Evaluation of Nitrogen Source Uptake by the Bivalve Nuttallia olivacea Fed with 15N-Labeled Diatoms.

Marine diatoms are an important food resource for bivalves, but few experimental studies have evaluated diatom assimilation by bivalves. We conducted a laboratory experiment to investigate the ability of the suspension-feeding bivalve Nuttallia olivacea to utilize three common diatom species (planktonic diatoms Thalassiosira pseudonana and Skeletonema dohrnii and the benthic diatom Entomoneis paludosa) as food labeled with heavy nitrogen stable isotope (15N) by incubation in medium containing Na15NO3. The percentage of food-derived nitrogen in the organs of the bivalves increased over time, confirming that the bivalves were taking up dietary nitrogen from diatoms. The proportion of food-derived nitrogen from diatoms to bivalves appeared to be higher in planktonic species than in benthic species. However, it is possible that the benthic diatom intake by the bivalves in this study was underestimated because the substrate was not disturbed as would occur under field conditions. The percentage of food-derived nitrogen in bivalve organs tended to be highest in the digestive diverticula, followed by the foot, mantle, and siphon, regardless of diatom type. These findings suggest that N. olivacea may preferentially distribute nitrogen to organs other than the siphon, which is prone to continuous loss by fish predation.

The tiny-leaved orchid Cephalanthera subaphylla obtains most of its carbon via mycoheterotrophy.

The evolution of mycoheterotrophy has been accompanied by extreme reductions in plant leaf size and photosynthetic capacity. Partially mycoheterotrophic plants, which obtain carbon from both photosynthesis and their mycorrhizal fungi, include species with leaves of normal size and others that are tiny-leaved. Thus, plant species may lose their leaves in a gradual process of size reduction rather than through a single step mutation. Little is known about how the degree of mycoheterotrophy changes during reductions in leaf size. We compared the degree of mycoheterotrophy among five Japanese Cephalanthera species, four with leaves of normal size (Cephalanthera falcata, Cephalanthera erecta, Cephalanthera longibracteata and Cephalanthera longifolia), one with tiny leaves (Cephalanthera subaphylla), and one albino form of C. falcata (as reference specimens for fully mycoheterotrophic plants). The levels of mycoheterotrophy were determined by stable isotope natural abundance analysis. All Cephalanthera species were relatively enriched in 13C and 15N in comparison with surrounding autotrophic plants. Cephalanthera subaphylla was strongly enriched in 13C and 15N to levels similar to the albinos. Species with leaves of normal size were significantly less enriched in 13C than C. subaphylla and the albinos. Thus, C. subaphylla was strongly mycoheterotrophic, obtaining most of its carbon from mycorrhizal fungi even though it has tiny leaves; species with leaves of normal size were partially mycoheterotrophic. Hence, during the evolutionary pathway to full mycoheterotrophy, some plant species appear to have gained strong mycoheterotrophic abilities before completely losing foliage leaves.

Influence of submarine topography and sediment environment on microbial assemblages in a coastal lagoon in northeastern Japan.

The relationships among eutrophication, anoxia, and microbial distribution were investigated for Nagatsura-Ura Lagoon on the northeastern Pacific coast of Japan. In September 2017, the bottom environment in a small area of the inner part of the lagoon (which has a basin-shaped bottom topology) was eutrophic and anoxic, with high carbon, nitrogen, phosphate, acid-volatile sulfide, and low dissolved oxygen and oxidation-reduction potential. Dissolved oxygen levels improved during the winter. Bacillariophyta (diatoms) were the main organic component according to pigment analysis and next-generation sequencing of nucleic acids in seawater samples. Phylum Proteobacteria was dominant among the bacterial flora in the sediment but the proportions of Class Epsilon-proteobacteria and Chlorobium (a green sulfur-utilizing bacterium) were high in the inner part of the lagoon compared to other stations, and these groups were also present in winter. Apparently groups able to thrive in both anoxic and aerobic conditions were predominant in the inner part of the lagoon.