Effects of an invasive bivalve on the zooplankton community of the Hudson River

1. Previous studies documented that zebra mussels became abundant in the Hudson River during 1992 causing an 80–90% reduction in phytoplankton biomass. This study used intervention time series analysis of abundance, biomass and reproduction over the period 1987–95 to assess changes in zooplankton in response to the invasion.
2. Zebra mussels caused a size-dependent decline in zooplankton. Microzooplankton, including tintinnid ciliates, rotifers and copepod nauplii all declined in 1992 and were scarce thereafter. Mean abundances of post-naupliar copepods and of cladocerans were also lower following the invasion but these changes were not statistically significant ( P > 0.05). Egg ratios and clutch sizes for the dominant cladoceran, Bosmina freyi , were not significantly related to zebra mussels, even though relatively low egg ratios were observed after the invasion.
3. The strong declines in microzooplankton were probably caused by direct zebra mussel predation. Estimated consumption rates by mussels were roughly equivalent to maximum microzooplankton growth rates.
4. The total biomass of zooplankton in the Hudson River declined by more than 70% following the invasion. Annual average zooplankton biomass was correlated with chlorophyll, but biomass per unit chlorophyll in the Hudson River was much lower than in lakes. The present study hypothesizes that this lower biomass reflects limitations by riverine flow and by predation during summer.     

Identification of the sites in opsin modified by photoactivated azido[125I]iodobenzene.

Opsin labelled with photoactivated 1-azido-4-[125I]iodobenzene was proteolysed in situ with Staphylococcus aureus V8 proteinase to yield two radioactive membrane-bound fragments. These were separated, cleaved with CNBr and the resultant peptides sequenced in order to locate the radiolabelled residues. In the whole molecule, there was clear evidence for modification of at least 20 sites, identified as derivatives of cysteine, tryptophan, tyrosine, histidine and lysine residues. The probe primary reacted, therefore, with nucleophilic substituents. The positions of the modified sites relative to the confines of the phospholipid bilayer were consistent with all other studies on the disposition of the polypeptide chain. The location of these sites substantiated an earlier suggestion that not all the transmembrane segments should be regarded as continuous regular alpha-helices.     

A single-residue deletion alters the lipid selectivity of a K+ channel-associated peptide in the beta-conformation: spin label electron spin resonance studies.

Lipid-peptide interactions with the 27-residue peptide of sequence KLEALYILMVLGFFGFFTLGIMLSYIR reconstituted as beta-sheet assemblies in dimyristoylphosphatidylcholine bilayers have been studied by electron spin resonance (ESR) spectroscopy with spin-labeled lipids. The peptide corresponds to residues 42-68 of the IsK voltage-gated K+ channel protein and contains the single putative transmembrane span of this protein. Lipid-peptide interactions give rise to a second component in the ESR spectra of lipids spin-labeled on the 14C atom of the chain that corresponds to restriction of the lipid mobility by direct interaction with the peptide assemblies. From the dependence on the lipid/peptide ratio, the stoichiometry of lipid interaction is found to be about two phospholipids/peptide monomer. The sequence of selectivity for lipid association with the peptide assemblies is in the order phosphatidic acid > stearic acid = phosphatidylserine > phosphatidylglycerol = phosphatidylcholine. Comparison with previous data for a corresponding 26-residue mutant peptide with a single deletion of the apolar residue Leu2 (Horvath et al., 1995. Biochemistry 34:3893-3898), indicates a very similar mode of membrane incorporation for native and mutant peptides, but a strongly modified pattern and degree of specificity for the interaction with negatively charged lipids. The latter is interpreted in terms of the relative orientations of the charged amino acid side chains in the beta-sheet assemblies of the native and deletion-mutant peptides.