Adaptation to High-Intensity, Low-Wavelength Light among Surface Blooms of the Cyanobacterium Microcystis aeruginosa

Natural populations of the nuisance bloom cyanobacterium Microcystis aeruginosa obtained from the eutrophic Neuse River, N.C., revealed optimal chlorophyll a-normalized photosynthetic rates and resistance to photoinhibition at surface photosynthetically active radiation (PAR) intensities. At saturating PAR levels these populations exhibited higher photosynthetic rates in quartz than in Pyrex vessels. Eucaryotic algal populations obtained from the same river failed to counteract photoinhibition. At saturating PAR levels, such populations generally yielded lower photosynthetic rates in quartz containers than they did in Pyrex containers. Cultivation of natural Microcystis populations under laboratory conditions led to physiologically distinct populations which had photoinhibitory characteristics similar to those of other cultured cyanobacterial and eucaryotic algae. Our findings indicate that (i) photosynthetic production among natural surface populations is best characterized and quantified in quartz rather than Pyrex incubation vessels; (ii) extrapolation of natural photoinhibitory trends from laboratory populations is highly subjective to culture and PAR histories and may yield contradictory results; and (iii) buoyant surface-dwelling populations, rather than exhibiting senescence, are poised at optimizing PAR utilization, thereby maintaining numerical dominance in eutrophic waters when physico-chemical conditions favor bloom formation.     

Partitioning of CO2 Fixation in the Colonial Cyanobacterium Microcystis aeruginosa: Mechanism Promoting Formation of Surface Scums

Constraints on inorganic carbon (C_i) availability stimulated buoyancy in natural, photosynthetically active populations of the colonial blue-green alga (cyanobacterium) Microcystis aeruginosa. In nonmixed eutrophic river water and cultures, O₂ evolution determinations indicated C_i limitation of photosynthesis, which was overcome either by CO₂ additions to the aqueous phase or by exposure of buoyant colonies to atmospheric CO₂. Microautoradiographs of M. aeruginosa colonies revealed partitioning of ¹⁴CO₂ fixation and photosynthate accumulation between peripheral and internal cells, particularly in large colonies. When illuminated colonies were suspended in the aqueous phase, peripheral cells accounted for at least 90% of the ¹⁴CO₂ assimilation, whereas internal cells remained unlabeled. However, when ¹⁴CO₂ was allowed to diffuse into colonies 15 min before illumination, a more uniform distribution of labeling was observed. Resultant differences in labeling patterns were most likely due to peripheral cells more exclusively utilizing CO₂ when ambient C_i concentrations were low. Among colonies located at the air-water interface, internal cells showed an increased share of photosynthate production when atmospheric ¹⁴CO₂ was supplied. This indicated that C_i transport was restricted in large colonies below the water surface, forcing internal cells to maintain a high degree of buoyancy, thus promoting the formation of surface scums. At the surface, C_i restrictions were alleviated. Accordingly, scum formation appears to have an ecological function, allowing cyanobacteria access to atmospheric CO₂ when the C_i concentration is growth limiting in the water column.     

A mini-review of microbial consortia: Their roles in aquatic production and biogeochemical cycling

Molecular oxygen (O₂) is a potent inhibitor of key microbial processes, including photosynthesis, N₂ fixation, denitrification, sulfate reduction, methanogenesis, iron, and metal reduction reactions. Prokaryote survival and proliferation in aquatic environments is often controlled by the ability to tolerate exposure to oxic conditions. Many prokaryotes do not have subcellular organelles for isolating O₂-producing from O₂-consuming processes and have developed consortial associations with other prokaryotes and eukaryotes that alleviate metabolic constraints of high O₂. Nutrient transformations often rely on appropriate cellular and microenvironmental, or microzonal, redox conditions. The spatial and temporal requirements for microenvironmental overlap among microbial groups involved in nutrient transformations necessitates close proximity and diffusional exchange with other biogeochemically distinct, yet complementary, microbial groups. Microbial consortia exist at different levels of community and metabolic complexity, as shown for detrital, microbial mat, biofilm, and planktonic microalgal-bacterial assemblages. To assess the macroscale impacts of consortial interactions, studies should focus on the range of relevant temporal (minutes to hours) and spatial (microns to centimeters) scales controlling microbial production, nutrient exchange, and cycling. In this review, we discuss the utility and application of techniques suitable for determining microscale consortial activity, production, community composition, and interactions in the context of larger scale aquatic ecosystem structure and function. Correspondence to: Hans W. Paerl.     

