Module 5: Diatoms

Diatoms

Photosynthetic eukaryotic microorganisms enclosed in porous silica shells called frustules. They are major primary producers and help move fixed carbon into the deep ocean when they sink.

Phycosphere

The region surrounding an algal cell, chain, or colony where algal secretions stimulate bacterial growth. It is a concentrated zone for nutrient exchange and chemical interactions.

Rhizosphere

The soil region around plant roots where root-derived compounds influence associated microbes; the terrestrial analogy used to explain the phycosphere.

Diffusive boundary layer

A thin layer of fluid around a small aquatic cell in which diffusion controls molecular movement because turbulent mixing is ineffective at that scale. It helps maintain chemical gradients around the cell.

What features make diatoms important to marine ecosystems and global nutrient cycles?

Diatoms are photosynthetic eukaryotes encased in porous silica frustules. The lecture attributes about 20% of Earth's photosynthesis to them, making them major primary producers at the base of marine food webs. They fix CO2 into organic matter that supports consumers and associated bacteria. After death they can sink rapidly, transporting organic carbon to deeper water and contributing to the carbon cycle. Their growth and interactions also affect nitrogen, silicon, and iron cycling, linking microscopic cell processes to ocean productivity.

How does cell movement change the diffusive boundary layer, and why does that matter?

A diffusive boundary layer is a thin layer of water around a small aquatic cell where diffusion controls molecular movement because turbulent mixing is ineffective. Secreted compounds remain concentrated near the cell, helping sustain the phycosphere. A still cell has a more even, approximately symmetric concentration field. When the cell swims, rises, or sinks, relative water motion stretches and distorts that field, making it uneven. This changes where nutrients and signals are concentrated. Bacteria seeking the phycosphere must therefore track a moving, changing resource patch; chemotaxis or attachment can help.

Where are the Antarctic ice diatoms discussed in class found?

They occur in and on sea ice, including liquid brine channels within the ice and the ice-water interface on its underside. These sites can provide light from above and nutrients from seawater below. The lecture case study involved Antarctic ice diatoms from the Weddell Sea; Amphiprora kufferathii was examined with its attached bacterial epibionts. Ice-associated does not mean the cells are embedded in completely solid, liquid-free ice.

How can bacterial epibionts protect Antarctic ice diatoms from oxidative stress?

Photosynthesis can generate reactive oxygen species, including superoxide. Superoxide dismutase converts superoxide into products including hydrogen peroxide (H2O2). In the lecture study, diatom catalase activity was not detected, whereas bacterial epibionts had catalase, which breaks down H2O2 into water and oxygen. Removing peroxide limits formation of highly damaging reactive species and helps protect lipids, proteins, and DNA. This explains why the bacteria can improve diatom growth, particularly under high light when peroxide accumulates more strongly in axenic cultures.

What evidence supports a peroxide-detoxifying role for Antarctic diatom epibionts?

Researchers compared Amphiprora kufferathii cultures with bacterial epibionts against axenic cultures produced by antibiotic treatment. Diatoms with bacteria grew better, and added H2O2 declined faster in cultures containing epibionts. Under increased light, axenic cultures accumulated more peroxide. Enzyme assays detected catalase in the bacterial isolates but not in the diatom preparation. These results support the detoxification model more specifically than growth improvement alone, because they connect the bacterial effect to peroxide removal and a plausible enzyme mechanism.

Why does a diatom create a phycosphere?

The diatom releases dissolved organic compounds into nearby water. Diffusion and limited local mixing allow concentrations near the cell to differ from the bulk water, producing a chemically structured zone. Bacteria can detect and exploit that zone; it is not a rigid anatomical organ or a separate membrane.

How are the phycosphere and diffusive boundary layer related but different?

The diffusive boundary layer describes the physical regime of molecular transport near the cell. The phycosphere describes the chemically influenced neighborhood shaped by the diatom and used by associated microbes. Limited diffusion-driven transport helps preserve local gradients, but the terms do not name the same feature.

How can bacteria remain associated with a moving diatom?

Chemotaxis can guide swimming bacteria along gradients of diatom-released compounds, while attachment can hold bacteria near the source. Because cell motion deforms and moves the surrounding concentration field, successful association involves tracking or remaining attached to a changing resource patch.

Why does bacterial rescue of growth in B12-free medium not identify the exchanged metabolite by itself?

Live bacteria may supply B12, but they could also provide another growth-promoting compound or alter conditions. Growth restoration supports bacterial facilitation; identifying B12 as the actual causal factor requires more specific evidence. Separate the observed growth effect from the proposed nutritional explanation.

How can an iron-binding bacterial product create cooperation or competition?

