Krill (Euphausia superba) provide a direct link between primary producers and higher trophic levels in the Antarctic marine food web1,2,3,4,5,6. The pelagic tunicate Salpa thompsoni can also be important during spring and summer through the formation of extensive and dense blooms6,7,8,9. Although salps are not a major dietary item for Antarctic vertebrate predators7,10, their blooms can affect adult krill reproduction and survival of krill larvae. Here we provide data from 1995 and 1996 that support hypothesized relationships between krill, salps and region-wide sea-ice conditions11,12. We have assessed salp consumption as a proportion of net primary production, and found correlations between herbivore densities and integrated chlorophyll-a that indicate that there is a degree of competition between krill and salps. Our analysis of the relationship between annual sea-ice cover and a longer time series of air temperature measurements12,13 indicates a decreased frequency of winters with extensive sea-ice development over the last five decades. Our data suggest that decreased krill availability may affect the levels of their vertebrate predators. Regional warming and reduced krill abundance therefore affect the marine food web and krill resource management.

Effects of sea-ice extent and krill or salp dominance on the Antarctic food web

By controlling nutrient cycling and biomass production at the base of the food web, interactions between phytoplankton and bacteria represent a fundamental ecological relationship in aquatic environments Although typically studied over large spatiotemporal scales, emerging evidence indicates that this relationship is often governed by microscale interactions played out within the region immediately surrounding individual phytoplankton cells This microenvironment, known as the phycosphere, is the planktonic analogue of the rhizosphere in plants The exchange of metabolites and infochemicals at this interface governs phytoplankton-bacteria relationships, which span mutualism, commensalism, antagonism, parasitism and competition The importance of the phycosphere has been postulated for four decades, yet only recently have new technological and conceptual frameworks made it possible to start teasing apart the complex nature of this unique microbial habitat It has subsequently become apparent that the chemical exchanges and ecological interactions between phytoplankton and bacteria are far more sophisticated than previously thought and often require close proximity of the two partners, which is facilitated by bacterial colonization of the phycosphere It is also becoming increasingly clear that while interactions taking place within the phycosphere occur at the scale of individual microorganisms, they exert an ecosystem-scale influence on fundamental processes including nutrient provision and regeneration, primary production, toxin biosynthesis and biogeochemical cycling Here we review the fundamental physical, chemical and ecological features of the phycosphere, with the goal of delivering a fresh perspective on the nature and importance of phytoplankton-bacteria interactions in aquatic ecosystems

Zooming in on the phycosphere: the ecological interface for phytoplankton-bacteria relationships.

Marine plankton use a variety of defences against predators, some of which affect trophic structure and biogeochemistry1. We have previously shown2 that, during grazing by the protozoan Oxyrrhis marina on the alga Emiliania huxleyi, dimethylsulphoniopropionate (DMSP) from the prey is converted to dimethyl sulphide (DMS) when lysis of ingested prey cells initiates mixing of algal DMSP and the enzyme DMSP lyase. Such a mechanism is similar to macrophyte defence reactions3,4. Here we show that this reaction deters protozoan herbivores, presumably through the production of highly concentrated acrylate, which has antimicrobial activity5. Protozoan predators differ in their ability to ingest and survive on prey with high-activity DMSP lyase, but all grazers preferentially select strains with low enzyme activity when offered prey mixtures. This defence system involves investment in a chemical precursor, DMSP, which is not self-toxic and has other useful metabolic functions. We believe this is the first report of grazing-activated chemical defence in unicellular microorganisms.

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Grazing-activated chemical defence in a unicellular marine alga

Many bacteria swim in liquid or swarm over solid surfaces by synthesizing rotary flagella The same bacteria that are motile also commonly form nonmotile multicellular aggregates called biofilms Biofilms are an important part of the lifestyle of pathogenic bacteria, and it is assumed that there is a motility-to-biofilm transition wherein the inhibition of motility promotes biofilm formation The transition is largely inferred from regulatory mutants that reveal the opposite regulation of the two phenotypes Here, we review the regulation of motility during biofilm formation in Bacillus, Pseudomonas, Vibrio, and Escherichia, and we conclude that the motility-to-biofilm transition, if necessary, likely involves two steps In the short term, flagella are functionally regulated to either inhibit rotation or modulate the basal flagellar reversal frequency Over the long term, flagellar gene transcription is inhibited and in the absence of de novo synthesis, flagella are diluted to extinction through growth Both short-term and long-term motility inhibition is likely important to stabilize cell aggregates and optimize resource investment We emphasize the newly discovered flagellar functional regulators and speculate that others await discovery in the context of biofilm formation

