankton communities

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the "macroscopic" realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute approximately 50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

Microbial diversity–productivity relationships in aquatic ecosystems

Thanks to recent advances in molecular biology, one's knowledge of microbial co-occurrence patterns, microbial biogeography and microbial biodiversity is expanding rapidly. This MiniReview explores microbial diversity-productivity relationships in the light of what is known from the general ecology literature. Analyses of microbial diversity-productivity relationships from 70 natural, experimental, and engineered aquatic ecosystems reveal patterns that are strikingly similar to those that have long been documented for communities of macroorganisms. Microbial ecology and the general science of ecology are thus continuing to converge.

Microbial community composition of the Danshui river estuary of Northern Taiwan and the practicality of the phylogenetic method in microbial barcoding

In this study, the microbial community in a mangrove ecosystem was surveyed and used to test the eligibility of 16S rDNA library and neighbor-joining method for the purpose of estimating microbial composition. Genetic diversity (pi) and four other diversity indices (Simpson's unbiased, Shannon-Wiener, Evenness, and Chao1 indices) were applied to estimate the adaptive lineages of microorganisms in the mangrove ecosystem. The results indicated that gamma-Proteobacteria is the most diverse taxon, while the most abundant family is Rhodobacteraceae (alpha-Proteobacteria), followed by Comamonadaceae (beta-Proteobacteria). This result may imply the existence of a graded distribution of microbial diversity across a spectrum of different salinities in the waterbody of this estuary ecosystem. Furthermore, at least 500-1,000 bps of the posterior portion of 16S rDNA is required as a marker to profile the microbial diversity in a microcosm of interest using phylogenetic methods, according to the results of our sliding window analyses for the measurements of pi, consistency index, and retention index.

Independent gradients of producer, consumer, and microbial diversity in lake plankton

Interactions between trophic levels during food web assembly can drive positive correlations in diversity between producers, consumers, and decomposers. However, the contribution of trophic interactions relative to local environmental factors in promoting species diversity is poorly understood, with many studies only considering two trophic levels. Here we examine correlations in diversity among zooplankton, phytoplankton, and bacteria in the pelagic zone of 31 lakes in British Columbia, Canada. We sampled species diversity of zooplankton and phytoplankton through morphological identification, and bacterial genetic diversity was estimated by denaturing gradient gel electrophoresis (DGGE) of 16S rDNA polymorphisms. We looked for correlations in diversity that were independent of the abiotic environment by statistically controlling for 18 limnological variables. No significant correlations were found between the diversity of zooplankton, phytoplankton, and bacteria. In addition, the physical factors that were associated with species composition in one trophic level were independent of those that were important for another. Our results provide no support for the importance of direct feedbacks between producers, consumers, and decomposers in maintaining diversity. Zooplankton, phytoplankton, and bacterial diversity and composition are regulated independently from one another and respond to different environmental variables. These results suggest that species of lake plankton show loose trophic associations with one another due to broad diets in consumers and decomposers.

Chemical cues induce consumer-specific defenses in a bloom-forming marine phytoplankton

Blooms of the phytoplankton Phaeocystis can comprise 85% of total production and generate major biogeochemical signals across broad oceanic regions. The success of Phaeocystis may result from its ability to change size by many orders of magnitude when it shifts from small cells of 4-6 microm to large colonies of up to 30,000 microm in diameter. Single cells are consumed by ciliates but not copepods, whereas colonies are consumed by copepods but not ciliates. We demonstrate that chemical cues associated with each of these grazers induce consumer-specific, but opposing, morphological transformations in the bloom-forming species Phaeocystis globosa. Chemical cues from grazing copepods suppress colony formation by a significant 60-90%, a response that should be adaptive because copepods feed four times more on colonies versus solitary cells. In contrast, chemical cues from grazing ciliates enhance colony formation by >25%, a response that should be adaptive because ciliates grow three times faster when fed solitary cells versus colonies. Because size-selective predation fundamentally alters community structure and ecosystem function, this chemically cued shift may redirect energy and nutrients from food webs supporting fisheries to those fueling detrital pathways, thus potentially altering ecosystem-level processes such as productivity, carbon storage, and nutrient release.

Chemical cues, defence metabolites and the shaping of pelagic interspecific interactions

Several observations and model calculations suggest that chemically mediated interactions can structure planktonic food webs. However, only recently have improvements in chemical methods, coupled with ecological assays, led to the characterization of chemical cues that affect the behaviour and/or physiology of planktonic organisms. We are currently beginning to elucidate if or how chemical signals can directly affect the interactions between species and even shape complex community structures in aquatic systems. Here, we highlight recent research on the nature and action of chemical signals in the pelagic marine and freshwater environments, with an emphasis on kairomones and defence metabolites.

