Unravelling microalgal-bacterial interactions in aquatic ecosystems through 16S co-occurrence networks

Microalgae and bacteria have a wide spectrum of associations in aquatic environments. Since their interactions can directly influence global carbon and nutrient cycling, understanding these associations help us evaluate their influence on ecosystem productivity. Algal biodiversity is large, and bacterial associations have been characterised for a small fraction of them. While experiments based on algal-bacterial co-culturing are commonly used to infer interactions, deciphering all associations present in nature through such methods is impractical and approaches based on co-occurrence network analysis can help infer associations. In this study, we used microbial co-occurrence networks built from Earth microbiome project 16S metabarcoding data to detect microalgal-bacterial associations in aquatic environments. We analysed marine and freshwater environments to understand what groups of bacteria are tightly co-occurring with different algal groups in both aquatic environments, to see patterns of interactions, and to evaluate the overall use of co-occurrence networks to infer meaningful algal-bacterial interactions. In line with expectations from co-culturing work, our results show that the phyla Proteobacteria and Bacteroidetes are the major bacterial associates of microalgae and the co-occurring bacteria may be specific to the algal host. From the independent analysis of environments, we also show that sample origin may be an important determinant of these interactions. By unravelling previously established microalgal-bacterial links as well as identifying a range of previously unknown interactions, we show that co-occurrence network analysis is a promising hypothesis-generating framework to study microalgal-bacterial interactions that can guide future research into the functional nature of interactions.

The Influences of Nutrients and Snow on the Spatial and Temporal Variability of Sea Ice Algae

<p>Polar marine regions are dominated by sea ice, where large gradients in temperature, salinity, nutrients and light occur. Despite this, a rich community exists within the sea ice, consisting of prokaryotic organisms, several algal groups and small zooplankton. Prokaryotes are present in the largest abundance in the sea ice; however, diatoms dominate in biomass. Diatoms are the main primary producers within the ice and they form a vital food source for many organisms. However, factors determining species composition, abundance, spatial and temporal variability and nutrient requirement are relatively poorly understood. In order to increase understanding of these processes, an integrated approach was used in this thesis to provide an insight into the potential changes to the ecology of the Southern Ocean in relation to predicted climate change. In this thesis, I studied ice algal community structure, diversity and nutrient requirements at several locations in the sea ice of the Ross Sea, Antarctica. Though many previous studies have focussed on these organisms, this is the first study to I) integrate recent and historical data collected over 30 years and to compare spatial and temporal differences in sea ice communities, II) use the near real time nutrient induced fluorescence transient (NIFT) method to study nutrient limitation in sea ice and further develop this method for use with the imaging pulse amplitude modulator (I-PAM), III) show that Antarctic diatoms may be more susceptible to silica limitation than previously thought, despite the fact that the silica concentration in the Southern Ocean are relatively high. Results from these studies provide important new information on community structure and how it is influenced by and responds to the environment ...</p>

The Influences of Nutrients and Snow on the Spatial and Temporal Variability of Sea Ice Algae

<p>Polar marine regions are dominated by sea ice, where large gradients in temperature, salinity, nutrients and light occur. Despite this, a rich community exists within the sea ice, consisting of prokaryotic organisms, several algal groups and small zooplankton. Prokaryotes are present in the largest abundance in the sea ice; however, diatoms dominate in biomass. Diatoms are the main primary producers within the ice and they form a vital food source for many organisms. However, factors determining species composition, abundance, spatial and temporal variability and nutrient requirement are relatively poorly understood. In order to increase understanding of these processes, an integrated approach was used in this thesis to provide an insight into the potential changes to the ecology of the Southern Ocean in relation to predicted climate change. In this thesis, I studied ice algal community structure, diversity and nutrient requirements at several locations in the sea ice of the Ross Sea, Antarctica. Though many previous studies have focussed on these organisms, this is the first study to I) integrate recent and historical data collected over 30 years and to compare spatial and temporal differences in sea ice communities, II) use the near real time nutrient induced fluorescence transient (NIFT) method to study nutrient limitation in sea ice and further develop this method for use with the imaging pulse amplitude modulator (I-PAM), III) show that Antarctic diatoms may be more susceptible to silica limitation than previously thought, despite the fact that the silica concentration in the Southern Ocean are relatively high. Results from these studies provide important new information on community structure and how it is influenced by and responds to the environment ...</p>

