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Abstract—For the first time, the landscape approach was used to conduct a comparative analysis of the macrophytobenthic species diversity and changes in its biomass in the coastal zone of Karanskii Nature and Landscape Reserve over the period of 1964–2016. A landscape map of the submarine nearshore slope was produced, and three benthic natural complexes (BNCs) have been distinguished. The main such complex is the BNC of psephitic deposits with the predomination of Carpodesmia crinita and Treptacantha barbata (up to 89.7% of the total macrophyte biomass). The complex is distinguished by a high floristic diversity and the maximum macrophytobenthic biomass. The BNC of psephitic deposits predominated by C. crinita and Tr. barbata with alternating pebble–gravel deposits and broken shells predominated by Phyllophora crispa is distinguished by the highest floristic diversity and lower shares of C. crinita and Tr. barbata; the share of Ph. crispa in the total macrophyte biomass amounts to 14.4%. The BNC of psammitic deposits with the predomination of Ph. crispa and with individual randomly scattered blocks where Nereia filiformis and Zanardinia typus predominate features a low floristic diversity, the predominance of Phyllophora crispa (74.3% the total macrophyte biomass), and good preservation of perennial species. Over the course of more than 50 years, the plant component underwent restructuring and degradation in all BNCs of the reserve (at depths of 0.5–10 m). Apparently, this can be explained by the intensification of anthropogenic activities in the coastal zone. Eudesme virescens, Dictyota dichotoma, Feldmania irrgularis, Dasya baillouviana, and Rhodochorton purpureum inhabiting clean open coastal areas disappeared from the benthic vegetation; while macrophytes typical for eutrophic waters (Cladophora laetevirens, Ectocarpus siliculosus, and C. virgatum) appeared there. The biomass of epiphytes has sharply increased from 0.1 to 42.6% of the total macrophyte biomass, while the role of dominant species decreased: the share of C. crinita and Tr. barbata dropped from 99.9 to 78.6% of the total macrophyte biomass; the share of Ph. crispa, from 52.9 to 14.4%. The designation of Karanskii Reserve contributed to the preservation and partial restoration of the unique natural complex of marine flora and benthic vegetation; the increased floristic diversity of algae acting as indicators of clean waters and the increased share of Red Data Book species confirm this.

The Mediterranean Sea harbors more than 17,000 eukaryotic marine species, with several ecosystems recognized as biodiversity hotspots, such as Posidonia oceanica meadows. Recent research indicates that benthic mats formed by the fleshy red alga Phyllophora crispa are also associated with high species richness. Among key groups found in these mats are sessile polychaetes, which live as epiphytes on the red algae thalli. Knowledge of abundance, species richness, and spatial variation of polychaetes associated with these habitats is still scarce. We carried out a comparative assessment focusing on serpulid polychaetes within samples from P. crispa mats and neighboring P. oceanica meadows at six different sampling sites around Giglio Island (Tyrrhenian Sea, Italy). A total of 17 serpulid taxa were identified. The abundance of serpulids (5665 individuals m−2 of P. crispa mat) were similar to neighboring P. oceanica meadows (2304 individuals m−2 leaves and 5890 individuals m−2 shoots). The number of serpulid taxa was significantly higher in P. crispa mats (average 6.63 ± 1.32 taxa) compared to P. oceanica beds (average 1.56 ± 0.63 and 1.84 ± 1.04 taxa in leaves and shoots, respectively). Within habitat type, there were no significant differences in species richness between sites. The most abundant species found was Josephella marenzelleri (61% of individuals), while Vermiliopsis spp. and Bathyvermilia sp. were exclusively found in P. crispa samples. Our results highlight that P. crispa mats host an exceptional diversity and that these habitats should be included in conservation strategies. Further research should focus on the significance of other important taxonomic groups within these mats and evaluate the distribution of P. crispa in different regions of the Mediterranean Sea.

