Calanus the cannibal

Recent modelling efforts have suggested that cannibalism oneggs and young nauplii is an important factor controlling thepopulation dynamics of Calanus. We measured cannibalistic feedingrates on eggs and nauplii of Calanus helgolandicus in the laboratory.Clearance rates on nauplii were     

Importance of Viral Lysis and Dissolved DNA for Bacterioplankton Activity in a P-Limited Estuary,Northern Baltic Sea

Through lysis of bacterioplankton cells, viruses mediate an important, but poorly understood, pathway of carbon and nutrients from the particulate to the dissolved form. Via this activity, nutrient-rich cell lysates may become available to noninfected cells and support significant growth. However, the nutritional value of lysates for noninfected bacteria presumably depends on the prevailing nutrient limitation. In the present study, we examined dynamics of dissolved DNA (D-DNA) and viruses along a transect in the phosphorus (P)-limited Ore Estuary, northern Baltic Sea. We found that viruses were an important mortality factor for bacterioplankton and that their activity mediated a significant recycling of carbon and especially of P. Uptake of dissolved DNA accounted for up to 70% of the bacterioplankton P demand, and about a quarter of the D-DNA pool was supplied through viral lysis of bacterial cells. Generally, the importance of viral lysates and uptake of D-DNA was highest at the estuarine and offshore stations and was positively correlated with P limitation measured as alkaline phosphatase activity. Our results highlight the importance of viral activity for the internal recycling of principal nutrients and pinpoints D-DNA as a particularly relevant compound in microbial P dynamics.     

Diversity and abundance of freshwater Actinobacteria along environmental gradients in the brackish northern Baltic Sea

Actinobacteria are highly abundant in pelagic freshwater habitats and also occur in estuarine environments such as the Baltic Sea. Because of gradients in salinity and other environmental variables estuaries offer natural systems for examining factors that determine Actinobacteria distribution. We studied abundance and community structure of Bacteria and Actinobacteria along two transects in the northern Baltic Sea. Quantitative (CARD-FISH) and qualitative (DGGE and clone libraries) analyses of community composition were compared with environmental parameters. Actinobacteria accounted for 22–27% of all bacteria and the abundance changed with temperature. Analysis of 549 actinobacterial 16S rRNA sequences from four clone libraries revealed a dominance of the freshwater clusters ac I and ac IV, and two new subclusters ( ac I-B scB-5 and ac IV-E) were assigned. Whereas ac I was present at all stations, occurrence of ac II and ac IV differed between stations and was related to dissolved organic carbon (DOC) and chlorophyll a (Chl a ) respectively. The prevalence of the ac I-A and ac I-B subclusters changed in relation to total phosphorus (Tot-P) and Chl a respectively. Community structure of Bacteria and Actinobacteria differed between the river station and all other stations, responding to differences in DOC, Chl a and bacterial production. In contrast, the composition of active Actinobacteria (analysis based on reversely transcribed RNA) changed in relation to salinity and Tot-P. Our study suggests an important ecological role of Actinobacteria in the brackish northern Baltic Sea. It highlights the need to address dynamics at the cluster or subcluster phylogenetic levels to gain insights into the factors regulating distribution and composition of Actinobacteria in aquatic environments.     

Virus Production and Lysate Recycling in Different Sub-basins of the Northern Baltic Sea

In the Gulf of Bothnia, northern Baltic Sea, a large freshwater inflow creates north-southerly gradients in physico-chemical and biological factors across the two sub-basins, the Bothnian Bay (BB) and the Bothnian Sea. In particular, the sub-basins differ in nutrient limitation (nitrogen vs. phosphorus; P). Since viruses are rich in P, and virus production is commonly connected with bacterial abundance and growth, we hypothesized that the role of viral lysis differs between the sub-basins. Thus, we examined virus production and the potential importance of lysate recycling in surface waters along a transect in the Gulf of Bothnia. Surprisingly, virus production and total P were negatively correlated. In the BB, virus production rates were double those elsewhere in the system, although bacterial abundance and production were the lowest. In the BB, virus-mediated cell lysates could account for 70-180% and 100-250% of the bacterial carbon and P demand, respectively, while only 4-15% and 8-21% at the other stations. Low concentrations of dissolved DNA (D-DNA) with a high proportion of encapsulated DNA (viruses) in the BB suggested rapid turnover and high uptake of free DNA. The correlation of D-DNA and total P indicates that D-DNA is a particularly important nutrient source in the P-limited BB. Our study demonstrates large and counterintuitive differences in virus-mediated recycling of carbon and nutrients in two basins of the Gulf of Bothnia, which differ in microbial community composition and nutrient limitation.     

