Co-occurring morphologically distinct algae support a diverse associated fauna in the intertidal zone of Araçá Bay, Brazil

Low molecular weight carbohydrate patterns of mangrove macroalgae from different climatic niches under ocean acidification, warming and salinity variation

Ocean acidification has increased due to the enhanced solubility of CO2 in seawater. Mangrove macroalgae in tropical and subtropical coastal regions can benefit from the higher availability of CO2 for photosynthesis and primary production. However, they can be negatively affected by the simultaneously occurring warming and increased salinity in estuaries. Thus, we analyzed the isolated effects of ocean acidification and the interactive effects of increased temperature and salinity on the low molecular weight carbohydrate (LMWC) contents of the mangrove red macroalgae Bostrychia montagnei and Bostrychia calliptera from Brazilian tropical and subtropical populations. Specimens from both climatic niches were tolerant to pH decreased by CO2 enrichment and enhanced their LMWC contents under increased availability of CO2. Specimens from both climatic niches also accumulated their dulcitol and sorbitol contents to cope with warming and salt stress. Nevertheless, temperature of 34 °C was lethal for tropical macroalgae, while 29 °C and 31 °C were lethal for subtropical B. calliptera under salinity of 35. Tropical and subtropical B. montagnei synthesized dulcitol (5-110 mmol kg-1 dry weight) and sorbitol (5-100 mmol kg-1 dry weight) as osmoregulatory, energy and thermal protection compounds, whereas tropical and subtropical B. calliptera synthesized mainly dulcitol (10-210 mmol kg-1 dry weight). Although digeneaside has an energy function in Bostrychia spp., it is not an osmolyte or thermal protection compound. Our data demonstrated that both tropical and subtropical Bostrychia spp. benefit from ocean acidification by CO2 enrichment, increasing their LMWC contents. However, warming and increased salinity in estuaries will be detrimental to them, even they producing protective metabolites. Multifactorial approaches are recommended to investigate whether negative effects of increased temperature and salinity nullify positive effects of ocean acidification on these Bostrychia species/populations.

Ocean warming and increased salinity threaten Bostrychia (Rhodophyta) species from genetically divergent populations

Increased greenhouse gas concentrations in the Earth's atmosphere have resulted in global change, such as ocean warming and sea level rise. Increased salinity in estuaries is expected as a result of sea level rise and warming. Thus, we analysed the interactive effects of increased temperature and salinity on multiple physiological responses of Bostrychia montagnei and B. calliptera from two biogeographic provinces, Tropical Southwestern Atlantic (TSA) and Warm Temperate Southwestern Atlantic (WTSA). Macroalgae were cultured under three salinities (15, 25 and 35 PSU) and three temperatures: mean sea surface temperature (SST: 27 °C for TSA and 24 °C for WTSA), an RCP8.5 ocean warming scenario (SST + 5 °C), and a maximum temperature to test the algal upper thermal tolerance limits (RCP8.5 + 2 °C). Macroalgae from both localities decreased their growth under increased temperature and salinity. RCP8.5 + 2 °C was lethal for both macroalgae from TSA. RCP8.5 and RCP8.5 + 2 °C at 35 PSU were lethal for B. calliptera from WTSA, due to the interactive effects between increased temperature and salinity. Overall, increased salinity decreased the effective quantum yield and relative electron transport rate in algal photosynthesis. Our results demonstrated that the macroalgae synthesized proteins, carbohydrates (polysaccharides and low molecular weight carbohydrates), and antioxidants to tolerate detrimental temperatures and salinities. Our results also demonstrated that the macroalgae adjusted their pigment contents (phycobiliproteins, total carotenoids, and chlorophyll a) for efficient light-harvesting under thermal and saline stress. Our findings suggest that ocean warming and increased salinity in estuaries will be detrimental to B. montagnei and B. calliptera populations from both biogeographic provinces, especially to those from TSA that already live closer to their upper thermal tolerance limits.

The Effects of Habitat Heterogeneity at Distinct Spatial Scales on Hard-Bottom-Associated Communities

For marine benthic communities, environmental heterogeneity at small spatial scales are mostly due to biologically produced habitat heterogeneity and biotic interactions, while at larger spatial scales environmental factors may prevails over biotic features. In this study, we investigated how community structure and β-diversity of hard-bottom-associated meio- and macrofauna varied in relation to small-scale (cm–m) changes in biological substrate (an algae “turf” dominated by the macroalgae Gelidium sp., the macroalgae Caulerpa racemosa and the sponge Hymeniacidon heliophile) in a rocky shore and in relation to larger-scale (10’s m) changes in environmental conditions of the same biological substrate (the macroalgae Bostrychia sp) in different habitats (rocky shore vs. mangrove roots). Results showed that both substrate identity and the surrounding environment were important in structuring the smaller-sized meiofauna, particularly the nematode assemblages, whereas the larger and more motile macrofauna was influenced only by larger-scale changes in the surrounding ecosystem. This implies that the macrofauna explores the environment in a larger spatial scale compared to the meiofauna, suggesting that effects of spatial heterogeneity on communities are dependent on organism size and mobility. Changes in taxa composition between environments and substrates highlight the importance of habitat diversity at different scales for maintaining the diversity of the associated fauna.