The Mediterranean Sea comprises habitats such as Posidonia oceanica seagrass meadows that exhibit high associated biodiversity of sessile organisms. Recent pilot research indicates that benthic mats formed by the scarcely investigated fleshy red alga Phyllophora crispa also host a high diversity of benthic fauna. Among the key taxa found in these mats in the recent pilot studies are benthic foraminifera that live as epiphytes on the red algae thalli. Knowledge about their abundance and species richness associated with this habitat in relation to reference habitats is missing. We thus carried out a comparative assessment focusing on foraminifera within samples from P. crispa mats and neighboring P. oceanica meadows on five different sampling sites around Giglio Island in the Tuscan Archipelago (Tyrrhenian Sea, Italy). A total of 104 different foraminiferal taxa were identified, of which a total of 85 taxa were found in P. crispa samples (46 exclusively in this habitat). This biodiversity was higher compared to other studies on phytal habitats in the Mediterranean Sea. The number of foraminiferal taxa associated with P. crispa was significantly higher (average 27.5 ± 8.1 taxa) compared to P. oceanica (leaves average 7.0 ± 3.6, shoots average 7.9 ± 3.4 taxa). The abundance of foraminifera (12,000 individuals m−2 surface area of P. crispa mat) was also higher than in the neighboring P. oceanica meadows (7792 individuals m−2 leaf and 8171 individuals m−2 shoot surface area). The most frequently found taxa across habitats were Miniacina miniacea, Lobatula lobatula, and Sejunctella sp. (24%, 20%, and 6% of the total population, respectively). Our results imply that P. crispa mats host an exceptional diversity of associated foraminifera that is even higher than those associated with seagrass meadows. Red algae mats built by P. crispa may thus be considered as potential refuge habitats and biodiversity reservoirs in management and conservation.

The Mediterranean Sea hosts highly diverse habitats such as Posidonia oceanica meadows, coralligenous communities, and gorgonian forests. Stressors including warming, eutrophication, pollution, and overfishing are driving shifts towards algae-dominated systems, often with reduced biodiversity. Among these, recent research surprisingly revealed that the mat-forming red alga Phyllophora crispa, which overgrows seagrass and gorgonian habitats, supports high sessile invertebrate diversity. However, little is known about its associated non-sessile fauna. This study thus investigated non-sessile invertebrates in P. crispa using a newly designed appropriate sampling technique at two study sites around Giglio Island (Italy), Fenaio, and Secca II (distance ca. 600 m from each other). Across all samples, 5464 organisms were identified, mostly to family level. We recorded 169 non-sessile taxa, including 96 families, 41 copepod morphotypes, 21 ostracod morphotypes, and 11 unclassified taxa. The dominant phyla were Arthropoda (67%), Mollusca (14%), Annelida (9%), and Nematoda (5%). The most abundant families were Calliopiidae (Amphipoda), Leptognathiidae (Malacostraca), and Mytilidae (Bivalvia). Of the 169 taxa, 128 occurred at both sites, while 20 were unique to Fenaio and 21 to Secca II, suggesting high connectivity likely linked to mobility. Organism abundances ranged from 1315 to 5759 individuals per m2 seafloor. Diversity indices were as follows: Shannon 1.5–3.4, Simpson 0.6–1.0, and Pielou 0.6–0.9. These values are similar or even exceed previously reported values for sessile invertebrates (Shannon 2.2–2.5). Notably, P. crispa supported diversity levels higher than those reported for seagrass meadows (Shannon 2.0–2.1) and even tropical coral reefs (2.0). Our study thus confirms P. crispa as a biodiversity hotspot and suggests that these algae mats should be considered in biodiversity conservation strategies.

The role of habitat degradation on the spread of the alien green alga Caulerpa cylindracea is reported here by comparing observations achieved through a multi-year assessment on three Mediterraneans habitats, namely Posidonia oceanica meadows, Phyllophora crispa turf, and coralligenous reefs. Due to the peculiarity of the study site, both natural-reference and impacted conditions were investigated. C. cylindracea occurred in all the studied habitats under impacted conditions. High susceptibility to the invasion characterized impacted P. oceanica, where Caulerpa cover reached 70.0% in summer months. C. cylindracea cover did not differ significantly among conditions in P. crispa turf, where values never exceeded 5.0%. Conversely, the invasive green algae was low in abundance and patchily distributed in coralligenous reefs. Our results confirmed that habitat loss enhances the spread of C. cylindracea, although with different magnitudes among habitats. Dead matte areas of P. oceanica represented the most vulnerable habitat among those analyzed, whereas coralligenous reefs were less susceptible to the invasion under both the studied conditions.

