Freshwater Benthic Diatom Research Articles

Freezing stress tolerance of benthic freshwater diatoms from the genus Pinnularia: Comparison of strains from polar, alpine, and temperate habitats.

Diatoms are among the most important primary producers in alpine and polar freshwaters. Although temperate diatoms are sensitive to freezing, polar diatoms often exhibit more resistance. This is particularly true for members of the (predominantly terrestrial) Pinnularia borealis species complex. However, it remains unclear to what extent this resistance applies to other representatives of the genus. Here, we compare the freezing-stress tolerance of 11 freshwater, benthic strains representing different species of Pinnularia (including Caloneis) from polar, alpine, and temperate habitats. As vegetative cells, strains were exposed to freezing temperatures of -4, -10, -20, -40, -80, and -196°C. Survival was assessed by light microscopy and photosynthetic measurements. We observed vegetative cells to be sensitive to low freezing temperatures; only "mild" freezing was survived by all tested strains, and most tested strains did not survive treatments ≤-10°C. However, individual strain sensitivities appeared related to their original habitats. For example, polar and alpine strains better withstood "mild" and "moderate" freezing (-4 and -10°C, respectively); although temperate strains were significantly affected by the "mild" freezing treatment, polar and alpine strains were not. The -10°C treatment was survived exclusively by polar strains, and only P. catenaborealis survived all treatments. Interestingly, this species exhibited the lowest survival in the -10°C treatment, potentially implying some metabolic activity even at freezing temperatures. Thus, despite more extensive sampling throughout the genus and finer temperature scaling compared to previous studies, the remarkable freezing-stress tolerance of the P. borealis species complex remains unique within the genus.

Cellular uptake of multi-walled carbon nanotubes is associated to genotoxic and teratogenic effects towards the freshwater diatom Nitzschia linearis

The increase in industrial production of multi-walled carbon nanotubes (MWCNTs) raises concerns about their potential adverse effects associated to environmental releases, especially in aquatic environments where they are likely to accumulate. This study focuses on the environmental impact of MWCNTs, specifically on a benthic freshwater diatom (Nitzschia linearis), which plays a major role in the primary production of water bodies. The obtained results indicate that exposure to MWCNTs in the presence of natural organic matter (NOM) inhibits diatom's growth in a dose-dependent manner after 72 h of exposure. Interestingly, the photosystem II quantum yield (PSIIQY) in diatoms remains unaffected even after exposure to MWCNTs at 10 mg/L. After 48 h of exposure, MWCNTs are found to bind preferentially to extracellular polymeric substances (EPS) produced by diatoms, which could decrease their toxicity by limiting their interaction with this organism. However, measurement of genotoxicity and teratogenicity in diatoms exposed to MWCNTs revealed that the exposure to MWCNTs increased the occurrence of cells with micronuclei and abnormal frustules. Microscopy analyses including two-photon excitation microscopy (TPEM) revealed the internalization of MWCNTs. Investigations of the diatom's frustule structure using Scanning electron microscopy (SEM) indicated that the presence of pore structures constitutes a pathway allowing MWCNTs uptake. The presence in the diatom's cytoplasm of MWCNTs might possibly induce disturbances of the cellular components, leading to the observed genotoxic and teratogenic effects. In view of previous studies, this work underscores the need for further studies on the interaction between nanomaterials and different diatom species, given the species-specific nature of the interactions.

Geology, mineralogy, geochemistry and genesis of volcano-sedimentary hosted Lake Abiyata diatomite in central main Ethiopian Rift

