Hadal zone microbial communities
Hadal zone microbial communities are the groups of microorganisms which reside within hadal zones, which consist of many individual deep oceanic trenches found around the world.
The environmental conditions of the trench environments select for the microbe communities. Generally, the environmental properties of the hadal zone are similar to the abyssal; temperature varies between different trenches, as do salinity and nutrients. A unique characteristic of these hadal zones is the large abundance and accumulation of particulate organic matter in these trenches. These high concentrations correspond directly to high microbial productivity. The physical bathymetry of the trenches is also critical to understandings of microbial communities as this influences the distribution of organic matter. Microbes play a great role in the cycling of this organic matter and other nutrients within the hadal zone.
Hadal zone microbial communities are highly diverse, and include bacteria, archaea as well as viruses. Hadal zone bacterial sediment communities are much more abundant than adjacent abyssal communities thus implicating the importance of distinguishing these two microbial communities. Archaeal microbes also make up a substantial portion of hadal microbial communities and are in fact found to be the most abundant within hadal sediments. Although viral communities have been found to be in a greater amount than nearby abyssal zones, a large proportion of viral genomes are unknown and thus suggest a large gap in viral knowledge within hadal zones.
As the microbial community composition in the hadal zones is incredibly diverse, so is their metabolic potential. The accumulation of organic matter results in the hadal zone being a metabolically rich environment. The most dominant form of metabolism utilized by hadal zone microbes is heterotrophic microbial metabolism. Other forms of dominant metabolism include fermentation, anammox, denitrification, sulfate reductive metabolism, as well as chemolithotrophic pathways such as sulfur oxidation, iron oxidation, nitrification and carbon fixation. Surprisingly, despite the low oxygen conditions, aerobic metabolic potential has also been identified. No phototrophic lifestyles have been found within hadal zone microbes.
With 37 hadal trenches around the globe, the importance of understanding hadal microbial communities and community composition can not be overstated. However, a deep understanding of the microbial ecosystem in the hadal zone still remains largely uncharacterized. It remains one of the least studied and explored microbial habitats due to difficulties with sampling. Exploring the hadal zone and its communities can shed light on the dark biosphere as well as possible aid scientists in exploring oceans even beyond Earth.
Environmental properties
Temperature, salinity, and flows
The environmental properties of the hadal zone are similar to those of the deep end of the abyssal zone. Temperature in the hadal zone mostly ranges between 1 and 2.5 °C and varies between different trenches. Measurements from the Izu-Ogasawara Trench suggest that the hadal zone has slightly higher salinity and dissolved oxygen than the abyssal zone. Rising pressure with depth causes the in-situ temperature of water in trenches to increase without heat exchange with the surrounding water. Movement of water in hadal trenches is driven by deep water currents, such as the North Pacific Deep Water and Lower Circum-Polar Water currents.Particulate organic matter (POM)
Research has found an abundance of particulate organic matter within various hadal zone environments, such as the Mariana Trench and Atacama Trench. POM has been found both in the water column and sediments of the Mariana Trench. Measurements taken in sediments at the deepest depths of the Mariana Trench show higher amounts of organic carbon than in shallower depths of the abyssal zone.Additionally, large amounts of organic matter sink down and accumulate in the sediments of the Atacama trench. Sediments here had high amounts of organic carbon and proteins that exceed the amount found in the Abyssal zone sediments. These concentrations were also comparable to coastal areas where productivity is high. Most organic matter deposited in the hadal zone comprises phytoplankton from the surface which sinks down to deeper depths.
Hadal sediments appear to have higher turnover rates of carbon compared to sediments at abyssal depths due to high microbial activity, which is enabled by the large amount of organic matter deposited onto the sediments. POM deposited in the Atacama trench is consumed in microbial activity and mineralization, rendering the Atacama trench a nutrient-dense hadal environment. Additionally, sediments in the hadal zone of the Mariana Trench had higher concentrations of Chlorophyll a and breakdown products than sediments at 6000m depth, suggesting that the deeper sediments may contain organic matter that is of higher quality and more readily accessible to microbes.
Biological oxygen consumption rate was found to be higher in sediments in the hadal zone compared to those at 6000m depth in the Mariana Trench. There is also a higher rate of organic matter diagenesis compared to the abyssal zone. Dark carbon fixation, or chemosynthesis, may also be a source of organic matter for microbes in trenches in the hadal zone.
Physical bathymetry and transport
The physical bathymetry of trench environments influences how organic matter is distributed. In contrast to the notion that biomass reduces with depth due to remineralisation of organic carbon, biomass production in the hadal zone is supported by abundant organic matter sinking from shallower depths. Trenches are characterised by slopes, which may result in a downward movement of organic material and sediment deposited on the trenches. Downslope movement of material could also be driven by tectonic activity and materials being suspended and redeposited over time.Downslope transport of sediments influences the distribution of benthic biomass. This was seen in modelling of the Kermadec Trench which showed how biomass is variably distributed across a trench due to the incline and positioning of the trench slopes. Additionally, isotopic carbon composition measurements of organic carbon in hadal zone sediments in the New Britain Trench reveal that organic matter from terrestrial environments within sediments is carried downslope. The surface characteristics of the trench, such as how rugged or flat it is, can also affect the distribution of biomass. Sediments of the Mariana Trench appear to have an uneven and variable profile, suggesting that sediments are frequently deposited and accumulated at the bottom of the hadal zone.
Nutrient cycling
Studies have found that varying types of microbes are found in different trenches, suggesting that nutrient cycle regimes vary between environments in the hadal zone. The presence of nitrifying microbes has been detected in the transition depths between the abyssal and hadal zone, suggesting that microbes in the sediment may utilize nitrification as a source of energy. Sulfur cycling plays an important role in sulfur reduction and oxidation within the hadal zone, and microbes that facilitate both processes have been discovered in the Mariana Trench. These microbes may also contribute to carbon fixation in hadal zone sediments.Distribution
Other geological units may fall into the hadal zone, however, Hadal zones are almost exclusively confined to the 37 hadal trenches. Hadal trenches are thought to have formed from the slow subduction of older, dense lithosphere. This is supported by the distribution of hadal zones, most of which in continent-ocean subduction zones. The most well known hadal trench is the Mariana trench, but there are hadal trenches distributed around the world's oceans.Spatial distribution
Microbes that live in the hadal zone are more dependent on recycled organic matter, hypothesised to be due to the trench geomorphology, than their abyssal counterparts that are more dependent on chemolithotrophy. The dependence on organic matter is supported by results that suggest that some combination of hadal currents, rapid burial, and vertical deposition from surface waters drives the organic matter cycle in the hadal zone. The deposition of sediments reflects the overlying ocean's productivity, and is much higher at hadal zones than the abyssal zones due to the input of landslides funnelling into the hadal zone. The organic matter in the hadal zone is more quickly buried than in the abyssal zone, allowing it to avoid oxidative degradation and become available for subsurface microbes. This sedimentation of labile, organic matter is the reason why microbial communities distinct to the hadal zone may be observed globally. These microbial communities have been observed to have different abundances and compositions based on sea surface conditions.Hadal depths have unique relative phylum abundances even when compared to abyssal depths. It has been found that the hadal depth's prokaryotic SSU rRNA gene communities in the challenger deep are dominated by the bacteria SAR406 and bacteroidetes, and the deepest depths have an increase in gammaproteobacteria, which both contrast with the aphotic waters above that are dominated by SAR11 and thaumarchaeota. In general the abundance of microbes in the hadal zone has been observed to be higher than the abyssal zone, and in the Mariana Trench it was found to increase with depth.