With its geographically isolated location and geological history, the Mediterranean Sea harbors well-known biodiversity hotspots, such as Posidonia oceanica seagrass meadows. Recently, long-living mats formed by the fleshy red alga Phyllophora crispa have been described to be associated with a high diversity of sessile invertebrates in the Tyrrhenian Sea. One of the key taxa among these sessile invertebrates are bryozoans: their abundance, diversity, and spatial distribution in P. crispa mats represent a gap in scientific knowledge. Thus, we conducted a pilot study on bryozoan assemblages associated with P. crispa mats around Giglio Island (Tuscan Archipelago, Italy) in 2018, followed by a comparative study on four sites distributed around the island in the subsequent year, 2019. We compared these findings to bryozoan abundance and diversity on P. oceanica shoots and leaves during the second expedition. The findings revealed more than 46 families, with a significantly higher number of taxa identified in P. crispa mats (33) than in P. oceanica meadows (29). The Shannon diversity index was similar between P. crispa and P. oceanica shoots, while Pielou’s evenness index was lower in P. crispa mats. The most abundant families reported across all habitats were Crisiidae, Aetidae, and Lichenoporidae; but the most abundant family on P. crispa was Chlidoniidae (Chlidonia pyriformis). The assemblages associated with P. crispa differed among sites, with higher abundances but lower diversity on the exposed southernmost site. The total bryozoan abundance was significantly higher on P. crispa (average 2.83 × 106 ± 1.99 × 106 colonies per m2 seafloor) compared to P. oceanica meadows (average 0.54 × 106 ± 0.34 × 106 colonies per m2 seafloor). Our results show a high diversity of bryozoans on P. crispa thalli compared to P. oceanica meadows, which was consistent throughout the study. These findings confirm the value of the red alga-generated habitat for associated bryozoans and may have implications for future biodiversity assessments and conservation measures.

In the Mediterranean, the fleshy, mat-forming red alga Phyllophora crispa creates high-biodiversity habitats that influence light availability, water movement, and temperature. However, knowledge about its influence on other key environmental parameters, such as oxygen availability, chlorophyll, and turbidity, is missing. Therefore, we conducted an in situ study in the Western Mediterranean Sea using multiparameter probes that were placed multiple times on algal P. crispa mats, in adjacent Posidonia oceanica seagrass meadows, and on bare hard bottoms. We acquired a total of 17 full diel measurements for dissolved oxygen (DO), chlorophyll, and turbidity in September and October 2019. Results showed that P. crispa mats influence the investigated parameters differently when compared to P. oceanica meadows and that a monthly effect was observed. In September, general DO patterns measured for P. crispa mats and P. oceanica meadows follow the daily cycle depending on light availability, with the measured DO being lower in the P. oceanica meadows compared to the P. crispa mats and the hard-bottom habitats. In October, however, no significant difference in DO concentrations was observed between P. crispa mats and P. oceanica meadows. Results of this study corroborate that P. crispa mats can be viewed as an ecosystem engineering species, influencing environmental parameters and, hence, providing a habitat for outstanding associated biodiversity.

Research into ion-exchange properties of cell walls isolated from thallus of red seaweed Phyllophora crispa was carried out. Ion-exchange capacity and the swelling coefficient of the red alga cell walls were estimated at various pH values (from 2 to 12) and at constant ionic strength of a solution (10 mM). It was established that behavior of cell walls as ion-exchangers is caused by the presence in their matrix of two types of cation-exchange groups and amino groups. The amount of the functional group of each type was estimated, and the corresponding values of p K a were calculated. It can be assumed that ionogenic groups with p K a ∼5 are carboxyl groups of uronic acids, and ionogenic groups with p K a ∼7.5 are carboxyl groups of the proteins. Intervals of pH in which cation-exchange groups are ionized and can take part in exchange reactions with cations in the environment are defined. It was found that protein was a major component of cell wall polymeric matrix because its content was 36%.