The Abiyata Diatomite deposit is located in the Main Ethiopian Rift, which is characterized by strong extensional tectonics. The deposit is mostly made up of diatomaceous earth, which is a sedimentary rock made up of the fossilized remnants of diatoms, which are tiny algae. Diatomite's geological, geochemical, and mineralogical features, as well as its formation, are discussed in this research. To characterize diatomite from Abiyata, chemical, mineralogical, technological, and micro paleontological examinations were conducted on samples collected from outcrops and stream sections. The XRD characteristic peaks of diatomite demonstrate that it is primarily made up of Opal A silica, however certain crystalline phases were discovered in adequate amounts. Quartz and feldspar were the predominant crystalline phases, with lesser amounts of Calcite, Cristobalite, Illite, Mordinite, Wairakite, Halloysite, Clinoaptilolite, Adularia, and Tridymite. From SEM photomicrographs diatomites are primarily formed of benthic freshwater diatom species such as Staurosirella pinnata, Staurosira construens, Pseudostaurosira brevistriata, Epithemia Sorex and surirella pinnata. Diatom species, sedimentary profile sections and mineralogical data suggest that diatomite was deposited in lacustrine-type freshwater shallow lake environment. Chemical data obtained from 10 diatomite samples show that while silica is the bodybuilding material for diatomite. i.e., Silica (SiO2), 76.9 %; Alumina (Al2O3), 3.49 %; Sodium Oxide (Na2O), 1.52 %; Potassium Oxide (K2O), 1.107 %; Iron Oxide (Fe2O3), 1.1 %; Loss on ignition (LOI) 13.7 and other oxides are below 1 %. Studies from technological properties like physical tests, chemistry, and mineralogy and micropaleontology of Abiyata diatomite suggest that calcined diatomite can be used for waste treatment processes in the filter aid industry and as filler material.

Gomphonema incrassata sp. n., a new benthic diatom species (Bacillariophyceae: Gomphonemataceae) from Walayar Dam, Palakkad district, Kerala, India

ABSTRACT A new freshwater benthic diatom species, Gomphonema incrassata sp. n. (Mohan and Prabha), is described from the Walayar Dam, Palakkad district, Kerala, India. The description is based on data derived from light and scanning electron microscopic studies. The main features of Gomphonema incrassata are: valves are weakly heteropolar; linear-lanceolate; head pole is bluntly round; basal or foot pole is sharply rounded; and the frustule is club-shaped and flexed (bent) in the girdle view. In addition, the analysed physicochemical characteristics revealed that Walayar Dam is moderately polluted, due to various human activities. The newly identified freshwater diatom species adds to the knowledge of diatom diversity in the Indian subcontinent.

Revisiting global diversity and biogeography of freshwater diatoms: New insights from molecular data

AbstractThe distribution of microorganisms has long been assumed to be cosmopolitan and primarily controlled by the environment, but recent studies suggest that microbes may also exhibit strong biogeographical patterns driven by dispersal limitation. Past attempts to study the global biogeography of freshwater diatoms have always encountered the great difficulty of collecting taxonomically harmonized large‐scale data. However, developments in molecular techniques and DNA metabarcoding provide a unique opportunity to overcome these limitations and to disclose diatom biodiversity at an unprecedented scale and resolution. Here, we assembled DNA metabarcoding data of freshwater benthic diatom communities sampled in seven geographic regions across the world to investigate how diatom diversity varies along latitude and to assess the proportion of genetic variants of these microorganisms which are exclusive or shared across regions. We observed significant differences in assemblages among climate zones and found that genetic richness is not affected by latitude, but by an island effect. The genetic resolution directly impacts the proportion of variants shared across regions; however, the majority of taxa remained specific to a single geographic region. Freshwater diatoms disperse over long distances and across oceans but at rates that allow the appearance of local genetic variants and the regionalization of assemblages. Future work should focus on putting these diversity dynamics into a temporal context, an approach that should be possible by bringing together new sequencing techniques and phylogeography.

No lukewarm diatom communities—the response of freshwater benthic diatoms to phosphorus in streams as basis for a new phosphorus diatom index (PDISE)

In the present study, we developed a new Swedish phosphorus diatom index (PDISE) to improve the poor fit of existing indices to match the needs of water managers to detect and mitigate eutrophication. We took advantage of a large amount of data (820 Swedish stream sites) collected in recent years. During our work, we found an unexpected bimodal response of the diatom assemblages to phosphorus. The taxa clustered either into an assemblage with a low or with a high site-specific averaged TP optimum (a calculated value comprised of the diatom taxa-specific optima). We could not find a characteristic diatom assemblage for sites with intermediate site-specific averaged TP optima. To our knowledge, this bimodal community response has not been shown earlier. The PDISE correlated more strongly than the currently used TDI to changes in TP concentrations. Thus, the PDISE should replace the TDI in the Swedish standard method. The modeled TP optima (expressed as categories) were different compared to the TDI for most of the taxa included in the index, indicating that the realized niche for these morphotaxa was different between Sweden and the UK where the TDI was developed originally. With a r2 of 0.68, the correlation of the PDISE to TP is among the highest reported for other diatom nutrient indices globally; thus, we believe that it might be worth to test it for other bioregions with similar geography and climate.