A siderophore can mobilize iron and make it available to a partner that can use the iron complex. It can also restrict access if another organism cannot obtain iron from that complex. The result depends on partner capabilities and resource availability, not simply on whether iron binding occurs.

Why does nitrogen fixation help a diatom that already photosynthesizes?

Photosynthesis fixes carbon but does not automatically supply usable nitrogen. Nitrogen-fixing cyanobacteria convert N2 into ammonia, providing a different limiting resource. Heterocysts help separate nitrogen fixation from oxygen-producing photosynthesis within the cyanobacterial partner.

Why are superoxide dismutase and catalase different steps in oxidative-stress protection?

Superoxide dismutase acts on superoxide and produces hydrogen peroxide among its products. Catalase then removes hydrogen peroxide by converting it to water and oxygen. Detecting the first enzyme does not show that the second step is sufficient. The ice-diatom study detected bacterial catalase while diatom catalase was not detected under the tested conditions.

What is the difference between a bloom-associated bacterial shift and a proven growth-promoting mechanism?

A community shift shows that certain bacteria are enriched with a host or under particular bloom conditions. Gene profiles suggest possible activities such as carbohydrate processing. A causal growth mechanism requires evidence that a particular bacterial activity changes host performance. Site differences and host identity must also be considered.

Diatom + vitamin B12-associated bacteria — Describe this relationship.

Partners: A vitamin-requiring diatom and associated bacteria; the experiment does not supply a universally applicable named pair. | Habitat: Aquatic diatom habitat and the laboratory B12-limitation experiment. | Anatomy / location: Bacteria occur in the diatom-associated environment/phycosphere; intracellular residence is not established for this example. | Transmission: Environmental association is consistent with the model; a specific inherited route is not established. | Interaction type: Growth-promoting association, consistent with nutrient cooperation; the exact exchanged factor needs confirmation. | Effect on host / recipient: Many diatoms require B12 but cannot synthesize it. Live bacteria improve growth under B12 limitation more than dead bacteria in the illustrated experiment. | Effect on symbiont / other partner: Diatom-derived organic carbon is a possible resource, based on the broader lecture model; a reciprocal fitness benefit is not directly measured in this experiment. | Age: Not specified in the provided sources. | Key distinction: Growth rescue alone does not prove that bacteria supplied B12 rather than another helpful metabolite.

Diatom + iron-mobilizing bacteria — Describe this relationship.

Partners: Diatoms and associated bacteria capable of producing iron-binding siderophores. | Habitat: Aquatic habitats where iron availability can limit growth. | Anatomy / location: Extracellular chemical exchange in the phycosphere and surrounding water; the interaction does not require an intracellular bacterium. | Transmission: Partners associate through the shared environment; pair-specific inheritance is not established. | Interaction type: Potential cooperation through iron mobilization, with possible competition for the same limiting resource. | Effect on host / recipient: Diatoms can benefit from bacterial siderophore-associated iron acquisition. Diatom-released saccharides can also improve iron accessibility. | Effect on symbiont / other partner: Bacteria bind/access iron; diatom-associated carbon may support them. The outcome depends on resource access and the partner combination. | Age: Not specified in the provided sources. | Key distinction: An iron-binding molecule can facilitate another organism’s uptake or withhold iron from it. The lecture also treats domoic-acid metal binding as a proposed mechanism, not a universal demonstrated exchange.

Diatom + consumers of released organic carbon — Describe this relationship.

Partners: Diatoms and heterotrophic bacteria able to use their dissolved organic carbon, including glycolate-utilizing bacteria. | Habitat: The phycosphere and broader aquatic bloom environment. | Anatomy / location: Extracellular exchange: diatoms release dissolved organic compounds and extracellular polymeric substances that bacteria can use. | Transmission: Environmental encounter/association; inherited transmission is not demonstrated. | Interaction type: Carbon provisioning benefits bacteria. A mutualistic classification requires a demonstrated return benefit to the diatom. | Effect on host / recipient: The diatom releases organic carbon; a specific benefit from each bacterial consumer is not established. | Effect on symbiont / other partner: Bacteria gain carbon and energy. Only some community members carry relevant capabilities, such as glcD-associated glycolate use. | Age: Not specified in the provided sources. | Key distinction: Carbon release alone does not prove mutualism. Community competition determines which bacteria exploit the released material.

Diatom + nitrogen-fixing cyanobacteria — Describe this relationship.

