Pollution in the seas and oceans is a global problem, which highlights emerging pollutants and plastics, specifically microplastics (MPs), which are tiny (1 μm to 5 mm) ubiquitous plastic particles present in marine environments that can be ingested by a wide range of organisms. Holothurians are benthic organisms that feed on sediment; therefore, they can be exposed to contaminants present in the particles they ingest. The objective was to evaluate the effects of human activity on Holothuria tubulosa through the study of biomarkers. Specimens were collected in three different areas throughout the island of Eivissa, Spain: (1) a highly urbanized area, with tourist uses and a marina; (2) an urbanized area close to the mouth of a torrent; (3) an area devoid of human activity and considered clean. The results showed a higher presence of microplastics (MPs) in the sediments from the highly urbanized area in relation to the other two areas studied. Similarly, a higher number of MPs were observed in the digestive tract of H. tubulosa from the most affected area, decreasing with the degree of anthropic influence. Both in the sediment and in the holothurians, fibers predominated with more than 75% of the items. In the three areas, mesoplastics were analyzed by means of FTIR, showing that the main polymer was polypropylene (27%) followed by low-density polyethylene (17%) and polystyrene (16%). Regarding the biomarkers of oxidative stress, the intestine of H. tubulosa from the most impacted areas showed higher catalase, superoxide dismutase (SOD), glutathione reductase (GRd), and glutathione S-transferase (GST) activities and reduced glutathione (GSH) levels compared to the control area. The intermediate area only presented significant differences in GRd and GST with respect to the clean area. The activities of acetylcholinesterase and the levels and malondialdehyde presented similar values in all areas. In conclusion, human activity evaluated with the presence of MPs induced an antioxidant response in H. tubulosa, although without evidence of oxidative damage or neurotoxicity. H. tubulosa, due to its benthic animal characteristics and easy handling, can be a useful species for monitoring purposes.

This study provides the first extensive data on in situ rates of N₂ fixation in minimally disturbed Posidonia oceanica meadows and their temporal and spatial variability along the Mallorcan coasts in the Mediterranean Sea. Benthic bell‐jar chambers were deployed underwater enclosing (1) natural P. oceanica shoots, (2) matted sediment of P. oceanica with the leaves carefully removed by clipping to assess the contribution of N₂ fixation associated with leaves, and (3) adjacent unvegetated sediment to assess the contribution of N₂ fixation by the sediment. In all of the chambers, N₂ fixation varied seasonally, with highest rates during summer and lowest in winter. Average N₂ fixation rates in chambers enclosing natural P. oceanica shoots ranged from 0.06 mg N m⁻² d⁻¹ to 1.51 mg N m⁻² d⁻¹ (0.004–0.107 mmol N m⁻² d⁻¹) in summer. Careful removal of leaves generally reduced N₂ fixation rates by 16–89%. The N₂ fixation rates in unvegetated sediments ranging from 0.01 mg N m⁻² d⁻¹ to 1.99 to mg N m⁻² d⁻¹ (7.1 × 10⁻⁴–0.14 mmol N m⁻² d⁻¹) have similar magnitudes in sediments with P. oceanica. This suggests that these sediments are “hot spots” for N₂ fixation. Exploratory analyses of environmental factors that may influence N₂ fixation rates highlight temperature and pore water total phosphorus concentrations as important factors.

The use of marine sponges dates back thousands of years, and interest in these animals is increasing as new applications are discovered. Their potential is extensive, both in their ancient and still popular use as bath sponges for cosmetics and regarding the more recent discovery of bioactive secondary metabolites mainly of interest for the pharmaceutical industry and the less developed aquariology. Despite their proven biofiltration and ecosystem restoration ability and the biomass supply problem for the interested industries, few integrated multi-trophic aquaculture (IMTA) systems incorporate these invertebrates in their facilities. Therefore, in this brief review, the benefits that marine sponges could bring to rapidly growing IMTA systems are summarized, highlighting their suitability for a circular blue economy.