Co-occurrence, ecological profiles and geographical distribution based on unique molecular identifiers of the common freshwater diatoms Fragilaria and Ulnaria

Diatom taxonomy has evolved in recent years, with many new species described and new approaches such as molecular genetics showing the existence of cryptic diversity within currently accepted species. This cryptic diversity is not well understood even for common freshwater genera such as Fragilaria and Ulnaria. The purpose of our study was to define taxon-specific ecological profiles and geographical distributions for unique molecular identifiers (amplicon sequence variants, ASVs) linked to curated taxonomy for these genera. Our goal is to contribute to the development of ecological assessment methods, and to the understanding why we often observe so many diatom species co-occurring in a single sample. We filtered a large (770 samples) metabarcoding dataset with linked environmental data covering several countries in Europe for genetic variants (ASVs) assigned to currently accepted species of our target genera. We studied the geographical distribution of the ASVs, and tested for ASV-pair co-occurrence. We modelled ASV-specific preferences for pH, alkalinity, total nitrogen, total phosphorus and conductivity, and analysed their preference for lakes or streams as habitat. Our study confirmed that there seems to be no general geographical barrier for the distribution of freshwater benthic diatom ASVs in Europe, but that dispersal is not rapid enough to hide historical events. The Fragilaria and Ulnaria ASVs in our study showed considerable overlap in geographical distribution, habitat and ecological preferences. We found evidence that only large differences in preferences for the analysed water chemistry variables prevented the co-occurrence of ASVs at the same sites. Instead, Fragilaria and Ulnaria ASVs co-occurred frequently in samples. We found subtle differences in ecological preferences for some ASV pairs, which might in part explain the co-occurrence by the avoidance of direct competition. However, the great overlap in distribution and ecological preferences suggests that other factors not studied here were also responsible for the observed co-occurrences and high richness of ASVs found at many sites. To our knowledge, we are the first to use ASVs in combination with a curated taxonomy to understand co-occurrence, specific ecological profiles and large-scale geographical distribution for unique identifiers not biased by the quality of reference databases, clustering methods, or non-harmonized morphological identification. Thus, our results can now be used in subsequent projects to interpret ASV occurrences, e.g. for development of ecological assessment methods.

Morphology and molecular phylogeny of Amphora baotuensis sp. nov., a new freshwater benthic diatom from a karst spring in China

A new freshwater benthic diatom Amphora baotuensis is described from a karst spring in China, based on morphological and molecular phylogenetic data. Based on SSU rRNA and rbcL sequences the molecular phylogeny indicates that this species is closely related to A. copulata (Kützing) Schoeman & Archibald within the Amphora sensu stricto clade. Amphora baotuensis differs from congeners by a combination of morphological features including smaller cell size, a wide, rectangular to fan-shaped dorsal fascia, a well-developed raphe ledge on the ventral side and uniseriate ventral striae.

Same same, but different: The response of diatoms to environmental gradients in Fennoscandian streams and lakes – barcodes, traits and microscope data compared

We developed and compared the performance of freshwater benthic diatom indices calculated from (i) traditional morphological species identification, (ii) Amplicon Sequence Variants (ASVs) obtained via DNA metabarcoding, and (iii) morphological traits to indicate eutrophication in rivers and lakes in Fennoscandia. Based on the results, we provided recommendations for the future routine use of diatom bioassessment tools in environmental monitoring and assessment. Our results show that ASVs are the most promising candidates to be used in environmental assessment. Indices based on ASVs correlated better with TotP concentrations than morphological taxa data, whereas the trait indices correlated least. We could see by studying the taxonomic assignments of the ASVs that traditional morphotaxa were divided up into several ASVs with different ecological profiles, which explained part of the better index performance and also encourages further studies on diatom diversity and ecological preferences. In general, ASV- and morphotaxon-specific optima differed slightly between streams and lakes, but were significantly correlated with each other. This means that it should be possible to develop a common index that is applicable in both streams and lakes, but boundary values with respect to TotP might need to be set separately for them. More knowledge on diatom traits is required to enable their use for environmental assessment.