Feeding, prey selection and prey encounter mechanisms in the heterotrophic dinoflagellate Noctiluca scintillans

The heterotrophic dinoflagellate Noctiluca scintillans has anegligible swimming ability and feeds predominantly on immobileprey. How, then, does it encounter prey? Noctiluca scintillansis positively buoyant and, therefore, we hypothesized that itintercepts prey particles during ascent and/or that microscaleshear brings it into contact with prey. Noctiluca scintillanshas a specific carbon content 1–2 orders of magnitudeless than that typical for protists and, thus, an inflated volume.It also has a density slightly less than that of the ambientwater and therefore ascends at high velocities (-1 m h^–1).In stagnant water, clearance rates of latex spheres (5–80µm) increased approximately with prey particle size squared.This scaling is consistent with N.scintillans being an interceptionfeeder. However, absolute clearance rates were substantiallylower than those predicted by modeling N.scintillans both asa spherical and as a cylindrical collector. The latter modelassumes that prey particles are collected on the string of mucusthat may form at the tip of the tentacle. Feeding, growth andprey selection experiments all demonstrated that diatoms arecleared at substantially higher rates than latex beads and otherphytoplankters, particularly dinoflagellates. We propose thatdiatoms stick more efficiently than latex beads to the mucusof N.scintillans and that dinoflagellates reduce fatal contactbehaviorally. We conclude that N.scintillans is an interceptionfeeder and that the high ascent velocity accounts for encounterswith prey. However, the flow field around the cell-mucus complexis too complicated to be described accurately by simple geometricmodels. Fluid shear (0.7–1.8 s^–1 had a negativeimpact on feeding rates, which were much less than predictedby models. Noctiluca scintillans can survive starvation forlong periods (>3 weeks), it can grow at low concentrationsof prey (-15 µg C l^–1), but growth saturates onlyat very high prey concentrations of 500–1000 µgC l^–1 or more. We demonstrate how the functional biologyof N.scintillans is consistent with its spatial and seasonaldistribution, which is characterized by persistence in the plankton,blooms in association with high concentrations of diatoms, andsurface accumulation during quiescent periods or exponentialdecline in abundance with depth during periods of turbulentmixing.     

MICROSENSOR MEASUREMENTS OF THE EXTERNAL AND INTERNAL MICROENVIRONMENT OF FUCUS VESICULOSUS (PHAEOPHYCEAE)1

We investigated the O₂, pH, and irradiance microenvironment in and around the tissue of the brown alga Fucus vesiculosus L. using microsensors. Microsensors are ideal tools for gaining new insights into what limits and controls macroalgal activity and growth at very fine spatial (<100 μm) and temporal (seconds) scales. This first microsensor investigation of a fucoid macroalga revealed differences in the microenvironment and metabolic activities at the level of different cell layers and thallus structures. F. vesiculosus responded quickly to rapid shifts in irradiance resulting in a highly dynamic microenvironment around and within its thallus. In combination with detailed morphological studies and molecular approaches, microsensors offer a promising toolbox to quantitatively describe structural and functional adaptations of macroalgae to environmental conditions, such as flow and light climate, as well as their physiological responses to environmental stressors.     

Copepod mating: chance or choice?

Mate choice is common in most animals, but the issue has largelybeen ignored in regard to pelagic copepods—the most abundantzooplankton group in the oceans. Contemporary demographic modelsof copepods assume that all encounters lead to successful copulationand viable egg production. We revisit the subject of copepodmating biology in the framework of sexual selection theory.We discuss, and point to, examples that suggest mate choicein both sexes. Sexual selection theory may add an importantdimension for understanding copepod population dynamics andpotentially lead to renewed interpretations of the diverse copepodbehaviours and life histories. In addition, the range of matingstrategies in copepods represents a rich, but unexplored, sourceof information for general behavioural ecologists.