A STUDY OF DIATOMS (BACILLARIOPHYTA) IN HOOR-AL-AZIM WETLAND, SOUTH-WEST IRAN, AND THEIR SEASONAL CHANGES

This study was aimed to identify the planktonic diatoms and their seasonal change in Hoor-AL-Azim wetlands, Iran. Diatoms are important algal groups in different aquatic ecosystem and considered as bioindicators for water quality and environmental health. Hoor-AL-Azim or Hoor-al-Hawizeh is one of the most extensive and ecologically important wetlands in the Middle East which is shared between Iran (south-west) and Iraq(south). A major part of the wetland area was exposed to drying completely or partially during last few decades due to two Country politics. The samples were taken on a monthly basis from four stations for year from January to end of December, for identification and seasonal changes of Diatoms (Bacillariophyta). In this study, totally 69 taxa belonging to 28 different genera were identified of which Nitzschia genus had the highest species diversity (13 species) and followed by Navicula genus (10 species) while the highest species diversity observed in autumn. The average density ranged from 4818 cells/ liter to 58625 cells /liter in spring and winter, respectively.

Assessment of phytoplankton composition and physicochemical parameters of Omasi rice field, Anambra State, Nigeria

The microalgae and physicochemical parameters of floodwater of Omasi rice field in Anambra State, Nigeria were studied. Samples for the studies were collected at monthly intervals. The microalgae were studied using light microscopy and identified with taxonomic keys, text books and photograph materials from the internet. The physicochemical parameters and coliform content of the floodwater were analysed using the methods described by the American Public Health Association. Simple means of the parameters and percentages of the algal populations were calculated, while Pearson correlation (p ≤ 0.05) was used to check for significance of the relationships between the investigated parameters. A total of 12 algal taxa belonging to Chlorophyta (48.99%), Cyanophyta (32.89%), Euglenophyta (10.07%), and Bacillariophyceae (8.05%) were recorded in decreasing order of abundance. Water temperature ranged from 26-38 °C with mean of 33.3 ± 2.56 °C; colour ranged from 15-175 Hazen units with mean of 86.25 ± 33.19 Hazen units; depth of water ranged from 7-10.5 cm with mean of 9.38 ± 0.8 cm. Ranges of nitrates and phosphates with their respective means were 0.5-1.8 mg/l (0.86±0.31 mg/l) and 0.79-1.96 mg/l (1.18±0.28 mg/l). Omasi rice field supported the growth of diverse algal groups and species; this may be as a result of available nutrients and good climate as can be deduced from the correlation analyses. Omasi rice field is typical of tropical freshwaters and some tropical rice fields that have been studied in terms of microalgal diversities and some physicochemistry

Recovery Responses of Freshwater Phytoplankton Assemblage After the Disappearance of Metal Toxicity in Semi-Continuous Cultures

The present study demonstrates the recovery of phytoplankton assemblage from metal stress. Phytoplankton assemblage consisting of different freshwater algal species isolated from a tropical pond was exposed to sublethal concentrations of Cu and Zn for 25 days in semi-continuous culture (Toxicity phase). Subsequently, algal assemblage grown in the toxicity phase were transferred to the fresh culture medium without elevated levels of the test metals for 25 days in semi-continuous culture (Recovery phase). We monitored the total biovolume of each algal species during the toxicity and recovery phases. The members of Cyanophyta were most sensitive against metal toxicity, followed by the members of Bascillariophyta. However, the members of Chlorophyta showed relatively lesser sensitivity against test metals. Among chlorophytes, Scendesmus opolinensis and Cosmarium bioculatum were tolerant to both the test metals. Metal-stressed algal species showed recovery after transferring to the basal medium depending on the concentration of metals during the toxicity phase. The members of Cyanophyta were unable to recover from metal stress. However, members of Chlorophyta showed faster recovery than others from Zn stress, and the members of Bascillariophyta showed quicker recovery from Cu stress. The differential abilities of various algal species to recover from metal stress perhaps depend on their ability to counterbalance metal toxicity. Further, research is warranted to characterize the differential ability of various algal groups to recover from metal stress. The present findings would help understand the efficiency of different algal groups to restore their position within the freshwater algal community after the disappearance for metal stress.

Transparent Exopolymer Particles (TEP), phytoplankton and picocyanobacteria a littoral-to-pelagic depth-gradient in a large subalpine lake

