What is Noctiluca scintillans?
In the last decade, the northern Arabian Sea has witnessed a radical shift in the composition of winter phytoplankton blooms, which previously comprised mainly of diatoms, the unicellular, siliceous photosynthetic organisms favored by nutrient-enriched waters from convective mixing. These trophically important diatom blooms have been replaced by widespread blooms of a large, green dinoflagellate, Noctiluca scintillans, which combines carbon fixation from its chlorophyll-containing endosymbiont with ingestion of prey. Here, we report that these massive outbreaks of N. scintillans during winter are being facilitated by an unprecedented influx of oxygen deficient waters into the euphotic zone and by the extraordinary ability of its endosymbiont Pedinomonas noctilucae to fix carbon more efficiently than other phytoplankton under hypoxic conditions. We contend that N. scintillans blooms could disrupt the traditional diatom-sustained food chain to the detriment of regional fisheries and long-term health of an ecosystem supporting a coastal population of nearly 120 million people.
Arabian Sea Context
Sea Level Anomolies
Arabian Sea Monsoon Winds and Sea Surface Temperature (℃)
Summer and Winter monsoon seasons greatly impact the Arabian Sea region. The summer monsoon brings very strong winds that travel northeast from the open ocean into the Northern Arabian Sea. Choppy and slightly turbid waters are its results.
Winter monsoons reverse the summer's winds. Weak winds travel out from the continent south into the Arabian Sea. With little power, they create minimal disturbance of the waters, turning them glassy.
Himalayan Snow Cover
The decrease in snow cover has led to greater differences in both temperature and pressure systems between the Indian subcontinent and the Arabian Sea. The pressure differences generate monsoon winds that mix the ocean water in the Western Arabian Sea.
Who would have thought that melting snow cover in the Himalayan Mountains could alter the ocean food chain over a thousand miles away?
Image to right: A graphic shows how snowmelt affects phytoplankton blooms. Click on image to enlarge. Oxygen depleted waters also provide the perfect environment for the growth of bacteria that convert nitrate in seawater into forms of nitrogen that most plants can't use. One of the latter is nitrous oxide, also known as laughing gas. In the atmosphere, nitrous oxide is 310 times more potent as a greenhouse gas than carbon dioxide. Thus, as very large phytoplankton blooms deplete more oxygen from the water, the creation of nitrous oxide in the Arabian Sea could exacerbate climate change. Credit: Joaquim Goes, Bigleow Laboratory for Ocean Sciences
The study finds a decline in winter and spring snow cover over Southwest Asia and the Himalayan mountain range is creating the right conditions for more widespread blooms of ocean plants in the Arabian Sea.
The decrease in snow cover has led to greater differences in both temperature and pressure systems between the Indian subcontinent and the Arabian Sea. The pressure differences generate monsoon winds that mix the ocean water in the Western Arabian Sea. This mixing leads to better growing conditions for tiny, free-floating ocean plants called phytoplankton. Phytoplankton serve as the base of the ocean food chain.
Joaquim Goes, a senior researcher at the Bigelow Laboratory for Ocean Sciences in Maine is the study's lead author. He and colleagues used data from OrbImage and NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite to show that phytoplankton concentrations in the Western Arabian Sea have increased by over 350 percent over the past 7 years.
"Climate change and warming are causing a decline in snow cover over the Eurasian region, especially over the Himalayas," said Goes. "The associated shifts in winds, and increased phytoplankton levels in the Arabian Sea, could have far reaching consequences for the ecosystem of the region."
Image left -- Reversal of Monsoon Wind Direction (Jun-Sept): The temperature difference between the landmass and the ocean creates a low pressure system over the Indian subcontinent and a high pressure system over the Arabian Sea. This difference causes the winds to blow from the Southwest Arabian Sea, bringing rainfall from June to September each year. The color bar represents the concentration of chlorophyll, with blue being the smallest and red being the largest. Click on image to enlarge. Credit: Joaquim Goes, Bigelow Laboratory for Ocean Sciences
When winter and spring snow cover is low over Eurasia, more of the Sun's rays are absorbed by Earth, rather than getting reflected back out to space. As a result the land mass heats up more in summer creating a larger temperature difference between the water of the Arabian Sea and the Indian subcontinent landmass.
