Bathymetric patterns of deep-sea benthic communities from bathyal to abyssal depths in the western South Pacific (Solomon and Coral Seas)

Distribution Patterns of Benthic Protist Communities Depending on Depth Revealed by Environmental Sequencing-From the Sublittoral to the Deep Sea

Protists are key components of the microbial food web in marine pelagic systems because they link algal and bacterial production to higher trophic levels. However, their functioning and bathymetric distribution in benthic deep-sea ecosystems are still only poorly understood. However, biogeographical patterns of communities can be coupled to the functioning of ecosystems and are therefore important to understand ecological and evolutionary processes. In this study, we investigated the diversity and distribution of benthic protist communities from the sublittoral down to the deep seafloor (50-2000 m) around three islands of the Azores in the North Atlantic Ocean. Using amplicon sequencing of the V9 region (18S rDNA) of 21 samples, we found that protist community compositions from different depths were significantly different. Three assemblages were separated along the following depths: 50 m, 150-500 m and 1000-2000 m, which indicate that deep-sea areas surrounding islands might act as isolating barriers for benthic protist species. A limited gene flow between the communities could favor speciation processes, leading to the unique protist communities found at the different investigated islands.

Body Size Versus Depth: Regional and Taxonomical Variation in Deep-Sea Meio- and Macrofaunal Organisms

Body size (weight per individual) is an important concept in ecology. It has been studied in the deep sea where a decrease in size with increasing depth has often been found. This has been explained as an adaptation to food limitation where size reduction results in a lowered metabolic rate and a decreased energetic requirement. However, observations vary, with some studies showing an increase in size with depth, and some finding no depth correlation at all. Here, we collected data from peer-reviewed studies on macro- and meiofaunal abundance and biomass, creating two datasets allowing statistical comparison of factors expected to influence body size in meio- and macrofaunal organisms. Our analyses examined the influence of region, taxonomic group and sampling method on the body size of meiofauna and macrofauna in the deep sea with increasing depth, and the resulting models are presented. At the global scale, meio- and macrofaunal communities show a decrease in body size with increasing depth as expected with the food limitation hypothesis. However, at the regional scale there were differences in trends of body size with depth, either showing a decrease (e.g. southwest Pacific Ocean; meio- and macrofauna) or increase (e.g. Gulf of Mexico; meiofauna only) compared to a global mean. Taxonomic groups also showed differences in body size trends compared to total community average (e.g. Crustacea and Bivalvia). Care must be taken when conducting these studies, as our analyses indicated that sampling method exerts a significant influence on research results. It is possible that differences in physiology, lifestyle and life history characteristics result in different responses to an increase in depth and/or decrease in food availability. This will have implications in the future as food supply to the deep sea changes as a result of climate change (e.g. increased ocean stratification at low to mid latitudes and reduced sea ice duration at high latitudes).

The coral sea: physical environment, ecosystem status and biodiversity assets

The Coral Sea, located at the southwestern rim of the Pacific Ocean, is the only tropical marginal sea where human impacts remain relatively minor. Patterns and processes identified within the region have global relevance as a baseline for understanding impacts in more disturbed tropical locations. Despite 70 years of documented research, the Coral Sea has been relatively neglected, with a slower rate of increase in publications over the past 20 years than total marine research globally. We review current knowledge of the Coral Sea to provide an overview of regional geology, oceanography, ecology and fisheries. Interactions between physical features and biological assemblages influence ecological processes and the direction and strength of connectivity among Coral Sea ecosystems. To inform management effectively, we will need to fill some major knowledge gaps, including geographic gaps in sampling and a lack of integration of research themes, which hinder the understanding of most ecosystem processes.

Molecular evidence demonstrating the basidiomycetous fungus Cryptococcus curvatus is the dominant microbial eukaryote in sediment at the Kuroshima Knoll methane seep

The Kuroshima Knoll, located in the southern Ryukyu Arc, is known to actively bubble with gas containing methane and hydrogen sulfide from numerous fissures in the large carbonate pavement. Although ecological studies regarding macrobenthos and bacteria from Kuroshima Knoll have been intensively conducted, the community structure and ecological importance of microbial eukaryotes (protists) have not yet been investigated. In the present study, we directly extracted DNA from sediment of the Kuroshima Knoll at a depth of 640 m and constructed genetic libraries of PCR-amplified eukaryotic small-subunit ribosomal DNA (SSU rDNA). Although the SSU rDNA sequences of several types of benthic foraminifers were retrieved from the surface of the sediment, all other sequences (just below the sediment surface to approximately 9 cm below sediment surface) were derived from the basidiomycetous yeast Cryptococcus curvatus. Furthermore, sequences of the internal transcribed spacer of rDNA (ITS-rDNA) retrieved from the same sediment were identical to that of C. curvatus originating from terrestrial habitats. The diversity of microbial eukaryotes in the Kuroshima Knoll sediment seems to be extremely low and significantly different from that of other marine environments previously reported.