Photophysiology of the first reported bleached crustose coralline alga, Clathromorphum sp. (Hapalidiales, Rhodophyta), from Antarctica

Abstract

During a 2019 Chilean Antarctic Scientific Expedition (ECA 55) studying crustose coralline algae (CCA) diversity on the Antarctic Peninsula, bleaching of these algae was observed for the first time in this region. Here, we present initial findings on the physiological state of bleached and normally pigmented CCA (Clathromorphum sp.) assessed using chlorophyll-a fluorescence induction pulse amplitude modulation. The study site experienced high light exposure and salinity in the water column. Our analyses found that bleached CCA have relatively healthy photophysiology responses but lower photosynthetic efficiency, which could be associated with the low salinities recorded in the study area. However, seasonal monitoring and mesocosm experiments across the southern polar latitudes are urgently required to confirm this hypothesis.

Introduction

Crustose coralline algae (CCA), calcifying multicellular red seaweeds, play critical ecological roles in marine environments, such as being one of the builders of coral reefs and providing habitat and settlement cues for larval and juvenile stages of invertebrates (Cornwall et al. 2019, Tâmega & Figueiredo 2019). CCA occur globally from the tropics to the cold waters of the polar regions (Nelson 2009). However, despite progress in describing their diversity and taxonomy (Sciuto et al. 2021), ecological knowledge of this group remains limited in polar latitudes, particularly in the Southern Hemisphere, including Antarctica.

The Antarctic region and the Southern Ocean, home to pristine ecosystems and unique biota, are pivotal to the stability of the global climate because of their ability to take up heat and CO₂ as well as providing important climate feedbacks through their influence on albedo and atmospheric and oceanic circulation (Jones et al. 2016, Convey & Peck 2019). Warming is a significant issue in this region, not least with the record highest air temperature being recorded on the continent in 2020 (18.3°C on 6 February 2020 at Argentina's Esperanza research station; World Meteorological Organization 2021). Warming also melts more land-based snow and ice, increasing freshwater runoff and causing localized lowered intertidal, subtidal and shallow water salinity (Janecki et al. 2010).

CCA are particularly sensitive to climate change, especially those inhabiting the Southern Ocean, due to the potential decline in their calcification rates resulting from decreased pH and carbonate ion (CO₃^2-) concentration as more CO₂ dissolves into ocean surface waters (Hofmann & Bischof 2014, McCoy & Kamenos 2015, Johnson et al. 2019, Sciuto et al. 2021). The photosynthetic performance of Antarctic intertidal CCA is yet to be investigated, as are the physiological traits that allow them to survive intertidal environmental conditions.

During a 2019 Antarctic expedition studying CCA diversity on the Antarctic Peninsula, bleaching of these algae was observed for the first time in this region. Such bleaching - the loss of pigmentation in algae - has been widely linked to thermal-stress events associated with climate change in tropical and subtropical regions (Martone et al. 2010, Cornwall et al. 2019, Montes-Herrera et al. 2024). Experiments on coralline algae have revealed a variety of causes for bleaching due to the single or combined effects of multiple environmental stressors, including water temperature change (Martone et al. 2010), acidification (Anthony et al. 2008), high irradiance (Dring et al. 1996, Martone et al. 2010), canopy and epiphyte loss (Figueiredo et al. 2000, Irving et al. 2005) and desiccation and salinity extremes (Sotka et al. 2018). Biological factors such as pathogen infection (Case et al. 2011) or changes in surface bacterial assemblages (Campbell et al. 2011) are other reported causes of bleaching events. Although the diversity in outcomes among different experimental studies, with both positive and negative physiological responses, blurs predictions of climate change impacts, this might be a result of species-specific differences (Chan et al. 2020, Sordo et al. 2020).

Over recent decades, West Antarctica and the Antarctic Peninsula have lost ice mass rapidly, three to four times more than the rest of Antarctica combined (Convey & Peck 2019). Therefore, it is relevant to study how photophysiology varies between CCA individuals undergoing different responses (bleaching phenotype) under the same environmental conditions, especially in the context of climate change and the development of future strategies for conservation in Antarctica led by Parties to the Antarctic Treaty.