Partners: Diatoms and nitrogen-fixing cyanobacteria; Richelia and Calothrix are associated examples named in the lecture. | Habitat: Aquatic diatom habitats where fixed nitrogen is valuable. | Anatomy / location: Cyanobacterial heterocysts allow nitrogen fixation to be separated from oxygen-producing photosynthesis. Exact placement relative to the diatom is not specified for every named association. | Transmission: Not established for this specific association in the provided sources. | Interaction type: Nutritional association providing fixed nitrogen; reciprocal details depend on the particular pair. | Effect on host / recipient: Cyanobacteria convert N2 into ammonia that the diatom can take up. | Effect on symbiont / other partner: The association provides a diatom-associated habitat; the specific reciprocal resource transfer is not directly established in these slides. | Age: Not specified in the provided sources. | Key distinction: Do not assign one host species or one intracellular/extracellular location to both Richelia and Calothrix without pair-specific evidence.

Diatom + bacteria releasing extracellular algicides — Describe this relationship.

Partners: A diatom and algicidal bacteria; a single universal bacterial species is not named for this mechanism. | Habitat: Aquatic diatom-associated habitat. | Anatomy / location: Bacteria release algicidal substances outside cells; the mechanism does not require invasion of the diatom. | Transmission: Environmental exposure to bacteria/their products; no inherited transmission is established. | Interaction type: Antagonism, presented among parasitic/defensive diatom–bacterial interactions. | Effect on host / recipient: Algicidal products damage or kill diatoms. | Effect on symbiont / other partner: Bacteria may gain access to material released by damaged diatoms; the exact fitness gain is not quantified in the slide. | Age: Not specified in the provided sources. | Key distinction: Quorum sensing can regulate extracellular attack. Keep this mechanism separate from direct Saprospira invasion.

Diatom + directly invading Saprospira — Describe this relationship.

Partners: Diatoms and Saprospira bacteria. | Habitat: Aquatic diatom habitat. | Anatomy / location: Direct physical attack/invasion of the diatom is illustrated, contrasting with diffusible extracellular algicides. | Transmission: Environmental encounter and attack; inherited colonization is not established. | Interaction type: Antagonistic, predatory/parasitic bacterial interaction. | Effect on host / recipient: The diatom is damaged or killed. | Effect on symbiont / other partner: The attacking bacterium can exploit diatom material; a quantitative fitness benefit is not given. | Age: Not specified in the provided sources. | Key distinction: This is a distinct bacterial attack strategy. Diatom fatty-acid products, polyunsaturated aldehydes, and altered quorum-sensing signals are discussed as defenses in the following slide, not as proof of protection in every encounter.

Antarctic ice diatom + peroxide-removing epibionts — Describe this relationship.

Partners: Amphiprora kufferathii and bacterial epibionts; tested isolates include MH1 Sulfitobacter sp., MH2 Colwellia sp., and MH3 Pibocella sp. | Habitat: Antarctic sea ice, including Weddell Sea brine channels and the underside of ice. | Anatomy / location: Ectosymbionts on the diatom exterior; bacteria were removed experimentally to compare cultures with and without epibionts. | Transmission: The study manipulates bacterial presence; it does not establish the natural inheritance mechanism. | Interaction type: Cooperation / bacterial facilitation of host growth; the diatom can be maintained without bacteria, so this is not demonstrated obligate dependence. | Effect on host / recipient: Bacteria promote growth and faster hydrogen-peroxide removal. Without epibionts, high-light cultures accumulate more peroxide, linking the association to oxidative-stress relief. | Effect on symbiont / other partner: Diatom organic carbon is a plausible resource from the broader phycosphere model, but this experiment does not directly establish the reciprocal exchange. | Age: Not specified in the provided sources. | Key distinction: All three isolates showed catalase activity. Diatom catalase was not detected under the assay conditions, although superoxide dismutase was detected; do not claim the diatom lacks all antioxidant defenses.

Bloom-forming TW diatom + associated bacterial community — Describe this relationship.

Partners: The diatom labeled TW in the lecture’s bloom experiments and a changing bacterial community; the comparison dinoflagellate is a different host. | Habitat: Hong Kong-area waters across an estuary-to-coastal gradient and simulated bloom experiments. | Anatomy / location: The study compares attached and free-living bacterial fractions; community association does not establish intracellular residence. | Transmission: Environmental community assembly during blooms; vertical transmission is not demonstrated. | Interaction type: Host-associated community shifts and possible growth-supporting functions, rather than a single proven obligate mutualism. | Effect on host / recipient: Bacterial communities may influence bloom growth and nutrient exchange, but community patterns alone do not establish the effect of each taxon. | Effect on symbiont / other partner: Bloom-derived organic matter can support bacteria; functional profiles include carbohydrate-processing and glycolate-use potential. | Age: No age for this specific association. The separate broad diatom lecture discusses diatom–bacterial coexistence over more than 200 million years, not the age of this TW community. | Key distinction: Host identity and sampling site both matter. Predicted metabolic functions are evidence of potential, not direct measurement of a specific metabolite transfer.