Regulation of flagellar motility during biofilm formation

Marine bacteria play a central role in the degradation of dimethylsulfoniopropionate (DMSP) to dimethyl sulfide (DMS) and acrylic acid, DMS being critical to cloud formation and thereby cooling effects on the climate. High concentrations of DMSP and DMS have been reported in scleractinian coral tissues although, to date, there have been no investigations into the influence of these organic sulfur compounds on coral-associated bacteria. Two coral species, Montipora aequituberculata and Acropora millepora, were sampled and their bacterial communities were characterized by both culture-dependent and molecular techniques. Four genera, Roseobacter, Spongiobacter, Vibrio, and Alteromonas, which were isolated on media with either DMSP or DMS as the sole carbon source, comprised the majority of clones retrieved from coral mucus and tissue 16S rRNA gene clone libraries. Clones affiliated with Roseobacter sp. constituted 28% of the M. aequituberculata tissue libraries, while 59% of the clones from the A. millepora libraries were affiliated with sequences related to the Spongiobacter genus. Vibrio spp. were commonly isolated from DMS and acrylic acid enrichments and were also present in 16S rRNA gene libraries from coral mucus, suggesting that under “normal” environmental conditions, they are a natural component of coral-associated communities. Genes homologous to dddD, and dddL, previously implicated in DMSP degradation, were also characterized from isolated strains, confirming that bacteria associated with corals have the potential to metabolize this sulfur compound when present in coral tissues. Our results demonstrate that DMSP, DMS, and acrylic acid potentially act as nutrient sources for coral-associated bacteria and that these sulfur compounds are likely to play a role in structuring bacterial communities in corals, with important consequences for the health of both corals and coral reef ecosystems.

Coral-Associated Bacteria and Their Role in the Biogeochemical Cycling of Sulfur

A capillary assay was used to evaluate the chemotactic responses of Vibrio alginolyticus to three common algal extracellular products. Acrylate and glycolate attracted the motile marine bacterium. The peak response occurred with 10(-2) M of each chemical. Acrylic and glycolic acid also attracted V. alginolyticus, with the peak response occurring at 5 x 10(-4) M of each chemical. Higher concentrations of the organic acids resulted in a decreased response. The bacteria also displayed positive chemotaxis to dimethyl sulfide.

Chemotactic responses of Vibrio alginolyticus to algal extracellular products.

Although bacterial flagellar sheaths were observed over 30 years ago, they may still be characterized as structures in search of a function. In addition to true sheaths, bacterial flagella may possess other adornments that cause an increase in the organelle's cross-sectional diameter. These "complex flagella" are sharply differentiated from sheathed flagella. Immunological and chemical distinctions have been found between flagellar sheaths, flagellar cores, and LPS layers inferred to be the sheath sensu stricto. Although complex flagella may serve as specific receptors for flagellotropic phages or in allowing for more efficient swimming in viscous environments, similar functions have not yet been attributed to true sheaths. It is postulated that flagellar sheaths may allow for specific interaction between a bacterium and a surface. In addition, there is a problem as to the relationship between a rapidly rotating flagellum and the sheath.

Invited review: bacterial flagellar sheaths: structures in search of a function.

Studies have been made of the initial stages in adsorption of several polarly flagellated marine bacteria to glass surfaces. Bacteria attach at the pole of flagellar insertion, and after a brief period (from a few seconds to a few minutes) of rotation around the attachment axis, become immobile. Soma do not spin or gyrate while the organisms rotate slowly. Flagellar activity continues for several minutes after soma immobilization. Tween 80 inhibits bacterial attachment, and deflagellated organisms do not adsorb to glass. Bacteria rendered nonmotile with sodium azide (NaN3) rapidly adsorb to glass and cannot be removed by washing with artificial seawater or a solution of 2.4% NaCl. It is proposed that both flagella-surface and somasurface interactions are involved in bacterial attachment. Bacterial flagella may play an important ecological role as attachment organelles.

Adsorption of polarly flagellated bacteria to surfaces

A capillary assay was employed to quantify positive chemotactic responses in the motile, unicellular, marine algaDunaliella tertiolecta. Among a wide range of inorganic and organic compounds tested, only ammonium ion,l-tyrosine,l-tryptophan, andl-phenylalanine were found to be major atractants for the chlorophyte.l-Methionine andl-cysteine weakly attracted the alga at 10−3 M. The minimum concentration of the major attractants needed to elicit an observable chemotactic response was approximately 10−6 M. The maximum response occurred when the capillaries contained 10−5 Ml-tyrosine orl-tryptophan, 10−4 Ml-phenylalanine, and 10−3 M ammonium chloride. The other amino acids, carbohydrates, B-vitamins, urea, and nitrate were among the chemicals that failed to attractD. tertiolecta. The alga apparently possesses one chemoreceptor that binds ammonium ion only, and another chemoreceptor that binds the three aromatic amino acids.

Chemoreception in the green algaDunaliella tertiolecta

Pseudomonas marina (ATCC 27 129) rapidly aggregates when suspended in buffered artificial seawater (ASW). Light microscopic observations of stained preparations, showed that flagella-flagella contact was responsible for this phenomenon. Aggregation did not occur if flagella were sheared off, or if motility was inhibited with NaN3. Aggregates were not observed when Mg2+ was omitted from ASW, even though the bacteria remained motile. Other divalent cations, including Ca2+, Mn2+, and Ba2+ could replace Mg2+. However, there is no absolute requirement for divalent cations, since aggregation occurred in ASW containing Cs+ or Li+ instead of Mg2+. P. marina aggregates developed from pH 5.8–8.4, but not below pH 5.8 even though motility continued unimpaired to pH 4.5.

Roy D. Sjoblad

Papers

Chemotactic responses of Vibrio alginolyticus to algal extracellular products.

Invited review: bacterial flagellar sheaths: structures in search of a function.

Adsorption of polarly flagellated bacteria to surfaces

Chemoreception in the green algaDunaliella tertiolecta

Novel function of eubacterial flagella: role in aggregation of a marine bacterium