Both hydrology and physicochemistry influence diatom morphometry

It has been shown that size of planktonic diatoms responds to changes in water chemistry, but little is known about the influence of different types of environmental factors upon benthic diatom size variation. Here we ask whether water chemistry or hydromorphological influences show a stronger effect on benthic diatom morphometry. 10,624 diatom valves / frustules were imaged, identified and measured from 48 samples across eight sites in the Ebro river basin obtained during ten years of biomonitoring work. We analysed how differences in surface-to-volume (S/V) ratio and other morphometric characteristics were related to spatial and temporal variability in environmental parameters. Our results show that both hydrological and physicochemical variables generated significant responses in interspecific and intraspecific morphometric variation. S/V ratio and transapical width showed the strongest responses, followed by (the predominantly used) cell volume. Freshwater benthic diatom morphology seems to respond to diverse environmental factors and could provide complementary ecological information to community composition.

Community phylogenetic structure reveals the imprint of dispersal-related dynamics and environmental filtering by nutrient availability in freshwater diatoms

Despite important progress, uncertainty persists regarding the ecological forces driving microbial community assembly. Here, we present the first study to use phylogenetic information to interpret the structure and diversity of diatom communities. We examined local phylogenetic divergence and beta- phylogenetic diversity in a large dataset of 595 freshwater benthic diatom communities and we investigated how this diversity is influenced by gradients in nutrients, pH, organic matter and catchment size. Overall, we found that diatom communities were phylogenetically clustered, i.e. species within communities were more closely related than expected by chance. Phylogenetic clustering was stronger in nutrient-poor environments and in sites with a small catchment area. The variation of the phylogenetic beta-diversity index was much better explained by space and environment than the variation of the taxonomic index was. Both approaches detected a significant effect of environment and space on diatom community turnover. Our results support the view that diatom communities are primarily shaped by environmental filtering, in particular by nutrient availability. Moreover, they highlight the importance of considering dispersal-related processes and the depth of phylogenetic signal in functional traits when interpreting patterns of diversity.

Physicochemical Controls on Spatiotemporal Distribution and Benthic Mat Severity of Didymosphenia geminata in Pine Creek, an Unregulated Watershed in Northern Pennsylvania

Didymosphenia geminata (Didymo) is a benthic freshwater diatom that has been globally expanding its range and extracellular stalk production in freshwater ecosystems. Didymo has been observed in reaches downstream of hypolimnetic reservoir releases in the northeastern US since 2007. This study focused on a newly observed (2013) Didymo occurrence in Pine Creek, a highly forested and unregulated watershed in north-central Pennsylvania. Study objectives included comparing contemporary distribution with historical data to provide insight on historical occurrence, quantifying physicochemical controls on Didymo distribution and benthic mat severity, and examining historical changes in water chemistry that might affect habitat suitability. At present, Didymo cellular distribution is limited to upper reaches of Pine Creek where median soluble reactive phosphorus (SRP) is 2.7 µg/L; median SRP was 4.8 µg/L at sites where Didymo was absent. At the epicenter of distribution in Pine Creek where SRP was consistently <2 µg/L, increased streamflow flashiness and water temperature were associated with decreased benthic mat severity. My results suggest SRP thresholds for Didymo proliferation may vary depending on whether streams are regulated by reservoirs with hypolimnetic releases. Mann–Kendall trends tests of a ∼20-y water chemistry dataset show that orthophosphate and sulfate concentrations decreased while pH increased within Pine Creek, which may have implications for Didymo habitat suitability. Further research is warranted to determine whether improving water quality following the industrial era may facilitate Didymo colonization.

Valve teratologies and Chl c in the freshwater diatom Tabellaria flocculosa as biomarkers for metal contamination

Freshwater benthic diatoms, routinely used for bioassessment, are potential metal indicators because of their known capacity to reflect changes in water chemistry. Nevertheless, the effect of metals on benthic freshwater diatoms is still poorly understood. Metals, such as Zn and Cu, are not only essential as micronutrients, but also one of the main anthropogenic pressures in aquatic ecosystems. However, metal stress cannot be detected with the current European standard methods using bioindicators for environmental assessment, so new tools need to be developed. We studied the effects of Zn and Cu on the freshwater diatom Tabellaria flocculosa. This diatom isolated from a Zn and Cu contaminated stream, was exposed to different concentrations of Zn and Cu, and the metal effects were assessed by measuring physiological ((growth, fluorescence, pigments’ content (Chl a, c, carotenoids)) and morphological parameters (teratological forms). We found that at environmental concentrations occurring in European streams, Zn and Cu showed toxic effects on T. flocculosa. Toxic effects induced by Zn were only observed at high concentrations (500 and 1000 µg/L), resulting in fluorescence changes, an increase of Chl c cellular content and an increase of teratological forms (with an increase of more heavily deformed valves with increasing metal concentrations). Increasing Cu exposure induced several changes, such as increasing the cellular content of Chl a and c, while carotenoids’ cellular content decreased. Cu also increased the number of T. flocculosa frustule deformations. Cu did not affect the cellular fluorescence. Our results thus show that the frequency of diatom teratologies could potentially be used to assess metal stress for the studied metals Zn and Cu. Additionally, the analysis of algal pigments could potentially separate the effects of different metals.