Here, we present initial findings on the physiological state (photosynthetic performance) of bleached and non-bleached CCA assessed using chlorophyll-a fluorescence induction measured using pulse amplitude modulation (PAM) fluorometry. This study contributes to our understanding of the physiological tolerance of coralline algae that is currently lacking, especially in extreme environments such as the polar regions.

Materials and methods

CCA bleaching was observed close to the Chilean Yelcho research station on Doumer Island in a protected bay composed of rocky platforms and surrounded by glaciers, adjacent to the Peltier Channel, western Antarctic Peninsula (64°52'33"S, 63°33'46"W; Fig. 1a–c), during the 2018/2019 summer (21 February 2019). The study site is characterized by the presence of hard substrate (pebbles, larger stones, bedrock) and is protected from large waves and swells. Seven specimens each of bleached (white) and normally coloured (pale violet-red) CCA (Fig. 1d,e) were randomly collected under permit (Special Permits 198/2019 and 200/2019 issued to Universidad de Magallanes by the Instituto Antártico Chileno - INACH) from the intertidal zone. These were used to perform identification and compare their photosynthetic performances. Sampling took place around solar noon on a sunny day. Additionally, a quadrat survey was performed to analyse the bleached CCA percentage cover. Ten quadrats of 50 × 50 cm were selected randomly in the study area.

Figure 1. a. Map of the western Antarctic Peninsula coastline showing the study site (white star) and point of environmental parameter measurements (black circle), both near the Chilean Yelcho research station (black square). b. The landscape of the collection area. c. Beds of crustose coralline algae (arrowheads) in the intertidal zone. d. Stone covered by bleached Clathromorphum sp. (arrowheads) surrounded by the red alga Palmaria decipiens, also showing some bleaching spots. e. Close-up of crustose coralline algae showing the initial stages of bleaching in the margins of the algae (arrowheads).

Identification of the collected CCA was performed using specialist literature (Mendoza & Cabioch 1985, Hommersand et al. 2009), and phylogenetic analyses were completed using standard markers from the plastid genome (psbA and rbcL). Molecular procedures were performed as described by Calderon et al. (2021). Primer pairs used for amplification and sequencing were F1- R2 (Yoon et al. 2002) for psbA and F57- 897cR and F645- R1150 (Freshwater & Rueness 1994, Lin et al. 2002, Torrano-Silva et al. 2014) for rbcL. In total, 13 new sequences (psbA = 6; rbcL = 7) were generated and have been deposited in GenBank (www.ncbi.nlm.nih.gov/genbank/; Table S1). Representative material has been deposited in the herbarium of Criptógamas Subantárticas (LEMAS) del Laboratorio de Ecosistemas Marinos Antárticos y Sub-antárticos of the Universidad de Magallanes, Punta Arenas, Chile (UMAG).

Photosynthetic activity was measured in situ on crusts using a pulse-amplitude-modulated chlorophyll fluorometer ‘MINI-PAM-II' (Walz GmbH, Germany). Photosynthetic performance was measured in vivo after 15 min of dark adaption, and rapid light curves (RLC) were recorded using an actinic light of 0–2950 μmol photons m^-2 s^-1 (Marambio et al. 2023). The standard photosynthetic parameters as relative maximum electron transport rate (rETR_max), electron transport efficiency (α, initial linear slope), light saturation point of photosynthesis (E _k) and quantum yield of photosystem II (F _v/F _m) were estimated for both bleached and coloured CCA following the procedures described by Wilson et al. (2004) and Méndez et al. (2018). Studies of photosynthetic performance commonly use a minimum of two to three measurements per group (see Gómez et al. 1997, Payri et al. 2001, Chisholm 2003). Here, the photosynthetic parameters rETR_max, α, E _k and the ratio of F _v/F _m were measured on three individuals each of bleached and coloured CCA. Temperature (°C) and salinity (psu) were measured using an SBE 19plus v2 CTD device (Sea-Bird Scientific, USA) in the water column, and pH was measured using a portable pH meter ProfiLine pH 3110 (WTW, Germany). Radiation data were obtained from WeatherOnline Ltd - Meteorological Services (www.weatheronline.co.uk) for Palmer Station (64°46'S, 64°03'W), 26 km north-west of our collection point. Photosynthetically active radiation (PAR) was calculated using the R package 'bigleaf' to convert radiation (W m^-2) to photosynthetic photon flux density (PPFD).