The temperature shift is responsible for a difference in pressure over land and the sea, creating a low pressure system over the Indian subcontinent and a high pressure system over the Arabian Sea. This difference in pressure causes the winds to blow from the Southwest Arabian Sea bringing rainfall to the subcontinent from June to September each year, known as the summer monsoon.
In the Western Arabian Sea these winds also cause upwelling of cooler nutrient-rich water, creating ideal conditions for phytoplankton to bloom every year during summer.
Image right: This graph depicts snow cover in Southwest Asia (SWA), and the Himalayas-Tibetan Plateau (HTP) between 1967 and 2003. The horizontal "zero" line represents the monthly average snow cover. Below that line indicates a drop, above represents an increase. The graph shows a decline in winter snow that has been particularly rapid over the past 6-7 years. Click on image to enlarge Credit: Joaquim Goes, Bigelow Laboratory for Ocean Sciences
Since 1997, a reduction in snow has led to wider temperature differences between the land and ocean during summer. As a consequence, sea surface winds over the Arabian Sea have strengthened leading to more intense upwelling and more widespread blooms of phytoplankton along the coasts of Somalia, Yemen and Oman.
According to Goes, while large blooms of phytoplankton can enhance fisheries, exceptionally large blooms could be detrimental to the ecosystem. Increases in phytoplankton amounts can lead to oxygen depletion in the water column and eventually to a decline in fish populations.
For more graphics and information visit: http://www.bigelow.org/climatechange/
Two major monsoon seasons greatly impact the Arabian Sea region. The summer monsoon brings very strong winds that travel northeast from the open ocean into the Northern Arabian Sea. These winds create slightly turbid and choppy waters throughout the Arabian Sea. The winter monsoon reverses the winds. During this season, weak winds travel from the continent to the northeast south into the Arabian Sea. These weak winds create little disturbance and the waters of the Northern Arabian Sea are often glassy during this time period.
Convective mixing and the presence of the oxygen minimum zone, or OMZ, play a key role in the creation of the conditions present in the Arabian Sea. When blooms or organisms die off, they begin to sink to lower depths where they are broken down by decomposers and bacteria. This decomposition process uses up most or all the available oxygen and creates OMZ. The OMZ is the zone or depth at which oxygen drastically decreases or is no longer present. These waters are, however, very high in nutrients from the decomposition process. The surface waters are typically much higher in oxygen as a result of photosynthesis, but far lower in necessary nutrients as they are taken up by the surrounding organisms. Convective mixing is the process in which density differences in the water column force these differing waters to mix and change locations. For example, in the Arabian Sea, the sub-oxic or anoxic OMZ nutrient rich waters are cooler and denser, whereas the oxygen rich and nutrient poor waters are warmer and less dense, so they are found at the surface of the water column. Convective mixing occurs during the summer and winter monsoon seasons when the winds cool the warm surface waters and make them more dense. They will then sink and be replaced by the nutrient rich waters and the upwelling cycle will continue and promote productivity. The high oxygen waters that sink also allow decomposers at lower water depths to continue the decomposition process.
This process that drives productivity in the region is changing as a result of climate change. Increased snow melt in the Himalayas and the warming of the Eurasian continent are promoting stratification of the water column. Less dense freshwater and warmer surrounding land is not allowing the surface layers to cool and sink. This is allowing for the OMZ to expand without upwelling, and prohibits the cycling of nutrients in the water column. This stratification is detrimental to the vast majority of marine organisms. As the natural upwelling cycle is slowed and even stalled, proper enrichment is not possible.
This trend, observed since the mid 1990’s, coincides with the advent of Noctiluca blooms. Photosynthetic phytoplankton and diatoms are poorly suited to survive in low nutrient conditions. Contrastingly, Noctiluca’s mixotrophic mode that gives it a competitive advantage, allows it to outcompete solely photosynthetic primary producers. Over the past few decades scientists have observed a large shift in the contents of primary producers in the Arabian Sea from phytoplankton and diatoms to Noctiluca.
Noctiluca has been observed in small amounts for decades during the summer monsoon season. During this time, they have been observed along the shore in coves, inlets, and bays where turbulence and oxygen levels are low. Noctiluca had not been observed at all in the winter monsoon season until the early 2000’s. Their presence in the summer monsoon seasons but not winter is a result of the less dramatic upwelling in winter. Noctiluca thrive and prefer low oxygen and low turbulence waters. The summer monsoon is dramatic enough to bring low oxygen waters to the surface, which is why Noctiluca were traditionally seen during this time but only in coves and such areas where turbulence was low. The winter upwelling was not strong enough as a result of smaller temperature differences to bring low oxygen waters to the surface, so Noctiluca was not seen there.
