Long-term warming destabilizes aquatic ecosystems through weakening biodiversity-mediated causal networks.

Understanding how ecosystems will respond to climate changes requires unravelling the network of functional responses and feedbacks among biodiversity, physicochemical environments, and productivity. These ecosystem components not only change over time but also interact with each other. Therefore, investigation of individual relationships may give limited insights into their interdependencies and limit ability to predict future ecosystem states. We address this problem by analyzing long-term (16-39 years) time series data from 10 aquatic ecosystems and using convergent cross mapping (CCM) to quantify the causal networks linking phytoplankton species richness, biomass, and physicochemical factors. We determined that individual quantities (e.g., total species richness or nutrients) were not significant predictors of ecosystem stability (quantified as long-term fluctuation of phytoplankton biomass); rather, the integrated causal pathway in the ecosystem network, composed of the interactions among species richness, nutrient cycling, and phytoplankton biomass, was the best predictor of stability. Furthermore, systems that experienced stronger warming over time had both weakened causal interactions and larger fluctuations. Thus, rather than thinking in terms of separate factors, a more holistic network view, that causally links species richness and the other ecosystem components, is required to understand and predict climate impacts on the temporal stability of aquatic ecosystems.

Effects of Algal Extracellular Polysaccharides on the Formation of Filamentous Manganese Oxide Particles in the Near-Bottom Layer of Lake Biwa.

Filamentous manganese (Mn) oxide particles, which occur in the suboxic zone of stratified waterbodies, are important drivers of diverse elemental cycles. These particles are considered to be bacteriogenic; despite the importance of biogeochemical implications, however, the environmental factor responsible for their formation has not been identified. The aim of this study was to demonstrate the involvement of algal extracellular polysaccharides in Mn oxide particle formation. Based on this study of laboratory cultures of a model Mn(II)-oxidizing bacterium, the supply of algal extracellular mucilage was shown to stimulate Mn(II) oxidation and thus the production of filamentous Mn oxide particles. This observation was consistent with the results obtained for naturally occurring particles collected from a near-bottom layer (depth of approximately 90 m) in the northern basin of Lake Biwa, Japan, that is, most Mn particles resembling δ-MnO2 were associated with an extracellular mucilage-like gelatinous matrix, which contained dead algal cells and was lectin-stainable. In the lake water column, polysaccharides produced by algal photosynthesis sank to the bottom layer. The analysis of the quality of water samples, which have been collected from the study site for 18 years, reveals that the annual average total phytoplankton biovolume in the surface layer correlates with the density of filamentous Mn particles in the near-bottom layer. Among different phytoplankton species, green algae appeared to be the key species. The results of this study suggest that algal extracellular polysaccharides serve as an important inducer for the formation of filamentous Mn oxide particles in the near-bottom layer of the northern basin of Lake Biwa.

Causal networks of phytoplankton diversity and biomass are modulated by environmental context.

Untangling causal links and feedbacks among biodiversity, ecosystem functioning, and environmental factors is challenging due to their complex and context-dependent interactions (e.g., a nutrient-dependent relationship between diversity and biomass). Consequently, studies that only consider separable, unidirectional effects can produce divergent conclusions and equivocal ecological implications. To address this complexity, we use empirical dynamic modeling to assemble causal networks for 19 natural aquatic ecosystems (N24◦~N58◦) and quantified strengths of feedbacks among phytoplankton diversity, phytoplankton biomass, and environmental factors. Through a cross-system comparison, we identify macroecological patterns; in more diverse, oligotrophic ecosystems, biodiversity effects are more important than environmental effects (nutrients and temperature) as drivers of biomass. Furthermore, feedback strengths vary with productivity. In warm, productive systems, strong nitrate-mediated feedbacks usually prevail, whereas there are strong, phosphate-mediated feedbacks in cold, less productive systems. Our findings, based on recovered feedbacks, highlight the importance of a network view in future ecosystem management.

Dominance of harmful algae, Microcystis spp. and Micrasterias hardyi, has negative consequences for bivalves in a freshwater lake.

The proliferation of cyanobacteria Microcystis spp. and the invading green alga Micrasterias hardyi in Lake Biwa has been increasing. However, the available knowledge on the dietary utilization of these cyanobacterial and algal species by bivalves, which are key species in lake ecosystems, is limited. In this study, we examined the dietary quality and utilization of these species by freshwater bivalves of the Corbicula spp., which are important fishery resources, by performing feeding experiments and field investigations based on fatty acid profiles and stomach content analysis. Although a significant increase in the dry weight and condition factor of the Corbicula spp. individuals fed on diatom was observed at the end of the experiment, for the individuals fed on Microcystis aeruginosa or M. hardyi, a dry weight increase was not observed and their condition factor decreased. Moreover, the fatty acid profile of the Corbicula spp. individuals fed on M. aeruginosa or M. hardyi indicated that they did not assimilate these diets, even though filtration was confirmed during the experiments. However, the stomach contents of wild Corbicula spp. specimens, collected from six sampling sites in Lake Biwa on four sampling occasions, showed that Microcystis spp. were the most abundant dietary items in all sites and on all occasions. Moreover, M. hardyi was detected during the analysis of stomach contents; this alga was the third most abundant algal species. As shown in the feeding experiments, they do not contribute to bivalve growth, indicating that the high occupation of Microcystis spp. and M. hardyi in the consumer's stomach may inhibit effective carbon transfer. The expansion of these unsuitable dietary organisms may affect the stability of lake ecosystems.

Whole-Genome Sequences of Two Manganese(II)-Oxidizing Bacteria, Bosea sp. Strain BIWAKO-01 and Alphaproteobacterium Strain U9-1i.

This report describes the whole-genome sequences of two Mn(II)-oxidizing bacteria, filamentous Mn oxide microparticle-forming Bosea sp. strain BIWAKO-01 and alphaproteobacterium strain U9-1i.

Effects of environmental factors on microalgal biomass production in wastewater using cyanobacteria Aphanothece clathrata and Microcystis wesenbergii.

Effects of temperature, light, coexistent microbes, and dissolved matter on the growth of cyanobacteria Aphanothece clathrata and Microcystis wesenbergii were discussed using a batch incubation system in this research for microbial biomass production in wastewater. As a result, water temperature in the effluents from the municipal wastewater treatment plants in Tokyo was suitable for the growth of these cyanobacteria, though preheating of wastewater may be required for M. wesenbergii in winter. The dissolved matter in the treated wastewater did not affect the growth of A. clathrata and enhanced the growth of M. wesenbergii. However, the microbes in the treated wastewater attacked the cyanobacteria and inhibited their growth. Therefore, pretreatment of microbes might be required for biomass production in the treated wastewater. The maximum methane production potentials of A. clathrata and M. wesenbergii in the treated wastewater were estimated to be 13.5 and 2.12 L-CH4•m-2•day-1, respectively, when the depth of the bioreactor was set at 4.8 m for A. clathrata and 0.7 m for M. wesenbergii. The potential of A. clathrata was higher than that of grasses. Consequently, cyanobacteria, especially A. clathrata, may become a good biomass for bioenergy production.