The deep-sea is the most extensive environment on the Earth and is defined as the water column and seafloor below 200 m water depth. The ecosystem services (E.S.) concept comprises ecological functions (e.g., climatic adaptation) and social and economic values (e.g., food security, job creation) that promote human well-being. Deep-sea E.S. comprise the provisioning of services such as industrial agents and fish catch, regulating services such as climatic and biological regulation, and cultural services such as aesthetics and inspiration for the arts. Several studies have shown that deep-sea ecosystems support a large variety of habitats and a wide array of faunal communities with various functions. These complex communities are threatened by the growth of human activities targeting precisely these provided ecosystem services.
1. Bibliometric Analysis Using VOSviewer
The visualization map of the documents published between 2012 and 2021 (Figure 1
a) shows the range of keywords and their co-occurrences related to deep-sea ecosystem services. Co-occurrences are utilized to understand the underlying patterns of the document set under investigation 
Figure 1. (a) Visualization map based on publication-weights, (b) visualization overlay map showing the current scientific production (2017–2021). The circle size is proportional to the number of published studies related to the items (terms or keywords), while the line represents the link between every item. The link between every term is strong when the line is short.
In total, 291 terms were presented and grouped into 5 clusters, with 29,643 links and a total link strength of 307,534. The main keyword co-occurrences for the red cluster (red circles in Figure 1a) are management (co-occurrence: 611), approach (co-occurrence: 654), ecosystem services (co-occurrence: 562) and assessment (co-occurrence: 475), followed by the green circles, for which the main keywords are diversity (co-occurrence: 636), abundance (co-occurrence: 592), concentration (co-occurrence: 565) and sediment (co-occurrence: 480). The main topic corresponding to the third cluster (blue circles) is climate change, where the main keywords are temperature (co-occurrence: 478) and warming (co-occurrence: 122). The main keywords related to the yellow cluster are carbon sequestration (co-occurrences: 240) and sea-level rise (co-occurrences: 176). The fifth cluster is mainly related to biodiversity (co-occurrence: 566), biomass (co-occurrence: 400) and ecosystem function (co-occurrence: 218).
Each cluster covers the following topics:
(i) The ecosystem services and goods, management and assessment tools and approaches cluster (red cluster in Figure 1a; 98 terms) reveals that the studies on ocean governance and conservation and the assessment of deep-sea ecosystem services are relatively advanced. Such studies mainly focus on laws and regulations regarding the exploration and exploitation of deep-sea resources, especially oil and gas and deep-sea minerals. However, the occurrence of keywords such as “knowledge” (co-occurrence: 286), “knowledge gap” (co-occurrence: 58), “gap” (co-occurrence: 110), “challenge” (co-occurrence: 242) and “uncertainty” (co-occurrence: 111) demonstrate that researchers are still investigating how to overcome the lack of knowledge related to the field of “deep-sea ecosystem services”, which is the main concern against, for instance, the development of deep-sea mining.
(ii) The deep-sea sediments, contamination and microplastic pollution cluster (green cluster in Figure 4a; 82 terms) mainly refers to studies related to deep-sea sediments analysis and contamination assessment. Such studies focus on texture, organic matter; carbonate content, geochemical composition (e.g., heavy metals), oil spills and contamination and nano- and micro-plastic pollution in deep-sea sediments.
(iii) The deep-sea role in climate change mitigation, ocean warming and temperature increase impact on marine animals cluster (blue cluster in Figure 1a; 67 terms) groups studies related to climate change impact on deep-sea fauna (e.g., migration, mortality rate). In particular, these studies aim to evaluate the impact of climate change on the Antarctic deep-sea life.
(iv) The fourth cluster (yellow cluster in Figure 1a; 31 terms) is about the impact of ocean chemistry variability and sea-level rise on carbon cycling, using sediment cores and stable isotopes to highlight changes and variations of the global carbon cycle.
(v) The biomass and biodiversity cluster (purple cluster in Figure 1a; 13 terms) identifies scientific research carried out to understand the ecosystem functioning by analyzing the species richness and composition.
Figure 1b displays the network map of the topic trend based on the keywords used from 2017 to 2021. This map is called overlay visualization, and the color bar shows the current scientific production. Purple keywords (e.g., ecosystem services, biodiversity) have been published in 2017, while green circles show that terms such as sediment, concentrations, abundance and conservation characterize the research work published in 2018. The more recently published works (between 2019 and 2020) mainly focus on micro-plastics, pollution and bacterial communities. However, the studies published in 2019 and 2020 show a small size circle reflecting the slight weight of items (keywords), which means that these fields are not well-studied while their occurrence proves there is a trend toward studies on micro-plastic pollution.
2. Evaluation of Deep-Sea Services
Deep-sea goods and services must be first identified and characterized in order to quantify their benefits for human well-being. Human profit should note the environmental component given that any change in an ecosystem will affect the sustainability of the provided services and goods. Few studies have examined deep-sea services because of the knowledge gap related to the functions, biodiversity and life in deep-sea ecosystems. The first study that analyzed deep-sea services and goods was carried out by Armstrong et al. 
and highlighted the crucial role played by the deep sea in the global biogeochemical cycle. Jobstvogt et al. 
reported the need for a better communication of the deep-sea ecological value to decision makers and the wider public for achieving ocean conservation targets, especially considering the general lack of knowledge and awareness about the deep-sea environment.
Currently, there is an increasing interest in exploring and exploiting deep-sea resources such as minerals, which carry a considerable economic potential 
. Thus, defining the supporting services of the deep-sea ecosystem that include habitats (e.g., seamount, abyssal plain, etc.) and biogeochemical cycling will allow researchers to quantify, on one hand, the benefits of these ecosystems and, on the other hand, to predict the potential risk of products used by humans and obtained from the habitat (provisioning services) such as the extraction of minerals.