Results

We report the presence of bleached CCA for the first time in the Antarctic Peninsula region. The specimens collected were consistent with the morphological description of the genus Clathromorphum (Hapalidiales, Rhodophyta). The phylogenetic reconstruction based on psbA and rbcL sequences (Fig. S2) did not group our specimens with the generitype Clathromorphum compactum, rather clustering them in an independent lineage within the order Hapalidiales (Figs S1 & S2). Since the identification of coralline algae is notoriously difficult, further studies are required to confirm the taxonomic position of these specimens, hereafter referred to as Clathromorphum sp.

Approximately 20% of intertidal CCA were bleached. Most of the instances of bleached CCA occurred at apparently random points on extensive solid rock surfaces and hemispherical boulders (~15–20 cm radius), the bleached area covering up to 70% of such boulders (Fig. 1d). Bleaching initiated at the periphery of the encrusting algae, advancing irregularly in the undulating margins (Fig. 1e). The surrounding red alga Palmaria decipiens also showed bleached regions at the margins and tips of its thalli (Fig. 1d). In situ environmental variables recorded at the study site included pH 8.1, temperatures between +4.5°C (at 0 m) and +1.7°C (at 25 m depth; Fig. 2a) and a gradient of salinity in the water column ranging from 0.15 (at 0 m) to 32.39 (at 25 m depth) psu (Fig. 2b), with the presence of a halocline between 10 and 12 m (3.12–16.3 psu; Table S2). PAR varied between 492.7 and 740.4 μmol photons m^-2 s^-1 (Table S3).

Figure 2. Variation in a. temperature and b. salinity in the water column. Photosynthetic parameters of healthy (black bars) and bleached (grey bars) crustose coralline algae calculated from chlorophyll-a fluorescence measurements for c. relative maximum electron transport rate, (rETR_max), d. light saturation point of photosynthesis (E _k), e. electron transport efficiency (α) and f. the quantum yield of photosystem II (F _v/F _m). r.u. = relative units.

Despite the limited number of specimens included in the photophysiological analysis, some interesting observations were apparent.

For instance, similar ranges of measurement were recorded for rETR_max, α and F _v/F _m in both CCA. rETR_max varied from 3.45 to 23.80 relative units (r.u.) in coloured CCA and from 15.48 to 16.48 r.u. in bleached CCA, while α ranged from 0.05 to 0.21 (μmol photons m^-2 s^-1)^-1 in coloured CCA and from 0.08 to 0.11 (μmol photons m^-2 s^-1)^-1 in bleached CCA. Furthermore, F _v/F _m varied from 0.28 to 0.38 and from 0.11 to 0.39 in coloured and bleached CCA, respectively. Conversely, E _k ranged from 69.87 to 115.99 μmol photons m^-2 s^-1 and from 146.08 to 209.36 μmol photons m^-2 s^-1 in coloured and bleached CCA, respectively (Fig. 2c–f & Table I).

Table I. Photosynthetic parameters of the crustose coralline algae (CCA) Clathromorphum sp. (bleached and coloured).

α = electron transport efficiency; E _k = light saturation point of photosynthesis; F _v/F _m = quantum yield of photosystem II; rETR_max = relative maximum electron transport rate.

Discussion

Photosynthetic performance of the intertidal calcareous coralline algae Clathromorphum sp. from Antarctica has not been explored previously. The genus Clathromorphum is very common along the Antarctic Peninsula, where it can dominate communities in intertidal pools and the subtidal seascape (Mendoza & Cabioch 1985, Hommersand et al. 2009). It typically occurs low on the shore (< 0.3 m tide height) and in mid-intertidal tidepools, where exposure to light stress and desiccation is likely to be reduced during low tide. Here, it was not possible to achieve species-level identification since three species of the genus Clathromorphum - C. annulatum, C. lemoineanum and C. obtectulum - have been reported in West Antarctica, and descriptions suggest that they share habitat and ecological features (Mendoza & Cabioch 1985, Hommersand et al. 2009). Thus, further taxonomic studies are required to confirm the phylogenic position of our specimens.

