Question

I was stunned at how the lake had changed in the 10 years since I had last at Grand Lake St.. No swimmers splashed in the water and hardly any boats floated across the lake, even though it was peak vacation season. As a lone speedboat zoomed past, Keith noticed its motor churning up lime green-colored water. A closer inspection revealed a foamy, green mat of scum clinging to the shore, with a few dead fish washed up.

Why is the water so green? What are cyanobacteria? What might cause a tremendous increase in their growth? Why would toxin secretion limit fish consumption? How would cyanobacterial photosynthesis affect the oxygen levels of the lake? Aidan indicated that aerobic cyanobacteria are using up oxygen in the water. How are they doing this? Identify the microbial imbalance in Grand Lake that has led to the oxygen depletion.

Please explain

Answer 1

What are cyanobacteria?

Cyanobacteria also known as Cyanophyta, are a phylum consisting of free-living bacteria and the endosymbiotic plastids, a sister group to Gloeomargarita, that are present in some eukaryotes. They commonly obtain their energy through oxygenic photosynthesis. The oxygen gas in the atmosphere of earth is produced by cyanobacteria of this phylum, either as free-living bacteria or as the endosymbiotic plastids. The name cyanobacteria comes from the color of the bacteria .  Cyanobacteria, which are prokaryotes, are also called "blue-green algae", though some modern botanists restrict the term algae to eukaryotes. Cyanobacteria appear to have originated in freshwater or a terrestrial environment.

Cyanobacteria are a group of photosynthetic bacteria, some of which are nitrogen-fixing, that live in a wide variety of moist soils and water either freely or in a symbiotic relationship with plants or lichen-forming fungi (as in the lichen genus Peltigera). They range from unicellular to filamentous and include colonial species. Colonies may form filaments, sheets, or even hollow spheres. Some filamentous species can differentiate into several different cell types: vegetative cells – the normal, photosynthetic cells that are formed under favorable growing conditions; akinetes – climate-resistant spores that may form when environmental conditions become harsh; and thick-walled heterocysts – which contain the enzyme nitrogenase, vital for nitrogen fixation in an anaerobic environment due to its sensitivity to oxygen.

Why is the water so green?

Blooms of Cyanobacteria, also known as blue-green algae, are affecting inland and coastal communities around the world. Cyanobacteria are aquatic bacteria, and are some of the oldest living organelles on Earth. Because these water-dwelling bacteria photosynthesize, they are also referred to as “blue-green algae.” Cyanobacteria can be found in many different environments, including freshwater and marine ecosystems. Despite being named “blue-green algae,” blooms may appear in many different colors including red, yellow, brown, blue, and green, and often form a scum on the water’s surface.

The relative abundance of phycobilin pigments, the reddish phycoerythrin and the blue phycocyanin, explain the color of cyanobacteria. Microscopically, the blue phycocyanin pigment, the green chlorophyll, and the accessory pigments give rise to blue-green algae. Species of cyanobacteria differ in their ratios of phyocyanin and phycoerythrin. The appearance of a body of water changes drastically during a "bloom" of cyanobacteria, but the color is also not always due to pigments alone.

What might cause a tremendous increase in their growth?

Cyanobacteria are a normal part of most aquatic ecosystems, including lakes, rivers, and oceans. However, when toxic algae are present in an ecosystem, or when there are “algal blooms” (the rapid, uncontrolled growth of algae) they can be harmful. There are factors that contribute to algal blooms, including limiting nutrients, climate change, and pollution. Two important contributing factors are climate change, which creates an environment in which cyanobacteria can thrive, and nutrient loading, which provides the cyanobacteria with and excess of limiting nutrients (nutrients necessary to the the growth of cyanobacteria).

The underlying cause of an individual cyanobacteria bloom can vary, but a major cause of cyanobacteria blooms is nutrient pollution. “Nutrient over enrichment of waters by urban, agricultural, and industrial development has promoted the growth of cyanobacteria as harmful algal blooms.”

Limiting Nutrients

Limiting nutrients refers to nutrients that are essential for an organisms growth and survival. For cyanobacteria, these limiting nutrients include phosphorus and nitrogen. In normal quantities, these nutrients are healthy for many environments. However, when there is an excess of these limiting nutrients, it causes an excess of growth—in the case of cyanobacteria, a “bloom.” Excessive limiting nutrients in lakes can cause eutrophication. Eutrophication is when bodies of water are overloaded with limiting nutrients. “As bodies of freshwater become enriched in nutrients, especially Phosphorus (P), there is often a shift in the phytoplankton community towards dominance by cyanobacteria. Examples of these changes are the dense blooms often found in newly euthrophied lakes, reservoirs, and rivers previously devoid of these events.

Agriculture

Agriculture is one of the main contributors to nutrient pollution. Farmers often use chemical fertilizers and pesticides to increase crop yields. These can contain limiting nutrients for cyanobacteria such as nitrogen and phosphorus. Since farmers usually have irrigated land, excess of rainwater is diverted away from the farm in order to keep crops from being flooded. Runoff from farms is often contaminated with chemicals picked up from the crops and soil. This nutrient rich water usually finds its way to another water source, such as a river or lake, where the excess nutrients can produce a cyanobacteria bloom.

Climate Change

Climate change contributes to excess cyanobacteria blooms by creating ideal conditions for cyanobacteria to grow. Cyanobacteria thrive in warm waters: as global temperatures rise, so too does global water temperatures. Cyanobacteria not only grow more rapidly in warm water from increased temperatures, but warmer waters also make it more difficult for water to mix, meaning the surface of the water remains much warmer than the rest of the body of water—and cyanobacteria grow more successfully on the surface. This is also disadvantageous because growing a thick cover on the surface of the water means that this photosynthetic organism can absorb sunlight easily, and grow even more rapidly.