Transparent Exopolymer Particles (TEP) play an important role in the organic carbon cycle of many aquatic systems but the production and distribution of TEP have been studied mainly in the marine environment, neglecting the large oligotrophic lakes. We selected Lake Maggiore, one of the most important freshwater reserve in Northern Italy, to study the horizontal and vertical distribution of TEP and of its possible drivers. Samplings along a transect in the Borromeo basin were performed in May, July and September 2019. Total Organic Carbon (TOC), TEP, chlorophyll-a (Chl) of different algal groups, picocyanobacteria, bacteria and eukaryotes counting, were measured at six stations and five depths. Our study showed that TEP exhibited a clear vertical heterogeneity from surface to the bottom related to the autotrophic microorganisms that are the main source of TEP and are prevalent in the euphotic zone of the lake. On the other hand, TEP was fairly evenly distributed along the horizontal transect from littoral to pelagic zone, although patches were present in spring, when TEP concentrations were low. In contrast to TEP, TOC and to a lesser extent Chl and bacteria showed horizontal heterogeneity, in some months. In Lake Maggiore TEP indeed was an important fraction of Total Organic Carbon (TOC), making up to 54% of TOC (in carbon units: 910 µg C L-1) and it was significantly correlated with Chl. The highest TEP concentration (1.44 mg GX eq L-1) was measured in September 2019, in coincidence with an episode of superficial foam appearance. Considering the biomass as Chl concentrations, the algal group mostly related to TEP was that of brown algae, particularly diatoms; but considering the numbers, the picocyanobacteria and bacteria were more significantly correlated to TEP. The presence of pennate diatoms in May and July, with their TEP-related chlorophyll, did not produce TEP in as high concentration as that observed in September in the presence of centric diatoms and of very high numbers of picocyanobacteria and bacteria.

Recovery Responses of Freshwater Phytoplankton Assemblage After the Disappearance of Metal Toxicity in Semi-Continuous Cultures

The present study demonstrates the recovery of phytoplankton assemblage from metal stress. Phytoplankton assemblage consisting of different freshwater algal species isolated from a tropical pond was exposed to sublethal concentrations of Cu and Zn for 25 days in semi-continuous culture (Toxicity phase). Subsequently, algal assemblage grown in the toxicity phase were transferred to the fresh culture medium without elevated levels of the test metals for 25 days in semi-continuous culture (Recovery phase). We monitored the total biovolume of each algal species during the toxicity and recovery phases. The members of Cyanophyta were most sensitive against metal toxicity, followed by the members of Bascillariophyta. However, the members of Chlorophyta showed relatively lesser sensitivity against test metals. Among chlorophytes, Scendesmus opolinensis and Cosmarium bioculatum were tolerant to both the test metals. Metal-stressed algal species showed recovery after transferring to the basal medium depending on the concentration of metals during the toxicity phase. The members of Cyanophyta were unable to recover from metal stress. However, members of Chlorophyta showed faster recovery than others from Zn stress, and the members of Bascillariophyta showed quicker recovery from Cu stress. The differential abilities of various algal species to recover from metal stress perhaps depend on their ability to counterbalance metal toxicity. Further, research is warranted to characterize the differential ability of various algal groups to recover from metal stress. The present findings would help understand the efficiency of different algal groups to restore their position within the freshwater algal community after the disappearance for metal stress.

Rediscovery of the terrestrial green alga Trentepohlia aurea (Linnaeus) C. Martius (Chlorophyta) in Croatia after 130 years

The first record of the terrestrial green alga Trentepohlia aurea for Croatia was made in 1890 in the area of the Krka River, and has remained until this day the only record of this globally widespread species in the country. During studies related to aquatic vegetation, the species was finally rediscovered from the area of Plitvice Lakes, in the Gorski kotar region and near Vukova Gorica village, after 130 years during which no records were made. All recorded populations formed round, woolly, orange-red patches on the surface of rocks and concrete walls, partly forming a community with bryophytes and lichens. In some cases in these mixed communities, Trentepohlia thalli overgrew bryophytes blocking their access to nutrients, water and light. This long period of lack of records of T. aurea in Croatia is mostly a consequence of the discontinuity in research on certain algal groups in Croatia.

The Ulvophyceae (Chlorophyta) of eastern Sorsogon, Philippines, including Halimeda magnicuneata sp. nov. (Bryopsidales)

The marine algal flora of eastern Sorsogon has been intensively collected and is generally considered as the richest in the Philippines. A trend of species records in the area has been dominated by rhodophytes (red algae) with significantly fewer similar studies for other algal groups (green and brown algae). In this study we present an updated catalogue of the green seaweeds (Ulvophyceae) of eastern Sorsogon. A checklist, including notes on taxonomy, is given of the 103 species. Twenty-six species are newly recorded locally of which five represent new records for the Philippines: Avrainvillea amadelpha, Caulerpa buginensis, an unidentified Caulerpa species, Codium cf. latum, and one taxon new to science. The new species is described as Halimeda magnicuneata Verbruggen et Dumilag based on morpho-anatomy and DNA sequence data. The number of ulvophycean species recorded in eastern Sorsogon is found to be the highest in the Philippines. This may be a result of the high collection effort in the region, as well as eastern Sorsogon’s diverse habitats providing favourable conditions for a wide range of seaweed species.