Our results derived from RLC measures, such as rETR_max (r.u.), E _k (μmol photons m^-2 s^-1) and α ((μmol photons m^-2 s^-1)^-1), of both coloured (3.45–23.80 r.u.; 69.87–115.99 μmol photons m^-2 s^-1; 0.05–0.21 (μmol photons m^-2 s^-1)^-1) and bleached CCA (15.48–16.48 r.u.; 146.08–209.36 μmol photons m^-2 s^-1; 0.08–0.11 (μmol photons m^-2 s^-1)^-1) were consistent with values reported in intertidal uncalcified red algae from Antarctica, such as Pyropia endiviifolia (10.82 ± 1.76 r.u.; 96.61 ± 26.13 μmol photons m^-2 s^-1; 0.11 ± 0.02 (μmol photons m^-2 s^-1)^-1), Palmaria decipiens (13.36 ± 3.97 r.u.; 78.58 ± 17.66 μmol photons m^-2 s^-1; 0.17 ± 0.05 (μmol photons m^-2 s^-1)^-1) and Iridaea cordata (18.61 ± 2.08 r.u.; 101.76 ± 15.17 μmol photons m^-2 s^-1; 0.18 ± 0.04 (μmol photons m^-2 s^-1)^-1; Gómez et al. 2019). The photosynthesis-irradiance (P-E) curve parameters (i.e. rETR_max and E _k), which vary in relation with depth (Gómez et al. 2019), are overall higher in eulittoral species than those collected from the subtidal zone and are not constrained by algal taxonomy (Huovinen & Gómez 2013). Additionally, the light requirements for photosynthesis (E _k) measured here in bleached CCA (E _k = 146.08–209.36 μmol photons m^-2 s^-1) were the highest values recorded for a shallow subtidal red alga in Antarctica, followed by Curdiea racovitzae (E _k = 92.73 ± 8.22 μmol photons m^-2 s^-1) and I. cordata (E _k = 121.26 ± 27.53 μmol photons m^-2 s^-1) and Pantoneura plocamioides (E _k = 149.39 ± 30.06 μmol photons m^-2 s^-1; Gómez et al. 2019), further supporting the notion of their strong resilience to exposure to high levels of PAR (Payri et al. 2001, Wiencke et al. 2007). Polar algae from the upper sublittoral or eulittoral typically show high values of saturation points for photosynthesis (E _k > 50 μmol photons m^-2 s^-1; Weykam et al. 1996, Gómez et al. 1997, Payri et al. 2001).

Photosynthetic studies of CCA in polar regions have focused to date on subtidal samples, complicating the comparison of parameters (Kühl et al. 2001, Roberts et al. 2002, Schwarz et al. 2005, Schoenrock et al. 2018). Despite this, the measured photosynthetic performance ranges (rETR_max; E _k) of both normally coloured and bleached CCA from Doumer Island were higher than those of other intertidal calcareous algae studied in temperate regions, such as Lithothamnion glaciale from the west coast of Scotland (3.83 ± 0.52 r.u.; 54.61 ± 5.29 μmol photons m^-2 s^-1) and Chamberlainium sp. from Garorim Bay in South Korea (8.2 ± 0.26 r.u.; 57.1 ± 2.6 μmol photons m^-2 s^-1; Burdett et al. 2012, Kim et al. 2020).

Most intertidal algal species examined in Antarctica have previously been shown to have high photosynthetic efficiency (α > 0.15 (μmol photons m^-2 s^-1)^-1; Gómez et al. 1997, Gómez & Huovinen 2011), while the quantum yield of photosystem II (PSII; F _v/F _m) for red algae is typically 0.5–0.6 (Dring et al. 1996, Burdett et al. 2012). The low values of F _v/F _m measured here in both bleached (0.043–0.363) and coloured CCA (0.277–0.377) might suggest physiological stress and inefficiency of energy transfer to the PSII reaction centres (Dring et al. 1996, Wilson et al. 2004, Schoenrock et al. 2018), perhaps associated with exposure to low salinities. Our observations showed a strong gradient in salinity in the water column from hyposaline conditions (1.37 psu at 0.5 m depth) to a halocline between 10 and 12 m depth (3.12–16.30 psu), so further experimental work, including increase sample size and effort, is required to explore whether a reduction in salinity has major implications for photosynthesis in Antarctic Clathromorphum sp., as metabolic processes that control salinity tolerance also remain poorly understood in Antarctic calcareous algae (Karsten 2012). Future in situ and laboratory experimental studies should also include short-term exposure to salinities close to freshwater values as well to reflect stresses associated with freshwater runoff across the shoreline from melt.