Furthermore, increasing concentrations of atmospheric carbon dioxide are also favorable to the growth of cyanobacteria.The combination of warmer water temperatures and carbon dioxide absorption further creates perfect conditions for cyanobacteria growth and blooms.

Why would toxin secretion limit fish consumption? How would cyanobacterial photosynthesis affect the oxygen levels of the lake?

When dissolved oxygen levels in aquatic environments plummet, it means that in extreme circumstances, aquatic flora and fauna can suffocate.

“High biomass blooms, whether of toxic or nontoxic species, can lead to very low oxygen levels in the water column (hypoxia), resulting in higher mortality rates in local fish, shellfish, invertebrate, and plant populations.” With low oxygen levels, fish and plants have less oxygen than they need to survive. Furthermore, “the blooms may also affect benthic flora and fauna due to decreased light penetration. Toxic blooms from some cyanobacteria genera may lead to inhibition of other phytoplankton and suppression of zooplankton grazing, leading to reduced growth and reproductive rates and changes in community structure and composition.”

There are many different types of Cyanobacteria, but not all produce toxins. Microcystin and Anatoxin are two of the more common toxins that are produced by Cyanobacteria, and in high concentrations can be very harmful to other organisms living in the same aquatic environment. Under optimal conditions such as warm temperatures, sunlight and plentiful nutrients such as nitrogen and phosphorous, cyanobacteria can grow in localized blooms. When these blooms form toxins, which is increasingly becoming problematic in areas of high nitrogen concentrations, there are a whole host of public health concerns as drinking contaminated water (see the Drinking Water Guide for more information), eating shellfish and or even swimming in affected waterways can cause serious health effects. For this reason, harmful algal blooms are monitored to protect drinking water and prevent recreational exposure.

Aidan indicated that aerobic cyanobacteria are using up oxygen in the water. How are they doing this?

When deprived of fixed nitrogen (fN), certain filamentous cyanobacteria differentiate nitrogen-fixing heterocysts. There is a large and dynamic fraction of stored fN in cyanobacterial cells, but its role in directing heterocyst commitment has not been identified. We present an integrated computational model of fN transport, cellular growth, and heterocyst commitment for filamentous cyanobacteria. By including fN storage proportional to cell length, but without any explicit cell-cycle effect, we are able to recover a broad and late range of heterocyst commitment times and we observe a strong indirect cell-cycle effect. We propose that fN storage is an important component of heterocyst commitment and patterning in filamentous cyanobacteria. The model allows us to explore both initial and steady-state heterocyst patterns. The developmental model is hierarchical after initial commitment: our only source of stochasticity is observed growth rate variability. Explicit lateral inhibition allows us to examine ΔpatS, ΔhetN, and ΔpatN phenotypes. We find that ΔpatS leads to adjacent heterocysts of the same generation, while ΔhetN leads to adjacent heterocysts only of different generations. With a shortened inhibition range, heterocyst spacing distributions are similar to those in experimental ΔpatN systems. Step-down to non-zero external fN concentrations is also investigated.

Under conditions of limited fixed-nitrogen, some filamentous cyanobacteria develop a regular pattern of heterocyst cells that fix nitrogen for the remaining vegetative cells. We examine three different heterocyst placement strategies by quantitatively modelling filament growth while varying both external fixed-nitrogen and leakage from the filament. We find that there is an optimum heterocyst frequency which maximizes the growth rate of the filament; the optimum frequency decreases as the external fixed-nitrogen concentration increases but increases as the leakage increases. In the presence of leakage, filaments implementing a local heterocyst placement strategy grow significantly faster than filaments implementing random heterocyst placement strategies. With no extracellular fixed-nitrogen, consistent with recent experimental studies of Anabaena sp. PCC 7120, the modelled heterocyst spacing distribution using our local heterocyst placement strategy is qualitatively similar to experimentally observed patterns. As external fixed-nitrogen is increased, the spacing distribution for our local placement strategy retains the same shape, while the average spacing between heterocysts continuously increases.

Identify the microbial imbalance in Grand Lake that has led to the oxygen depletion.

The Grand River Dam Authority is reporting that the most recent results from its water sample testing in Grand and Hudson Lakes have shown a negligible presence of blue green algae (BGA) toxins. Due to these results, GRDA is removing its public swim advisories for Grand Lake.Those advisories had been in place for the Fly Creek and Highway 85A bridge areas of Grand Lake. No BGA blooms have been confirmed in any other area of the lakes. While GRDA continues to monitor water quality on both Grand and Hudson, algae blooms are dynamic and develop rapidly.  The Grand River Dam Authority Ecosystems Management’s water lab has confirmed the presence of high levels of blue green algae (BGA) in a small area of Fly Creek (off the Horse Creek Arm of Grand Lake), near Bernice. Tests of the BGA showed microcystin toxin levels greater than 100 micrograms per liter with estimated counts of 577,000 cells per milliliter. BGA with more than 20 micrograms per liter of microcystin and more than 100,000 cells per milliliter is considered toxic. GRDA is advising the public to avoid bodily contact with water in the Fly Creek area.

Overall the algae do produce oxygen. They obtain their energy from photosynthesis, which does create oxygen. But, when blue-green algae die, they decompose by a process that uses cellular respiration, which uses up oxygen. Respiration disassembles carbon molecules and releases chemical energy. It is the decomposition of the cyanobacteria that causes oxygen depletion. Green algae also uses up oxygen in the same way when it decomposes.