Impacts of eutrophication on diatom life forms and species richness in coastal waters of the Baltic Sea

Increased nutrient and sediment loading can affect the functioning and biodiversity of coastal ecosystems. Lacking long-term monitoring data, paleolimnological techniques enable the estimation of habitat and diversity change through time. Using these methods we assessed the effects of eutrophication on diatom community structure and species richness over the past ca. 200 years in coastal waters of the Gulf of Finland. The abundance of planktonic diatoms has increased markedly because of increased eutrophication and turbidity. The loss of benthic habitats resulted in a clear decrease in diatom species richness after a threshold of 400-600 microg L(-1) total dissolved nitrogen; no unimodal pattern between diversity and productivity was observed in our data. The urban sites displayed a marked decrease in species richness starting in the late 19th century with increased urbanization. A clear recovery was visible after the cessation of point source loading by the mid-1980s at two sites, whereas at the third site no recovery was detected because of diffuse loading from the large catchment. Changes in the rural sites were minor and did not start until the 1940s.

Are algal communities driven toward maximum biomass?

In this continental-scale study, we show that in major benthic and planktonic stream habitats, algal biovolume--a proxy measure of biomass--is a unimodal function of species richness (SR). The biovolume peak is observed at intermediate to high SR in the benthos but at low richness in the phytoplankton. The unimodal nature of the biomass-diversity relationship implies that a decline in algal biomass with potential harmful effects on all higher trophic levels, from invertebrates to fish, can result from either excessive species gain or species loss, both being common consequences of human-induced habitat alterations. SR frequency distributions indicate that the most frequent richness is habitat-specific and significantly higher in the benthos than in the plankton. In all studied stream environments, the most frequent SR is lower than the SR that yields the highest biovolume, probably as a result of anthropogenic influences, but always within one standard deviation from it, i.e. they are statistically indistinguishable. This suggests that algal communities may be driven toward maximum biomass.

Spring bloom succession, grazing impact and herbivore selectivity of ciliate communities in response to winter warming

This study aimed at simulating different degrees of winter warming and at assessing its potential effects on ciliate succession and grazing-related patterns. By using indoor mesocosms filled with unfiltered water from Kiel Bight, natural light and four different temperature regimes, phytoplankton spring blooms were induced and the thermal responses of ciliates were quantified. Two distinct ciliate assemblages, a pre-spring and a spring bloom assemblage, could be detected, while their formation was strongly temperature-dependent. Both assemblages were dominated by Strobilidiids; the pre-spring bloom phase was dominated by the small Strobilidiids Lohmaniella oviformis, and the spring bloom was mainly dominated by large Strobilidiids of the genus Strobilidium. The numerical response of ciliates to increasing food concentrations showed a strong acceleration by temperature. Grazing rates of ciliates and copepods were low during the pre-spring bloom period and high during the bloom ranging from 0.06 (Delta0 degrees C) to 0.23 day(-1) (Delta4 degrees C) for ciliates and 0.09 (Delta0 degrees C) to 1.62 day(-1) (Delta4 degrees C) for copepods. During the spring bloom ciliates and copepods showed a strong dietary overlap characterized by a wide food spectrum consisting mainly of Chrysochromulina sp., diatom chains and large, single-celled diatoms.

EXTRINSIC AND INTRINSIC CONTROLS OF ZOOPLANKTON DIVERSITY IN LAKES

Pelagic crustacean zooplankton were collected from 336 Norwegian lakes covering a wide range of latitude, altitude, lake area, mean depth, production (as chlorophyll a), and fish community structure. Mean zooplankton species richness during the ice-free season was generally low at high latitudes and altitudes. Further, lower species richness was recorded in western lakes, possibly reflecting constraints on migration and dispersal. However, despite obvious spatial limitations, geographic boundaries were only weak predictors of mean zooplankton richness. Similarly, lake surface area did not contribute positively to mean richness such as seen in other ecosystem surveys. Rather, intrinsic factors such as primary production and fish community (planktivore) structure were identified by regression analysis as the major predictors of zooplankton diversity, while a positive correlation was observed between species richness and total zooplankton biomass. However, in spite of a large number of variables included in this study, the predictive power of multiple regression models was modest (<50% variance explained), pointing to a major role for within-lake properties, as yet unidentified intrinsic forces, stochasticity, or dispersal as constraints on zooplankton diversity in these lakes.

Diatom/copepod interactions in plankton: the indirect chemical defense of unicellular algae

Numerous coexisting species can be observed in the open oceans. This includes the complex community of the plankton, which comprises all free floating organisms in the sea. Traditionally, nutrient limitation, competition, predation, and abiotic factors have been assumed to shape the community structure in this environment. Only in recent years has the idea arisen that chemical signals and chemical defense can influence species interactions in the plankton as well. Key players at the base of the marine food web are diatoms (unicellular algae with silicified cell walls) and their main predators, the herbivorous copepods. It was assumed that diatoms represent a generally good food source for the grazers but recent work indicates that some species use chemical defenses. Secondary metabolites, released by these algae immediately after wounding, are targeted not against the predators themselves but rather at interfering with their reproductive success. This strategy allows diatoms to reduce the grazer population, thereby influencing the marine food web. This review addresses the chemical ecology of the defensive oxylipins formed by diatoms and the question of how these metabolites can act in such a dilute environment. Aspects of biosynthesis, bioassays, and the possible implications of such a chemical defense for the plankton community structure are also discussed.

