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The microenvironment surrounding individual phytoplankton cells is often rich in dissolved organic matter (DOM), which can attract bacteria by chemotaxis. These "phycospheres" may be prominent sources of resource heterogeneity in the ocean, affecting the growth of bacterial populations and the fate of DOM. However, these effects remain poorly quantified due to a lack of quantitative ecological frameworks. Here, we used video microscopy to dissect with unprecedented resolution the chemotactic accumulation of marine bacteria around individual Chaetoceros affinis diatoms undergoing lysis. The observed spatiotemporal distribution of bacteria was used in a resource utilization model to map the conditions under which competition between different bacterial groups favors chemotaxis. The model predicts that chemotactic, copiotrophic populations outcompete nonmotile, oligotrophic populations during diatom blooms and bloom collapse conditions, resulting in an increase in the ratio of motile to nonmotile cells and in the succession of populations. Partitioning of DOM between the two populations is strongly dependent on the overall concentration of bacteria and the diffusivity of different DOM substances, and within each population, the growth benefit from phycospheres is experienced by only a small fraction of cells. By informing a DOM utilization model with highly resolved behavioral data, the hybrid approach used here represents a new path toward the elusive goal of predicting the consequences of microscale interactions in the ocean.

A new high-resolution, in situ profiling fluorometer maps fluorescence distributions with a spatial resolution of 0.5 to 1.5 mm to a depth of 70 m in the open ocean. We report centimeter-scale patterns for phytoplankton distributions associated with gradients exhibiting 10- to 30-fold changes in fluorescence in contrasting marine ecosystems.

BackgroundDiatoms are important single-celled autotrophs that dominate most lit aquatic environments and are distinguished by surficial frustules with intricate designs of unknown function.Principal findingsWe show that some frustule designs constrain diffusion to positively alter nutrient uptake. In nutrient gradients of 4 to 160 times over

Abstract: Large temporal and spatial data series are increasingly available and easy to produce. This paper uses Zipf analysis to evaluate serial data sets from the HOTS, BATS, EquaPac and high-resolution vertical profiles of FluoroMAP. Zipf analysis produced Zipf exponents from best-fit lines that permitted comparison among data sets. It allows comparison of one-dimensional series despite differences in scale and missing data. Zipf exponents ranged from 0.043 to 0.83. Serial data with sampling intervals of milliseconds and months showed exponents that ranged around 0.3. To the extent that Zipf exponents measure structure and variation, the indication is that structure of distributions is similar over millimeters and hundreds of kilometers. Zipf analysis provides a means to quantify similarities and differences, and suggests that variation is linked across many length scales for phytoplankton. [Copyright &y& Elsevier]

Abstract: Marine phytoplankton experience competition, predation, infection and aggregation occurring across distances of micrometres to centimetres. However, the consequences of these interactions influence global processes, such as climate and fisheries productivity. There is a long-standing default assumption that these global processes cannot be traced to plankton distributions and interactions below a few metres because of the homogenising effect of turbulence [Hutchinson, G.E., 1961. The paradox of the plankton. Am. Nat. 95, 137–146.; Siegel, D.A., 1998. Resource competition in a discrete environment: Why are plankton distributions paradoxical? Limnol. Oceanogr. 43, 1133–1146.]. We show that, in active turbulence, phytoplankton patches, on the order of 10 cm, have repeatable asymmetry and regular spacing over distances of centimetres to tens of metres. The regularity and hierarchical nature of the patches in mixed ocean water means that phytoplankton are distributed in a dynamic, but definite seascape topography, where groups of patches coalesce between intermittent turbulent eddies. These patches may link large scale processes and microscale interactions, acting as fundamental components of marine ecosystems that influence grazing efficiency, taxonomic diversity, and the initiation of aggregation and subsequent carbon flux. [Copyright &y& Elsevier]

The standard method for counting phytoplankton requires 24–48 h of settling (Utermöhl, 1931). The modified Utermöhl method (plunger) described here reduces settling time to 2 h. Estimates of counts for phytoplankton using the plunger method plotted against counts from the Utermöhl method yield a good correlation (r2 = 0.97), and 95–100% of species (>2 μm) recorded in the Utermöhl method were recorded with the plunger method. [ABSTRACT FROM PUBLISHER]

Abstract: Coastal upwelling systems play an essential role in bringing cold and nutrient-rich water into the euphotic zone, hence enhancing the biological productivity of the world''s oceans. We describe a “cryptic” upwelling occurring in South Australian waters, in which cold upwelled waters do not reach surface waters and do not exhibit a sea surface temperature (SST) signature. Due to the wide continental shelf (ca. 100km), upwelled waters form a sub-surface cold water pool and are forced north-west after a secondary event. Using flow cytometry we investigated the abundance and composition of viruses, heterotrophic bacteria and pico-phytoplankton within upwelling affected and unaffected waters. Our results identified the presence of upwelled waters at and below the Deep-Chlorophyll Maximum (DCM), where water temperature was at least 4°C colder than surface waters. In contrast to previous studies, no significant differences were observed between upwelled and non-upwelled waters for most individual viral, bacterial and pico-phytoplankton sub-groups. However, one viral, one bacterial and two pico-phytoplankton sub-groups were significantly more abundant at the DCM. This indicates the presence of depth- and population-specific shifts in abundance and potential niche partitioning of these cytometrically-defined sub-groups that may be related to their host organisms and/or resource availability. [Copyright &y& Elsevier]

Abstract: Despite the long-standing belief that seawater viscosity is driven by temperature and salinity, biologically increased seawater viscosity has repeatedly been reported in relation to phytoplankton exudates in shallow, productive coastal waters. Here, seawater viscosity was investigated in relation to microbial and phytoplanktonic communities off the coast of East Antarctica along latitudinal transects located between 30°E and 80°E in sub-surface waters and at the deep chlorophyll maximum (DCM). The physical component of seawater viscosity observed along each transects ranged from 1.80 to 1.95cP, while the actual seawater viscosity ranged from 1.85 to 3.69cP. This resulted in biologically increased seawater viscosity reaching up to 84.9% in sub-surface waters and 77.6% at the DCM. Significant positive correlations were found between elevated seawater viscosity and (i) bacterial abundance in sub-surface waters and (ii) chlorophyll a concentration and the abundance of flow cytometrically-defined auto- and heterotrophic protists at the DCM. Among the 12 groups and 108 species of protists identified under light microscopy, dinoflagellates and more specifically Alexandrium tamarense and Prorocentrum sp. were the main contributors to the patterns observed for elevated seawater viscosity. Our observations, which generalised the link previously identified between seawater viscosity and phytoplankton composition and standing stock to the Southern Ocean, are the first demonstration of increases in seawater viscosity linked to marine bacterial communities, and suggest that the microbially-increased viscosity might quantitatively be at least as important as the one related to phytoplankton secretion. [Copyright &y& Elsevier]