As illustrated in Figure 2
, deep-sea ecosystems provide a wide variety of services to human welfare, which supports direct (provisioning, regulating and cultural services) and indirect (supporting services) services 
Deep-sea ecosystem goods and services (modified, source 
3. Combination Analysis of Deep-Sea Ecosystems Services
During the last four decades, the harvesting of deep-sea fisheries has increased due to the over-exploitation of continental shelf fish stocks 
. Seabed fisheries deploying bottom fishing gear to catch the target species put the benthic environment at risk 
. Numerous types of gear are used in deep-sea fisheries, such as bottom otter trawls, deep midwater trawls, bottom longlines, tangle nets, sink/anchor gillnets, pots and traps, which can destroy seabed habitats 
. Recent studies on the ecological effects of bottom-trawling focused on the physical impacts on soft sediments 
, the destruction of submarine features 
and the disturbance of benthic ecosystems, which concur to further decline the fish productivity 
Deep-sea species play an important role in biogeochemical cycling, which means that deep-sea fishery might affect the biogeochemistry of the global ocean. In addition, deep-sea fishes are slow-growing (some fish live > 100 years 
), and it can take hundreds of years to recover a species once damaged. According to an expert-based evaluation, bottom-trawling represents the highest threat to the marine benthic habitat, and seamounts shallower than 2000 m are significantly vulnerable to this fishing technique 
. Indeed, Ramalho et al. 
reported that bottom-trawling has negatively influenced the benthic community and associated ecosystem functions in the Western Iberian continental margin. In the Aotearoa seamounts, offshore of New Zealand, over the few past years, scientists have discovered 128 new species from fisheries bycatch 
, and the New Zealand fleet’s bottom nets dragged about 14.03 tons of corals in the 2018–2019 fishing year 
. Many other studies pointed out the adverse indirect and direct effects of trawling on benthic invertebrate communities and populations worldwide, with marked declines in biomass, abundance, species diversity and productivity 
3.2. Oil and Gas
The depletion of gas and oil resources on land was among the factors leading to the exploration and exploitation of gas and oil in the deep sea, the so-called offshore oil and gas industry 
. The operations consist of four stages: geological and geophysical investigation, exploration, production and decommissioning. Every step is associated with potential environmental impact, including chemical, physical and biological disturbance 
. Due to the absence of sufficient data related to deep-sea ecosystems, the environmental impact assessment of such activities is still limited 
. In addition, environmental management is challenging as deep-sea biological systems operate at a slower pace compared to shallow waters 
. Anthropogenic stressors resulting from deep-water oil and gas operations in such fragile ecosystems may influence habitats and species, making re-colonization and recovery difficult 
3.3. Deep-Sea Minerals
Deep-sea mineral extraction is identified as an alternative source of metals of economic interest and is claimed to be a future clean sector 
, unlike terrestrial mining, which generates pollutants into water and land 
. On the other hand, the risk and sustainability of such activities is still undefined because the ecological aspects of the deep-sea are unknown and studies are very few 
. The interest in this industry sector is substantially growing, but the risks associated with this kind of deep-sea operations remain immeasurable 
. Commercial mining tests and scientific investigations on the disturbance of polymetallic nodules have shown that the impact is severe after dredging operations, especially on habitat and biodiversity 
, and restoration is far from being implemented 
. The technologies and procedures for exploiting the deep sea for mining purposes could seriously harm the marine environment, including habitats, marine resources, biogeochemistry cycling and environmental quality and blue economy sectors (e.g., fisheries 
). Even subtle changes in the morphology of deep-sea abyssal plains have the potential to cause severe changes in benthic habitats 
. Furthermore, not only habitat and biodiversity in abyssal regions will be impacted by nodules operations, but the impact will also touch midwater and mesopelagic species together with biota through the entire water column, especially during the lifting of nodules to the surface 
3.4. Marine Renewable Energy (MRE)
MRE, the so-called ocean-based energy, looks promising in tackling dioxide emissions, meeting the growing energy demand, and reducing the human contribution to global warming 
. MRE include offshore winds farms (OWFs), solar energy, wave and tidal energy, in the latter case, the mattresses that stabilize submarine power cable may enhance benthic megafauna habitat capacity and increase artificial habitats for a range of fish and crustacean species 
. On the contrary, Dannheim et al. 
reported that MRE installations might impact the benthic compartment during the construction, operational or decommissioning stages.
The deep-sea OWE industry exerts potential associated risks and stressors on the environment that were defined by Boehlert and Gill 
and Copping et al. 
. These can be summarized as follows: (i) atmospheric and oceanic dynamics changes resulting from energy modification and removal; (ii) habitat alterations; (iii) electromagnetic field influence on deep-sea species from cables; (iv) underwater noise effects on marine species; (v) water quality changes.
3.5. Biotechnology and Chemical Compounds for Industrial and Pharmaceutical Uses
Chemicals from anthropogenic sources tend to harm the ocean and human health 
. The increase of human activities around and in the ocean, including oil and gas exploitation, deep-sea mining operations, fishing, coastal tourism and shipping contribute largely to the accumulation of toxic chemicals in marine ecosystems such as heavy metals 
, persistent organic chemicals (POC) 
and radioactive elements 
. In many coastal areas around the world, the concentrations of toxic chemicals are extremely high. Therefore, new biotechnology approaches represent the fundamental solution for tackling the challenge of “human need growing” vs. “pressure on marine resources” and are an efficient tool for environmental bioremediation