As we were not able to sample or monitor CCA over time, it was not possible to assess the duration of bleaching or recovery time (or if recovery occurred) of these bleached CCA. Thus, further studies that monitor pigment content, calcification rate and CaCO₃ skeleton thickness, singlet-oxygen (¹O₂) production and concentrations of antioxidant dimethylated sulphur compounds (dimethylsulphoniopropionate (DMSP) and dimethyl sulphoxide (DMSO)) over seasonal timescales are now required, as has been carried out in previous studies of other non-polar CCA (Latham 2008, Burdett et al. 2015a, 2015b, Schoenrock et al. 2018, Muth et al. 2020, Montes-Herrera et al. 2024).

Bleaching is often thought to be an indicator of death (Irving et al. 2004), although colour restoration has been observed after provision of shade, low irradiance exposure or salinity values > 30 psu (Dring et al. 1996, Figueiredo et al. 2000, Irving et al. 2004, Muth et al. 2020). However, measurements such as those reported here, confirming non-zero ETR values in bleached CCA, suggest bleaching is not necessarily associated with algal death. The study site was characterized by specific conditions (high irradiance and low salinity) that have been associated with bleaching and shifts in photosynthetic parameters of coralline algae in previous studies (Kirst 1989, Roberts et al. 2002, Thomas & Dieckmann 2002, Irving et al. 2005, Latham 2008, Burdett et al. 2015b, Schoenrock et al. 2018). Our observations highlight the importance of establishing appropriate monitoring of marine environmental variables in key locations (as proposed within the Scientific Committee on Antarctic Research (SCAR) Antarctic Near-Shore and Terrestrial Observation System (ANTOS) initiatives, www.scar.org/science/cross/antos) in order to provide data that will assist in identifying and tracking the origin and duration of marine anomalies, since the environmental conditions that could have caused the bleaching of CCA reported here could have initiated before our observations.

While bleaching has been documented in CCA at temperate and tropical latitudes (Figueiredo et al. 2000, Martone et al. 2010, Vargas-Ángel 2010, Campbell et al. 2011), there are no previous reports of its occurrence as a natural event in the polar regions. Subtidal video recordings (Supplemental Material) showed bleaching in almost 80% of CCA located at 8–11 m depth, where the halocline was observed. Historically, the area between Anvers Island and Adelaide Island along the coast of the western Antarctic Peninsula (64°S–67°35'S), where our observations were made, has been very poorly studied, with the existence or extent of any marine changes occurring being undocumented (Wiencke & Amsler 2012). Our initial study provides a starting point to both urgently draw attention to this bleaching event in the Antarctic Peninsula region and to improve understanding of bleaching in CCA and of its wider implications for Antarctic marine communities. It is already clear that bleached CCA have relatively healthy photophysiology responses (rETR_max, E _k), but with lower photosynthetic efficiency (F _v/F _m), possibly associated with the low salinities recorded in the study area; however, seasonal monitoring of key environmental parameters and mesocosm experiments across the southern polar latitudes are urgently required to confirm this hypothesis.

A range of consequences of climatic and other environmental changes in Antarctica have been reported (e.g. biological invasions, changing sea ice, ocean acidification and also now bleaching of CCA; Anthony et al. 2008, Abram et al. 2014, Convey & Peck 2019, Siegert et al. 2019, 2023), threats that are placing its unique and often highly endemic biodiversity at risk, yet are still waiting for political acceptance, reaction and effective response. Notwithstanding, an important element in any future strategy is that funding agencies from the national Antarctic Treaty Parties need now to develop ambitious commitments to tackle these growing concerns by investing in research, monitoring and protection programmes across Antarctica.