This trend has changed with climate change. Stratification has decreased the oxygen content in the surface layers of the water and has allowed for Noctiluca blooms to persist into the winter months like never before. The low turbulence, and now low oxygen, create perfect conditions for the organism to thrive, allowing Noctiluca to bloom extensively and dominate the region with their mixotrophy.
Our study is designed to understand how Noctiluca reacts to varying O2 concentrations with no CO2 manipulation, and varying CO2 and O2 concentrations. Additionally, we would like to understand the differences in growth, cell health, and more in these concentrations between fed and unfed cultures.
We will have three experiments running this summer. One experiment will manipulate CO2 and O2 conditions within the 1 Liter culture bottles. These conditions will replicate different time periods, including predicted conditions, to see how Noctiluca will react to past, current, and future oceanic conditions influenced by climate change. There will be seven culture bottles for each of the three CO2 and O2 concentrations. Three of seven bottles will be fed, meaning the dinoflagellate Peridinium which is common prey of Noctiluca will be present. The other four bottles will be unfed. More details concerning procedures and experimental set up will follow below in the methods section.
In addition to the CO2 and O2 manipulation experiment, we will be running two identical O2 manipulation experiments one after the other. CO2 will not be manipulated in these culture bottles. O2 conditions will vary in three conditions and the fed and unfed aspect from the CO2 and O2 experiment will remain the same. More details concerning procedures and experimental set up will follow below in the methods section.
Our research also aims to better comprehend the origins of such blooms and clearly define the confusing and complex symbiotic relationship between Noctiluca and Pedinomonas Noctilucae. The endosymbiont, Pedinomonas Noctilucae, likely originated about 1.2 billion years ago. At this point in time, Earth’s oceans were anoxic, high in CO2, and therefore highly acidic. This changed, however, when a group of cyanobacteria began to decompose organic matter and initiate an oxygen recycling process. Pedinomonas Noctilucae likely evolved in this time, which is the most likely reason that they are more productive in high CO2 and low O2 environments. This entails defining whether Pedinomonas Noctilucae actively enter Noctiluca as the high ammonia and low pH conditions within the cell are prime for Pedinomonas Noctilucae growth, or if Noctiluca feed on the endosymbiont, and harbor it safely in its central vacuole in order to capitalize on its photosynthetic abilities. Additionally, we would like to understand why the endosymbiont leaves the cell. The endosymbiont’s activities will exacerbate the low pH environment. In this case, Noctiluca may actively eject the endosymbionts in order to maintain homeostasis. Contrastingly, if the host cell is not properly functioning or dying, Pedinomonas Noctilucae may voluntarily leave the cell. Furthermore, if conditions outside the cell are preferable for Pedinomonas Noctilucae, they may also voluntarily leave the cell. Finally, Noctiluca may push Pedinomonas Noctilucae out of the cell in stressful conditions in order to reduce its cell size and therefore the amount of energy, metabolism, and nutrients it needs to survive and maintain homeostasis.
Questions: How will Noctiluca react to varying CO2 and O2 concentrations? Will fed and unfed bottles yield different results in the same concentrations? What parameters define the symbiotic relationship between Noctiluca and their endosymbionts? What is the role of limiting nutrients and salinity in the blooms of Noctiluca? What conditions initiate Noctiluca blooms and what conditions do they create? Is it hypoxia alone or a combination of hypoxia and certain CO2 levels that induce these blooms?
Hypothesis: Noctiluca will respond better to high CO2 and low O2 conditions in unfed bottles as photosynthesis of endosymbionts is more productive at these concentrations. This “better response” is defined as increased cell counts, high ammonium transport rates, and increased chlorophyll a production.
Because high CO2 and low O2 concentrations promote photosynthetic ability, Peridinium, a dinoflagellate which will serve as the food for Noctiluca, will outgrow and take over the Noctiluca in the fed bottles. Additionally, the endosymbionts will leave the host cell for favorable conditions outside the cell and because of decreased cell heath due to the Peridinium dominance and decrease Noctiluca’s health and cell counts while stripping Noctiluca of its competitive advantage.
As for the O2 manipulation experiment, Noctiluca will fare better, as defined by the factors above, in lower O2 concentrations, as this is where blooms have been found.