Transcriptomic response of the benthic freshwater diatomNitzschia paleaexposed to Few Layer Graphene

FLG induces strong physiological modifications as a survival strategy ofNitzschia palea, at cell and biofilm scale.

Late Holocene climate changes from diatom records in the historical Reservoir Gonggeomji, Korea

The paleoenvironmental history of the artificial Reservoir Gonggeomji in Sangju City, Korea, was reconstructed using fossil diatom analysis of four sediment cores: GG01, 02, 03, and 04. Accelerator mass spectrometer (AMS) radiocarbon dating results suggest that Reservoir Gonggeomji was maintained from ca. 1350 to 150 cal year BP, and the diatom record in core sediment samples revealed well-defined hydrological changes and paleoenvironmental conditions during this period. The fossil diatom assemblages were predominantly benthic freshwater diatoms. The highest diatom concentrations in all cores were in diatom zone II, in which species richness and diversity were also very high. Cymbella, Eunotia, Gomphonema, Gyrosigma, Navicula, and Pinnularia were the dominant genera in all core sediments. The long-term trend in diatom species abundance and species diversity showed a stronger relationship to temperature anomalies in the northern hemisphere during the past 2000 years with changes in precipitation. These diatom changes could have been controlled by natural climate change, despite anthropogenic activities linked to construction of the dyke in the artificial reservoir.

Toxicological effects of cadmium and lead on two freshwater diatoms

In recent years, there have been significant advances in the knowledge and understanding of the pollution attributed to effects of aquatic toxic metals on fresh water benthic diatoms. In this study, the cell growth, chlorophyll a content and superoxide dismutase activity in Halamphora veneta (Kützing) Levkov and Surirella crumena Brébisson exposed to cadmium and lead, were investigated. Furthermore, in order to explore the potential function of H. veneta on environmental monitoring and environmental remediation, expression of two genes (psbA, psaB) and morphological analysis of H. veneta were carried out. The cells growth of H. veneta and S. crumena were generally inhibited with cadmium and lead exposure during 96 h, while cells density of H. veneta was significantly increased under the low concentration at 24 h cadmium exposure. Our results indicated that H. veneta had a certain tolerance to toxic metals at initial treated time. In addition, the significantly changes of chlorophyll a content and SOD activities in H. veneta and S. crumena indicated that both photosynthetic system and the antioxidant system in benthic diatom might play important roles on the toxic metals tolerant mechanism. Meanwhile, it can be confirmed that the diatom photosynthetic systems play roles on toxic metals resistance inferred from the gene expression of psbA and psaB in H. veneta. Finally, the results of scanning electron microscopy showed that there was a slightly deformation on cells following the cadmium exposure in H. veneta, while obvious deformation with cell greatly widened after lead exposure. The present work will be helpful to understand the effect mechanisms of toxic metal by comparing two kinds of diatom on cell inhibition, biological response and morphological changes, which will provide more information for possible use of benthic diatoms in bioremediation.

Assessing Cu impacts on freshwater diatoms: biochemical and metabolomic responses of Tabellaria flocculosa (Roth) Kützing

Metals are a recognised threat to aquatic organisms but the impact of metals such as copper (Cu) on benthic freshwater diatoms is poorly understood, even if diatoms are commonly used as water quality indicators. Our study aimed to elucidate the cellular targets of Cu toxicity and the mechanisms cells resort to counteract toxicity and to increase tolerance to Cu. A concerted approach analysing the biochemical, physiological and metabolome alterations in diatom cells was conducted by exposing the freshwater diatom Tabellaria flocculosa to 0, 0.3, 6 and 10μgCu/L. Cu was already toxic to T. flocculosa at concentrations common in environments and which are not usually considered to be contaminated (0.3μgCu/L). Under Cu impact, the metabolome of T. flocculosa changed significantly, especially at high concentrations (6 and 10μgCu/L). Cu toxicity was counteracted by increasing extracellular immobilization (EPS, frustulins), antioxidant (SOD, CAT) and detoxifying (GSTs) enzymes activity and low molecular weight antioxidants (GSH). These mechanisms were fuelled by higher energy production (increased ETS activity). At the highest Cu concentration (10μg/L), these processes were specially enhanced in an attempt to restrain the oxidative stress generated by high intracellular Cu concentrations. However, these mechanisms were not able to fully protect cells, and damage in membranes and proteins increased. Moreover, the decrease of hydroxylamine and unsaturated fatty acids and the increase of saturated fatty acids, 2-palmitoylglycerol, glycerol and diterpenoid compounds should be tested as new specific markers of Cu toxicity in future studies. This information can support the prediction of diatom behaviour in different Cu contamination levels, including highly impacted environments, such as mining scenarios, and may assist in environmental risk assessment policies and restoration programs.