Adaptive evolution of phytoplankton cell size

We present a simple nutrient-phytoplankton-zooplankton (NPZ) model that incorporates adaptive evolution and allometric relations to examine the patterns and consequences of adaptive changes in plankton body size. Assuming stable environmental conditions, the model makes the following predictions. First, phytoplankton should evolve toward small sizes typical of picoplankton. Second, in the absence of grazers, nutrient concentration is minimized as phytoplankton reach their fitness maximum. Third, increasing nutrient flux tends to increase phytoplankton cell size in the presence of phytoplankton-zooplankton coevolution but has no effect in the absence of zooplankton. Fourth, phytoplankton reach their fitness maximum in the absence of grazers, and the evolutionary nutrient-phytoplankton system has a stable equilibrium. In contrast, phytoplankton may approach their fitness minimum in the evolutionary NPZ system where phytoplankton and zooplankton are allowed to coevolve, which may result in oscillatory (unstable) dynamics of the evolutionary NPZ system, compared with the otherwise stable nonevolutionary NPZ system. These results suggest that evolutionary interactions between phytoplankton and zooplankton may have contributed to observed changes in phytoplankton sizes and associated biogeochemical cycles over geological time scales.

Fishing for biodiversity: novel methanopterin-linked C1 transfer genes deduced from the Sargasso Sea metagenome

The recently generated database of microbial genes from an oligotrophic environment populated by a calculated 1800 major phylotypes (the Sargasso Sea metagenome-SSM) presents a great source for expanding local databases of genes indicative of a specific function. In this article we analyse the SSM for the presence of methanopterin-linked C1 transfer genes that are signature for methylotrophy. We conclude that more than 10 phylotypes possessing genes of interest are present in this environment. The sequences representative of these major phylotypes do not appear to belong to any known microbial group capable of methanopterin-linked C1 transfer. Instead, these sequences separate from all known sequences on phylogenetic trees, pointing toward their affiliation with novel microbial phyla. These data imply a broader distribution of methanopterin-linked functions in the microbial world than has been previously known.

Different measures of biodiversity

In their analysis of global trends of diversity in marine plankton, Irigoien et al. find that taxonomic diversity of zooplankton (consumer) is a unimodal function of community biomass that is unrelated to phytoplankton (producer) diversity. Their results are unexpected because in terrestrial organisms primary-producer diversity is a good predictor of consumer diversity. I contend that this apparent uncoupling of producer and consumer diversity in marine plankton is likely to be an artefact due to the authors' use of different measures of diversity for producers and for consumers.

Adaptive divergence in pigment composition promotes phytoplankton biodiversity

The dazzling diversity of the phytoplankton has puzzled biologists for decades. The puzzle has been enlarged rather than solved by the progressive discovery of new phototrophic microorganisms in the oceans, including picocyanobacteria, pico-eukaryotes, and bacteriochlorophyll-based and rhodopsin-based phototrophic bacteria. Physiological and genomic studies suggest that natural selection promotes niche differentiation among these phototrophic microorganisms, particularly with respect to their photosynthetic characteristics. We have analysed competition for light between two closely related picocyanobacteria of the Synechococcus group that we isolated from the Baltic Sea. One of these two has a red colour because it contains the pigment phycoerythrin, whereas the other is blue-green because it contains high contents of the pigment phycocyanin. Here we report theory and competition experiments that reveal stable coexistence of the two picocyanobacteria, owing to partitioning of the light spectrum. Further competition experiments with a third marine cyanobacterium, capable of adapting its pigment composition, show that this species persists by investing in the pigment that absorbs the colour not used by its competitors. These results demonstrate the adaptive significance of divergence in pigment composition of phototrophic microorganisms, which allows an efficient utilization of light energy and favours species coexistence.

Climate Impact on Plankton Ecosystems in the Northeast Atlantic

It is now widely accepted that global warming is occurring, yet its effects on the world's largest ecosystem, the marine pelagic realm, are largely unknown. We show that sea surface warming in the Northeast Atlantic is accompanied by increasing phytoplankton abundance in cooler regions and decreasing phytoplankton abundance in warmer regions. This impact propagates up the food web (bottom-up control) through copepod herbivores to zooplankton carnivores because of tight trophic coupling. Future warming is therefore likely to alter the spatial distribution of primary and secondary pelagic production, affecting ecosystem services and placing additional stress on already-depleted fish and mammal populations.