Estimating denitrification rates in estuarine sediments: A comparison of stoichiometric and acetylene based methods

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrient stoichiometry in the subtidal sediments of Tomales Bay, California (USA). By amending whole cores with acetylene and saturating nitrate concentrations, we obtained potential denitrification rates, which ranged between 4 and 30 mmol N m^–2 d^–1. We determined the apparent Michaelis constant (K_app) and the maximum potential rate (V_mp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the K_app and V_mp of the denitrifying community exhibited significant variation over both depth in the sediment column and time of sampling.Estimates ofin situ denitrification obtained using our kinetic-fix adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m^–1 d^–1. Denitrification rates obtained using benthic flux stoichiometry ranged between 0.7 and 4.1 mmol N m^–2 d^–1. Average denitrification rates obtained using the kinetic-fix acetylene block approach exceeded those obtained from net benthic flux stoichiometry; however, these differences were not significant. We conclude that our kinetic-fix adaptation of the acetylene block technique provides realistic estimates of denitrification in sediments, even when pore water nitrate concentrations are low and nitrification and denitrification are closely coupled.     

A highly conserved nuclear gene for low-level phylogenetics: elongation factor-1 alpha recovers morphology-based tree for heliothine moths

Molecular systematists need increased access to nuclear genes. Highly conserved, low copy number protein-encoding nuclear genes have attractive features for phylogenetic inference but have heretofore been applied mostly to very ancient divergences. By virtue of their synonymous substitutions, such genes should contain a wealth of information about lower-level taxonomic relationships as well, with the advantage that amino acid conservatism makes both alignment and primer definition straightforward. We tested this postulate for the elongation factor-1 alpha (EF-1 alpha) gene in the noctuid moth subfamily Heliothinae, which has probably diversified since the middle Tertiary. We sequenced 1,240 bp in 18 taxa representing heliothine groupings strongly supported by previous morphological and allozyme studies. The single most parsimonious gene tree and the neighbor-joining tree for all nucleotides show almost complete concordance with the morphological tree. Homoplasy and pairwise divergence levels are low, transition/transversion ratios are high, and phylogenetic information is spread evenly across gene regions. The EF-1 alpha gene and presumably other highly conserved genes hold much promise for phylogenetics of Tertiary age eukaryote groups.      

Role of heterotrophic bacteria in promoting N2 fixation byAnabaena in aquatic habitats

Anabaena species are commonly colonized by bacteria, especially during N₂-fixing blooms. Generally these associations do not represent bacterial attack on algal hosts. Instead, the algal N₂-fixing capabilities are increased in the presence of the bacteria. Possible mechanisms promoting the mutual growth of algae and attached bacteria were investigated by observing specific sites of bacterial attachment, by noting reduced microzones created by the bacteria, and by locating sites of bacterial uptake of organics representative of algal excretion products.Attached bacteria show preference for typical algal excretion products and their growth is enhanced by such products. In return, enhancement of algal nitrogenase activity occurs when bacteria create O₂-consuming microzones around the nitrogenase-bearing heterocysts.     

