During a bacterial survey of the Huon Estuary in southern Tasmania, Australia, we isolated a yellow-pigmented Pseudoalteromonasstrain (class Proteobacteria, gamma subdivision), designated strain Y, that had potent algicidal effects on harmful algal bloom species. This organism was identified by 16S rRNA sequencing as a strain with close affinities to Pseudoalteromonas peptidysin. This bacterium caused rapid cell lysis and death (within 3 h) of gymnodinoids (including Gymnodinium catenatum) and raphidophytes (Chattonella marina andHeterosigma akashiwo). It caused ecdysis of armored dinoflagellates (e.g., Alexandrium catenella,Alexandrium minutum, and Prorocentrum mexicanum), but the algal cultures then recovered over the subsequent 24 h. Strain Y had no effect on a cryptomonad (Chroomonas sp.), a diatom (Skeletonema sp.), a cyanobacterium (Oscillatoria sp.), and two aplastidic protozoans. The algicidal principle of strain Y was excreted into the seawater medium and lost its efficacy after heating. Another common bacterial species, Pseudoalteromonas carrageenovora, was isolated at the same time and did not have these algicidal effects. The minimum concentrations of strain Y required to kill G. catenatum were higher than the mean concentrations found in nature under nonbloom conditions. However, the new bacterium showed a chemotactic, swarming behavior that resulted in localized high concentrations around target organisms. These observations imply that certain bacteria could play an important role in regulating the onset and development of harmful algal blooms.

Mixotrophic protists (unicellular eukaryotes) that engage in both phototrophy (photosynthesis) and phago-heterotrophy (engulfment of particles)—are predicted to contribute substantially to energy fluxes and marine biogeochemical cycles. However, their impact remains largely unquantified. Here we describe the sophisticated foraging strategy of a widespread mixotrophic dinoflagellate, involving the production of carbon-rich ‘mucospheres’ that attract, capture, and immobilise microbial prey facilitating their consumption. We provide a detailed characterisation of this previously undescribed behaviour and reveal that it represents an overlooked, yet quantitatively significant mechanism for oceanic carbon fluxes. Following feeding, the mucospheres laden with surplus prey are discarded and sink, contributing an estimated 0.17–1.24 mg m−2 d−1 of particulate organic carbon, or 0.02–0.15 Gt to the biological pump annually, which represents 0.1–0.7% of the estimated total export from the euphotic zone. These findings demonstrate how the complex foraging behaviour of a single species of mixotrophic protist can disproportionally contribute to the vertical flux of carbon in the ocean.

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Blooms of phytoplankton (100-280 mg chlorophyll a m-1) occur on the continental shelf off Sydney in the spring of most years. These sudden chlorophyll increases (more than 10 times the normal algal biomass) are due to short-lived diatom blooms that evolve in a predictable sequence from small chainforming species (Nitzschia, Thalassiosira) to large centric species (Lauderia, Rhizosolenia) and eventually to large dinoflagellates (Protoperidinium). Two research cruises (October 1981, September 1984) were conducted to define the longshore extent of this phenomenon. Diatom blooms were widespread along the whole New South Wales coastline, occurring in the 700-km-long region from Cape Hawke in the north (32°S), where the East Australian Current separates from the coast, to Maria Island off Tasmania in the south (43°S). Hydrological mechanisms of these annually recurrent enrichments are related to the action of the East Australian Current and are unlike those triggering spring blooms in temperate European waters. Implications of these diatom blooms for coastal fisheries along the New South Wales coast are briefly discussed.

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