Validation of five diatom novelties published in “Freshwater Benthic Diatoms of Central Europe” and taxonomic treatment of the neglected species Tryblionella hantzschiana

In July 2017, Lange-Bertalot et al. (2017) published a revised and translated version of Hofmann et al. (2013)’s book on freshwater benthic diatoms of Central Europe. The revised edition includes a few nomenclatural novelties which neither conform to the Melbourne Code (McNeill et al. 2012) nor to the upcoming Shenzhen Code (see Turland et al. 2017). The treatment of Tryblionella gracilis W.Sm. and T. hantzschiana Grunow shows how nomenclatural and taxonomic data can sometimes be confusing. This paper validates five names invalidly published in Lange-Bertalot et al. (2017) whilst the valid but previously neglected name T. hantzschiana is proposed to replace Tryblionella tryblio Cantonati et Lange-Bert., nom. inval. The registration numbers refer to PhycoBank (Kusber et al. 2017) a registration system on algae (Barkworth et al. 2016) which recently went online.

Response of the freshwater diatom Halamphora veneta (Kützing) Levkov to copper and mercury and its potential for bioassessment of heavy metal toxicity in aquatic habitats

This study investigates the effects of copper and mercury on growth rate, chlorophyll a content, superoxide dismutase (SOD) activity, SOD mRNA gene expression, and frustule morphology of the benthic freshwater diatom Halamphora veneta (Kützing) Levkov and the potential utility of each for toxicity assessment in aquatic habitats. Results showed the following: (1) Compared to mercury, exposure to copper resulted in greater growth inhibition of H. veneta even at low concentrations and after short durations of exposure; (2) high accumulation of chlorophyll a in H. veneta is a stress response to the presence of heavy metals; (3) SOD activity and SOD gene expression varied in H. veneta according to the concentration, exposure time, and type of heavy metal; and (4) exposure to mercury resulted in deformity in the shape and an increase in size of the frustule of H. veneta. Growth rate, chlorophyll a content, SOD activity and gene expression, and frustule morphology of H. veneta are all potential candidates for the toxicological assessment of copper and mercury in aquatic habitats.

Chronic effects of atrazine exposure and recovery in freshwater benthic diatoms from two communities with different pollution histories

Diffuse agricultural runoff into rivers can result in contamination with herbicides for prolonged periods of time. Chronic exposure to herbicides has the potential to alter toxic impacts in primary producers such as benthic diatoms. Determining how individual diatom taxa respond to herbicide exposure over varied exposure durations is essential for assessing herbicide impacts. This study investigated the responses of various benthic diatom taxa and effects at the community level over 12days of atrazine exposure. Diatom communities were collected from two sites with differing exposure histories; a relatively unpolluted site (Alligator Creek) and an agricultural stream (Barratta Creek) known to be polluted by atrazine and other herbicides. Diatom community composition and the proportion of healthy cells per taxon were assessed at 0, 2, 3, 6, 9 and 12days of atrazine exposure. Pollution history altered the response of the diatom community to atrazine exposure. In the Alligator Creek diatom community there was a shift in composition towards more tolerant taxa and the loss of sensitive taxa in atrazine exposed treatments. The sensitive taxon (Gomphonema truncatum) was consistently affected by atrazine toxicity. Conversely, the polluted Barratta Creek diatom community was not strongly affected by atrazine exposure. Our study shows that during chronic atrazine exposure some taxa demonstrated the ability to recover despite initial toxicity response. Recovery could be an important trait for understanding the ecological effect of herbicide exposure on diatom species in nature and in applied circumstances such as biomonitoring indices.