Chick embryo fibroblasts produce two forms of hyaluronidase

Cultured chick embryo fibroblasts derived from skin and skeletal muscle exhibit hyaluronidase activity both associated with the cell layer and secreted into the medium. Although both forms of the enzyme have a number of similar characteristics (R.W. Orkin and B.P. Toole, 1980, J. Biol. CHem. 255), they differ in thermal stability at neutral pH and in behavior on ion-exchange chromatography. Both forms of the enzyme are equally stable at acidic pH for long intervals, but the cell-associated hyaluronidase is significantly less stable than the secreted froms at neutral pH and at temperatures more than or equal to 30 degrees C. Neither the presence of proteases nor inhibitors of hyaluronidase appear to be involved in the cell-asspcoated enzyme. Chromatography of the two forms of hyaluronidase on carboxymethyl cellulose reveals that most (60-90 percent) of the secreted form of the enzyme elutes at a lower ionic strength than the cell- associated enzyme. Treatment of the secreted form of hyaluronidase with neuraminidase shifts its elution profile on carboxymethyl cellulose toward that of the cell-associated form, and also decreases its thermal stability at neutral pH. In contrast, treatment of the secreted form of hyaluronidase with alkaline phosphatase has no detectable effect. These data suggest that the secreted hyaluronidase differs from the cellular form in possessing additional sialic acid residues which endow the former with increased stability in the extracellular milieu.     

Physiological Adaptations in Response to Environmental Stress During an N2-Fixing Anabaena Bloom

Anabaena spiroides has the ability to maintain intense biomass production for extensive periods in the epilimnion of a small eutrophic lake characterized by conditions shown to cause photooxidative death in a number of other phytoplankton. By the enhancement of carotenoid synthesis chlorophyll a was protected from photooxidation and prevented from catalyzing other photooxidative reactions within the cells. By temporally separating CO₂ and N₂ fixation, maximum utilization of photosynthetically active radiation was achieved. Because CO₂ fixation was more sensitive than N₂ fixation to a high oxygen concentration, the former was maximized during morning hours, before the afternoon buildup of dissolved oxygen. The diurnal partitioning of carbon and N₂ fixation has two additional advantages; possible competition for reductant-generating compounds is minimized, and adequate endogenous pools of carbon skeletons are assured to accept newly fixed ammonia. Hence, Anabaena, far from undergoing photooxidative death, appears to utilize a physiological strategy which allows optimization of radiant energy use for reductive processes and dominance of surface waters and shading of deeper phytoplankton during summer blooms.     

SPECIFIC ASSOCIATIONS OF THE BLUEGREEN ALGAE ANABAENA AND APHANIZOMENON WITH BACTERIA IN FRESHWATER BLOOMS1

Scanning electron micrographs and autoradiographs of Anabaena circinalis Rabenh. and Aphanizomenon flos-aquae (L.) Ralfs in samples from fresh-water communities show that bacteria are attached specifically at the polar region of heterocysts of these known N₂ fixers. This algal-bacterial association occurs most frequently during bloom conditions. The possible roles of this association in maintaining nuisance bloom conditions are discussed.     

Harmful Cyanobacterial Blooms: Causes, Consequences, and Controls

Cyanobacteria are the Earth’s oldest oxygenic photoautotrophs and have had major impacts on shaping its biosphere. Their long evolutionary history (～3.5 by) has enabled them to adapt to geochemical and climatic changes, and more recently anthropogenic modifications of aquatic environments, including nutrient over-enrichment (eutrophication), water diversions, withdrawals, and salinization. Many cyanobacterial genera exhibit optimal growth rates and bloom potentials at relatively high water temperatures; hence global warming plays a key role in their expansion and persistence. Bloom-forming cyanobacterial taxa can be harmful from environmental, organismal, and human health perspectives by outcompeting beneficial phytoplankton, depleting oxygen upon bloom senescence, and producing a variety of toxic secondary metabolites (e.g., cyanotoxins). How environmental factors impact cyanotoxin production is the subject of ongoing research, but nutrient (N, P and trace metals) supply rates, light, temperature, oxidative stressors, interactions with other biota (bacteria, viruses and animal grazers), and most likely, the combined effects of these factors are all involved. Accordingly, strategies aimed at controlling and mitigating harmful blooms have focused on manipulating these dynamic factors. The applicability and feasibility of various controls and management approaches is discussed for natural waters and drinking water supplies. Strategies based on physical, chemical, and biological manipulations of specific factors show promise; however, a key underlying approach that should be considered in almost all instances is nutrient (both N and P) input reductions; which have been shown to effectively reduce cyanobacterial biomass, and therefore limit health risks and frequencies of hypoxic events.